Swift's blog

By Swift, history, 3 years ago, In English,

**EDIT: A shorter error function **

WARNING: Many of these things belong to C++11 so use C++11 in order to test anything here :)

I just write a short version for this article, because it's now in the main page. I recommend you to click on "Read more »" and read more :) Here is a short trick for the short version:

I see lots of programmers write code like this one:

pair<int, int> p;
vector<int> v;
// ...
p = make_pair(3, 4);
v.push_back(4); v.push_back(5);

while you can just do this:

pair<int, int> p;
vector<int> v;
// ...
p = {3, 4};
v = {4, 5};

1. Assign value by a pair of {} to a container

I see lots of programmers write code like this one:

pair<int, int> p;
// ...
p = make_pair(3, 4);

while you can just do this:

pair<int, int> p;
// ...
p = {3, 4};

even a more complex pair

pair<int, pair<char, long long> > p;
// ...
p = {3, {'a', 8ll}};

What about vector, deque, set and other containers?


vector<int> v; v = {1, 2, 5, 2}; for (auto i: v) cout << i << ' '; cout << '\n'; // prints "1 2 5 2" deque<vector<pair<int, int>>> d; d = {{{3, 4}, {5, 6}}, {{1, 2}, {3, 4}}}; for (auto i: d) { for (auto j: i) cout << j.first << ' ' << j.second << '\n'; cout << "-\n"; } // prints "3 4 // 5 6 // - // 1 2 // 3 4 // -" set<int> s; s = {4, 6, 2, 7, 4}; for (auto i: s) cout << i << ' '; cout << '\n'; // prints "2 4 6 7" list<int> l; l = {5, 6, 9, 1}; for (auto i: l) cout << i << ' '; cout << '\n'; // prints "5 6 9 1" array<int, 4> a; a = {5, 8, 9, 2}; for (auto i: a) cout << i << ' '; cout << '\n'; // prints "5 8 9 2" tuple<int, int, char> t; t = {3, 4, 'f'}; cout << get<2>(t) << '\n';

Note that it doesn't work for stack and queue.

2. Name of argument in macros

You can use '#' sign to get exact name of an argument passed to a macro:

#define what_is(x) cerr << #x << " is " << x << endl;
// ...
int a_variable = 376;
what_is(a_variable);
// prints "a_variable is 376"
what_is(a_variable * 2 + 1)
// prints "a_variable * 2 + 1 is 753"

3. Get rid of those includes!

Simply use

#include <bits/stdc++.h>

This library includes many of libraries we do need in contest like algorithm, iostream, vector and many more. Believe me you don't need to include anything else!

4. Hidden function (not really hidden but not used often)

one)

__gcd(value1, value2)

You don't need to code Euclidean Algorithm for a gcd function, from now on we can use. This function returns gcd of two numbers.

e.g. __gcd(18, 27) = 9.

two)

__builtin_ffs(x)

This function returns 1 + least significant 1-bit of x. If x == 0, returns 0. Here x is int, this function with suffix 'l' gets a long argument and with suffix 'll' gets a long long argument.

e.g. __builtin_ffs(10) = 2 because 10 is '...10 1 0' in base 2 and first 1-bit from right is at index 1 (0-based) and function returns 1 + index.

three)

__builtin_clz(x)

This function returns number of leading 0-bits of x which starts from most significant bit position. x is unsigned int and like previous function this function with suffix 'l gets a unsigned long argument and with suffix 'll' gets a unsigned long long argument. If x == 0, returns an undefined value.

e.g. __builtin_clz(16) = 27 because 16 is ' ... 10000'. Number of bits in a unsigned int is 32. so function returns 32 — 5 = 27.

four)

__builtin_ctz(x)

This function returns number of trailing 0-bits of x which starts from least significant bit position. x is unsigned int and like previous function this function with suffix 'l' gets a unsigned long argument and with suffix 'll' gets a unsigned long long argument. If x == 0, returns an undefined value.

e.g. __builtin_ctz(16) = 4 because 16 is '...1 0000 '. Number of trailing 0-bits is 4.

five)

__builtin_popcount(x)

This function returns number of 1-bits of x. x is unsigned int and like previous function this function with suffix 'l' gets a unsigned long argument and with suffix 'll' gets a unsigned long long argument. If x == 0, returns an undefined value.

e.g. __builtin_popcount(14) = 3 because 14 is '... 111 0' and has three 1-bits.

Note: There are other __builtin functions too, but they are not as useful as these ones.

Note: Other functions are not unknown to bring them here but if you are interested to work with them, I suggest this website.

5. Variadic Functions and Macros

We can have a variadic function. I want to write a sum function which gets a number of ints, and returns sum of them. Look at the code below:

int sum() { return 0; }

template<typename... Args>
int sum(int a, Args... args) { return a + sum(args...); }

int main() { cout << sum(5, 7, 2, 2) + sum(3, 4); /* prints "23" */ }

In the code above I used a template. sum(5, 7, 2, 2) becomes 5 + sum(7, 2, 2) then sum(7, 2, 2), itself, becomes 7 + sum(2, 2) and so on... I also declare another sum function which gets 0 arguments and returns 0.

I can even define a any-type sum function:

int sum() { return 0; }

template<typename T, typename... Args>
T sum(T a, Args... args) { return a + sum(args...); }

int main() { cout << sum(5, 7, 2, 2) + sum(3.14, 4.89); /* prints "24.03" */ }

Here, I just changed int to T and added typename T to my template.

In C++14 you can also use auto sum(T a, Args... args) in order to get sum of mixed types. (Thanks to slycelote and Corei13)

We can also use variadic macros:

#define a_macro(args...) sum(args...)

int sum() { return 0; }

template<typename T, typename... Args>
auto sum(T a, Args... args) { return a + sum(args...); }

int main() { cout << a_macro(5, 7, 2, 2) + a_macro(3.14, 4.89); /* prints "24.03" */ }

Using these 2, we can have a great debugging function: (thanks to Igorjan94) — Updated!

#include <bits/stdc++.h>

using namespace std;

#define error(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }

void err(istream_iterator<string> it) {}
template<typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
	cerr << *it << " = " << a << endl;
	err(++it, args...);
}

int main() {
	int a = 4, b = 8, c = 9;
	error(a, b, c);
}

Output:

a = 4
b = 8
c = 9

This function helps a lot in debugging.

6. Here is C++0x in CF, why still C++?

Variadic functions also belong to C++11 or C++0x, In this section I want to show you some great features of C++11.

one) Range-based For-loop

Here is a piece of an old code:

set<int> s = {8, 2, 3, 1};
for (set<int>::iterator it = s.begin(); it != s.end(); ++it)
    cout << *it << ' ';
// prints "1 2 3 8"

Trust me, that's a lot of code for that, just use this:

set<int> s = {8, 2, 3, 1};
for (auto it: s)
    cout << it << ' ';
// prints "1 2 3 8"

We can also change the values just change auto with auto &:

vector<int> v = {8, 2, 3, 1};
for (auto &it: v)
    it *= 2;
for (auto it: v)
    cout << it << ' ';
// prints "16 4 6 2"

two) The Power of auto

You don't need to name the type you want to use, C++11 can infer it for you. If you need to loop over iterators of a set<pair<int, pair<int, int> > > from begin to end, you need to type set<pair<int, pair<int, int> > >::iterator for me it's so suffering! just use auto it = s.begin()

also x.begin() and x.end() now are accessible using begin(x) and end(x).

There are more things. I think I said useful features. Maybe I add somethings else to post. If you know anything useful please share with Codeforces community :)

From Ximera's comment:

this code:

for(i = 1; i <= n; i++) {
    for(j = 1; j <= m; j++)
        cout << a[i][j] << " ";
    cout << "\n";
}

is equivalent to this:

for(i = 1; i <= n; i++)
    for(j = 1; j <= m; j++)
        cout << a[i][j] << " \n"[j == m];

And here is the reason: " \n" is a char*, " \n"[0] is ' ' and " \n"[1] is '\n'.

From technetium28's comment:

Usage of tie and emplace_back:

#define mt make_tuple
#define eb emplace_back
typedef tuple<int,int,int> State; // operator< defined

int main(){
  int a,b,c;
  tie(a,b,c) = mt(1,2,3); // assign
  tie(a,b) = mt(b,a); // swap(a,b)

  vector<pair<int,int>> v;
  v.eb(a,b); // shorter and faster than pb(mp(a,b))

  // Dijkstra
  priority_queue<State> q;
  q.emplace(0,src,-1);
  while(q.size()){
    int dist, node, prev;
    tie(dist, ode, prev) = q.top(); q.pop();
    dist = -dist;
    // ~~ find next state ~~
    q.emplace(-new_dist, new_node, node);
  }
}

And that's why emplace_back faster: emplace_back is faster than push_back 'cause it just construct value at the end of vector but push_back construct it somewhere else and then move it to the vector.

Also in the code above you can see how tie(args...) works. You can also use ignore keyword in tie to ignore a value:

tuple<int, int, int, char> t (3, 4, 5, 'g');
int a, b;
tie(b, ignore, a, ignore) = t;
cout << a << ' ' << b << '\n';

Output: 5 3

I use this macro and I love it:

#define rep(i, begin, end) for (__typeof(end) i = (begin) - ((begin) > (end)); i != (end) - ((begin) > (end)); i += 1 - 2 * ((begin) > (end)))

First of all, you don't need to name the type you want to use. Second of all it goes forwards and backwards based on (begin > end) condition. e.g. rep(i, 1, 10) is 1, 2, ..., 8, 9 and rep(i, 10, 1) is 9, 8, ..., 2, 1

It works well with different types e.g.

vector<int> v = {4, 5, 6, 4, 8};
rep(it, end(v), begin(v))
    cout << *it << ' ';
// prints "8 4 6 5 4"

Also there is another great feature of C++11, lambda functions!

Lambdas are like other languages' closure. It defines like this:

[capture list](parameters) -> return value { body }

one) Capture List: simple! We don't need it here, so just put []

two) parameters: simple! e.g. int x, string s

three) return value: simple again! e.g. pair<int, int> which can be omitted most of the times (thanks to Jacob)

four) body: contains function bodies, and returns return value.

e.g.

auto f = [] (int a, int b) -> int { return a + b; };
cout << f(1, 2); // prints "3"

You can use lambdas in for_each, sort and many more STL functions:

vector<int> v = {3, 1, 2, 1, 8};
sort(begin(v), end(v), [] (int a, int b) { return a > b; });
for (auto i: v) cout << i << ' ';

Output:

8 3 2 1 1

From Igorjan94's comment:

Usage of move:

When you work with STL containers like vector, you can use move function to just move container, not to copy it all.

vector<int> v = {1, 2, 3, 4};
vector<int> w = move(v);

cout << "v: ";
for (auto i: v)
    cout << i << ' ';

cout << "\nw: ";
for (auto i: w)
    cout << i << ' ';

Output:

v: 
w: 1 2 3 4 

As you can see v moved to w and not copied.

7. C++0x Strings

one) Raw Strings (From IvayloS's comment)

You can have UTF-8 strings, Raw strings and more. Here I want to show raw strings. We define a raw string as below:

string s = R"(Hello, World!)"; // Stored: "Hello, World!"

A raw string skips all escape characters like \n or \". e.g.

string str = "Hello\tWorld\n";
string r_str = R"(Hello\tWorld\n)";
cout << str << r_str;

Output:

Hello	World
Hello\tWorld\n

You can also have multiple line raw string:

string r_str =
R"(Dear Programmers,
I'm using C++11
Regards, Swift!)";
cout << r_str;

Output:

Dear Programmer,
I'm using C++11
Regards, Swift!

two) Regular Expressions (regex)

Regular expressions are useful tools in programming, we can define a regular expression by regex e.g. regex r = "[a-z]+";. We will use raw string for them because sometimes they have \ and other characters. Look at the example:

regex email_pattern(R"(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$)"); // This email pattern is not totally correct! It's correct for most emails.

string
valid_email("swift@codeforces.com"),
invalid_email("hello world");

if (regex_match(valid_email, email_pattern))
    cout << valid_email << " is valid\n";
else
    cout << valid_email << " is invalid\n";

if (regex_match(invalid_email, email_pattern))
    cout << invalid_email << " is valid\n";
else
    cout << invalid_email << " is invalid\n";

Output:

swift@codeforces.com is valid
hello world is invalid

Note: You can learn Regex in this website.

three) User-defined literals

You already know literals from C++ like: 0xA, 1000ll, 3.14f and so on...

Now you can have your own custom literals! Sounds great :) So let's see an example:

long long operator "" _m(unsigned long long literal) {
	return literal;
}

long double operator "" _cm(unsigned long long literal) {
	return literal / 100.0;
}

long long operator "" _km(unsigned long long literal) {
	return literal * 1000;
}

int main() {
	// See results in meter:
	cout << 250_m << " meters \n"; // Prints 250 meters
	cout << 12_km << " meters \n"; // Prints 12000 meters
	cout << 421_cm << " meters \n"; // Prints 4.21 meters
}

Note that a literal should start with an underscore (_). We declare a new literal by this pattern:

[returnType] operator "" _[name]([parameters]) { [body] }

note that parameters only can be one of these:

(const char *)

(unsigned long long int)

(long double)

(char)

(wchar_t)

(char16_t)

(char32_t)

(const char *, size_t)

(const wchar_t *, size_t)

(const char16_t *, size_t)

(const char32_t *, size_t)

Literals also can used with templates.

To be continued :)

 
 
 
 
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3 years ago, # |
  Vote: I like it +7 Vote: I do not like it

Its awesome. Thanks Swift :)

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3 years ago, # |
  Vote: I like it +7 Vote: I do not like it

Your sum function returns an incorrect result for sum(1, 1.5). To fix, declare the return type as auto.

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3 years ago, # |
Rev. 2   Vote: I like it +5 Vote: I do not like it

It's better to use auto& in range-based loop when the object is not primitive (e.g pair, vector). UPD: I realized that you mention it at the end, but there are some code written poorly because of that in the first part.

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    21 month(s) ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    actually, compiler optimizations will get rid of the extra copy operations if you are not modifying the element. so I don't think it will be any slower in runtime compared to auto&.

    You can use auto& if you are too suspicious, but I don't think that the first part is categorized as 'written poorly'. it is just OK.

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    15 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    const auto& is even better if you want to be really strict about it.

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3 years ago, # |
  Vote: I like it +18 Vote: I do not like it

"these things are belong to C++11" — https://www.youtube.com/watch?v=8fvTxv46ano :)

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3 years ago, # |
  Vote: I like it -72 Vote: I do not like it

c++ is a very useless language

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3 years ago, # |
  Vote: I like it +4 Vote: I do not like it

mukel already has written nice "C++11 for dummies" tutorial http://codeforces.com/blog/entry/10124 . I think it's a good idea to provide that link directly in entry.

  • »
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    3 years ago, # ^ |
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    Excellent tutorial, I'll add it at top of blog.

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3 years ago, # |
  Vote: I like it +10 Vote: I do not like it

Could you give link to compiler that you use? Because I get CE on my GNU 4.7.1:)

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    3 years ago, # ^ |
    Rev. 2   Vote: I like it +5 Vote: I do not like it

    In CF, use GNU C++0x 4 instead of GNU C++ 4.7.

    Get latest GCC, and from your terminal/cmd use one of these flags -std=gnu++11 or -std=c++11 You can download it for your computer: Windows

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Thanks for such a nice explanation...

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3 years ago, # |
  Vote: I like it +5 Vote: I do not like it

Anyone knows how to include <bits/stdc++.h> on OS X? I am already using gcc but it cannot found that header...

  • »
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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it
    1. Go to: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1

    2. Create a folder named bits

    3. Add a file into that named stdc++.h

    4. Edit it and include libraries

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    21 month(s) ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    there is another way: install GCC using brew terminal package manager!

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3 years ago, # |
  Vote: I like it +4 Vote: I do not like it

The second sum function (with auto) is C++14 standard, not C++11. C++11 doesn't allow function without a return type.

  • »
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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Thanks for sharing your knowledge to us! That's why Xcode couldn't compile that. Now I tested it with C++14 and everything is OK. So let's make it clear in blog.

    • »
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      3 years ago, # ^ |
        Vote: I like it +32 Vote: I do not like it

      And it is still possible to write sum (or other) functions for mixed type using std::common_type

      template <typename A, typename B>
      auto sum(A a, B b) -> typename common_type<A, B>::type {
          return static_cast<typename common_type<A, B>::type>(a) + static_cast<typename common_type<A, B>::type>(b);
      }
      
      template <typename A, typename B, typename... Args>
      auto sum(A a, B b, Args... args) -> typename common_type <A, B, Args...>::type {
          return sum(sum(a, b), args...);
      }
      
      int main() {
          cout << sum(5, 7, 2, 2) + sum(3.14, 4.89) << endl;      // 24.03
          cout << sum (complex <double>(1, 2), 1.3, 2) << endl;   // (4.3,2)
      }
      
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3 years ago, # |
  Vote: I like it +3 Vote: I do not like it

As for __gcd(), it may be a little tricky at some compilers.

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3 years ago, # |
Rev. 2   Vote: I like it +30 Vote: I do not like it

The best thing is that you can write like this (C++11 vs C++) :D

vector<pair<int, int>> v;  

instead of this

vector<pair<int, int> > v;
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3 years ago, # |
Rev. 2   Vote: I like it +31 Vote: I do not like it

May be you can tell something more about this

for(i = 1; i <= n; i++) {
    for(j = 1; j <= m; j++)
        cout << a[i][j] << " ";
    cout << "\n";
}
for(i = 1; i <= n; i++)
    for(j = 1; j <= m; j++)
        cout << a[i][j] << " \n"[j == m];
  • »
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    3 years ago, # ^ |
    Rev. 3   Vote: I like it +32 Vote: I do not like it

    Well, Great creativity :)

    " \n" is a char*, " \n"[0] is ' ' and " \n"[1] is '\n'.

    Also this is a correct one too:

    for (int i = 1; i <= n; i++)
    		for (int j = 1; j <= m; j++)
    			cout << a[i][j] << (j == m)[" \n"];
    

    It's because e.g. a[8] and 8[a] are the same thing both of them are (a + 8)* and (8 + a)*.

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    3 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    For a while, I thought that this is Iverson's bracket :D

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3 years ago, # |
Rev. 2   Vote: I like it +14 Vote: I do not like it

Do you know tie and emplace ?

#define mt make_tuple
#define eb emplace_back
typedef tuple<int,int,int> State; // operator< defined

int main(){
  int a,b,c;
  tie(a,b,c) = mt(1,2,3); // assign
  tie(a,b) = mt(b,a); // swap(a,b)

  vector<pair<int,int>> v;
  v.eb(a,b); // shorter and faster than pb(mp(a,b))

  // Dijkstra
  priority_queue<State> q;
  q.emplace(0,src,-1);
  while(q.size()){
    int dist, node, prev;
    tie(dist, ode, prev) = q.top(); q.pop();
    dist = -dist;
    // ~~ find next state ~~
    q.emplace(-new_dist, new_node, node);
  }
}
  • »
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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Such a great feature.

    emplace_back is faster than push_back 'cause it just construct value at the end of vector but push_back construct it somewhere else and then move it to the vector.

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3 years ago, # |
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Can you get the previous element in an, let's say, vector using auto? Here is why auto is not the best option for dp-like tasks where you need information from the previous elements.

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    3 years ago, # ^ |
    Rev. 3   Vote: I like it +4 Vote: I do not like it

    Use this approach:

    vector<int> dp = {4, 5, 6, 4, 8};
    for (auto i = ++dp.begin(); i != dp.end(); ++i)
        *i += *(i - 1);
    for (auto i: dp)
        cout << i << '\n';
    

    Output:

    4
    9
    15
    19
    27
    

    Use range-based for-loop only when you want exact element, when you need to access other elements use normal for-loop, but this doesn't mean that you can't use auto in that for-loop.

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      3 years ago, # ^ |
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      Hm, I didn't know it could be done. Still, it is easier with normal for loop.

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        3 years ago, # ^ |
        Rev. 3   Vote: I like it +3 Vote: I do not like it

        Btw, using auto is just for inferring type you are working with. If your type is int, it's better to use that ('cause it's just 3 characters) but if your type is std::vector<std::pair<std::set<int>, bool>>::iterator so I think using auto is a must :)

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      12 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Just saying. Cumulative sum can be done only with this-

      vector<int> dp = {4, 5, 6, 4, 8};
      partial_sum(dp.begin(), dp.end(), dp.begin());
      
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3 years ago, # |
  Vote: I like it +13 Vote: I do not like it

In 2, I use:

#define DB(x) cerr << __LINE__ << ": " << #x << " = " << (x) << endl

In this way I get the number of the line in which this instruction is executed. It's useful when we have more than one variable with the same name. Also, x needs to be enclosed in parenthesis due to operators precedence.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

would you please tell me about vector ,i don't know anything about that !

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3 years ago, # |
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Its useful! Thanks for sharing.

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3 years ago, # |
Rev. 2   Vote: I like it +6 Vote: I do not like it

You say that "Variadic functions also belong to C++11", but that's not really correct. Even C had variadic functions. New feature in C++11 is variadic templates.

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    3 years ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    Yeah. You're right. Here I used variadic template so I said it's for C++11.

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3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

I thing you should consider defining short version of your blog post, now that it is on the main page.

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3 years ago, # |
  Vote: I like it +27 Vote: I do not like it

In my country, at this time, we are not allowed to use C++11 in national contest.

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    3 years ago, # ^ |
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    Is C++11 being used in IOI? If this is the case, I guess it should not be hard to convince the judge committee to change.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

if i have a vector < pair<int, pair<int, int> > > a;

could i use emplace_back to insert {1, {2, 3} }? i tries to emplace_back(1, 2, 3); but of course it's an error.

thanks in advance :-)

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    You could emplace_back(¹, mp(2,3))

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      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      thank you for replying. i was looking forward for a method like that above something like (1, 2, 3); as i don't like using macros, something that's faster to write.

      thanks in advance :)

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Don't use pair<int, pair<int, int>>! Code less and use tuple<int, int, int>:

    vector<tuple<int, int, int>> v;
    v.emplace_back(1, 2, 3);
    
    • »
      »
      »
      3 years ago, # ^ |
        Vote: I like it +3 Vote: I do not like it

      Well, actually sometimes pair<int, pair<int,int> > x; may make more sense than tuple<int,int,int> x;, for instance when x.second are coordinates of some point and x.first is some property of this point.

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        3 years ago, # ^ |
        Rev. 2   Vote: I like it +10 Vote: I do not like it

        When working with tuples, you don't really use get(tuple) you do use tie:

        tie(point_property, pointx, pointy) = some_tuple;
        

        And that makes sense.

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        3 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        then you probably have that point as a variable, not as two coordinates.

        • »
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          »
          »
          »
          3 years ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          I often use

          #define X first
          #define Y second
          #define pii pair<int, int>
          
          pii point;
          
      • »
        »
        »
        »
        3 years ago, # ^ |
          Vote: I like it +25 Vote: I do not like it

        Yeah let's write ugly unreadable code with nested pairs and macros instead of class/struct.

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          »
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          »
          3 years ago, # ^ |
            Vote: I like it +8 Vote: I do not like it

          I totally agree that classes/structs are more readable. I just wanted to point out that in some cases tuple<int,int,int> is less readable (at least for me) than pair<int, pair<int,int> >.

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          21 month(s) ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          The real solution to this would be something that lets us write

          struct dist_xy {
              const int dist, x, y;
          };
          

          and then would supply a commonsense bool operator< (..) automatically.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Thanks for this! I'm sure many of us would also be interested in a Java tricks article! :)

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    3 years ago, # ^ |
      Vote: I like it +38 Vote: I do not like it

    The advantage of Java is that there are no tricks.

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    3 years ago, # ^ |
    Rev. 2   Vote: I like it 0 Vote: I do not like it

    I can also write an article about Swift's tricks. But no one here, cares about that language :)

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3 years ago, # |
Rev. 2   Vote: I like it +3 Vote: I do not like it

your debugging function doesn't work for #args with spaces
so, I think it's better to rewrite split to more universal

vector<string> split(const string& s, char c) {
    vector<string> v;
    stringstream ss(s);
    string x;
    while (getline(ss, x, c))
        v.eb(x); //emplace_back
    return std::move(v);
}

(Note no copying because of move, another cpp trick)
and macro will be:

#define err(args...) {\
    vector<string> _v = split(#args, ',');\
    err(_v.begin(), args);\
}
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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    It also brings default space before arguments, e.g. err(a, b) outputs:

    a = value1
     b = value2
    

    but it's better for arguments like a + b so I'll replace it with my code.

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      »
      3 years ago, # ^ |
      Rev. 3   Vote: I like it 0 Vote: I do not like it

      oh, yep, I forgot I changed your err to

      void err(vector<string>::iterator it) {}
      template<typename T, typename... Args>
      void err(vector<string>::iterator it, T a, Args... args) {
      	cerr << it->substr((*it)[0] == ' ') << " = " << a << '\n';
      	err(++it, args...);
      }
      
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      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      if you are interested in it, I also have writeln and readln on variadic templates, which helps to write smth like this:

      int n; vector<pair<int, pair<int, long long>>> a; long long l; char c; string s; double d; // just any combination of fundamental types + vector/pair
      readln(n, a, l, c, s, d);
      writeln(n, a, l, c, s, d);
      

      you can find it here 9388829(I deleted all spaces for more compact view)
      if trailing space is unimportant, half of code can be deleted:)
      it can be simply extended on user's types by overloading ostream and istream operators
      this template is with cin/cout, and this->9316393 with scanf/printf
      yes, looks awful, and for only prewritten use:)

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    3 years ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    Actually this use of std::move is superfluous. The compiler will move the return value automatically (search for: return value optimization).

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3 years ago, # |
Rev. 3   Vote: I like it +1 Vote: I do not like it

One can omit return type in lambda expression in most cases.

P.S. I have to say, 'tie' looks awesome, I need to start using it.

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3 years ago, # |
  Vote: I like it +4 Vote: I do not like it

You haven't to specify return type in lambda functions if all return values are the same type.

auto f1 = [](int a, int b) {return a < b;}; // ok: return type is bool

auto f2 = [](int a, double b) {
             if (a == 0)
                 return b;
             else
                 return a;}; // error: is return type double or int?

auto f3 = [](int a, double b)->double {
             if (a == 0)
                 return b;
              else
                 return a;}; // ok: return type is double

auto f4 = [](double a, double b) {
             if (a < 0)
                 return a;
             else
                 return pow(a, b);}; // ok: return type is double

see more about lambda functions

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3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

you can even write your own recursive functions inside the main in lambdas, that's really cool and useful for less code.

But here instead of using auto you should specify the return type and the parameters type of the lambda expression.

see my submission here

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Thanks. Useful information.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Thank you so much :) I learned a lot :D

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3 years ago, # |
  Vote: I like it -16 Vote: I do not like it

+669 for vain' blog !why?

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    You are GiveMinus! Both of you have a comment "give a kiss baby :)"

    give a kiss baby :)

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    3 years ago, # ^ |
      Vote: I like it +21 Vote: I do not like it

    +726 for a lot of useful info, that's why.

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3 years ago, # |
Rev. 20   Vote: I like it +9 Vote: I do not like it
warning: ISO C does not permit named variadic macros [-Wvariadic-macros]
#define error(args...)
              ^

could write:

#define error(...) { vector<string> _v = split(#__VA_ARGS__, ','); err(_v.begin(), __VA_ARGS__);}
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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

The example which is now given for move (define w = move(v) and then output contents of v) is actually undefined behaviour. What the compiler will actually do in this situation is just swap the contents of the two vectors (v with the empty w); however, in theory v is now "junk" and should not be touched at all (it can not even be a vector with arbitrary contents, but just something referring to some arbitrary place in memory, which might, in theory, no longer correspond to any correct contents of a vector, and it can do basically anything when its methods (such as the range-based for loop) are called).

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3 years ago, # |
Rev. 2   Vote: I like it 0 Vote: I do not like it

Variadic functions and macros are awesome. Now I've got unique functions for debug, input and output, no more gi2, gi3, ... !!!

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3 years ago, # |
Rev. 3   Vote: I like it +20 Vote: I do not like it

I like the string literals fucntionality. Sometime it can make code much simpler, especially for competitions:

#include <iostream>
using namespace std;

int main() {
	string test = R"END(
		let's test a multiline string
		that can have special chars like ''
		or even ""
		and not to forget \
		and no need to escape!
		This rocks !)END";
	cout << test << endl;
	return 0;
}

And the result on ideone can be seen here.

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I didn't know about this! Thank you. Could you please write a tutorial about this, I'll move it to this post.

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      3 years ago, # ^ |
        Vote: I like it +5 Vote: I do not like it

      c++11 also introduces a set of new string literals. Some of them are really useful for professional programming, but not very helpful for competitions(like UTF-8, UTF-16 and UTF-32 literals) and thus they are not that much of an interest(you can read about them in the wiki article that I link to). However one type of string literal is particularly interesting — the raw string literal. To write a raw string literal you need to prefix the opening quotes with R and immediately after the quotes you should write some delimiter, the delimiter can be a string of up to 16 characters and should not contain whitespace or control characters, You should terminate the string with the same delimiter before the closing quote and also the string should be in brackets(after the delimiter). Here is an example usage:

      int main() {
      	string test = R"END(
      		let's test a multiline string
      		that can have special chars like ''
      		or even ""
      		and not to forget \
      		and no need to escape!
      		This rocks !
                      )END";
      	cout << test << endl;
      	return 0;
      }
      

      And the output can be seen here.

      Note that the string can span multiple lines and that you don't need to escape special characters in it. In this case I use END as my delimiter.

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3 years ago, # |
Rev. 4   Vote: I like it +17 Vote: I do not like it

Following is also useful for GCC. Very fast ASM bit operations:

Note, that offset can be >=32, any valid offset will work. However, I didn't know if inline assembly allowed in CF. Should work.

/* Read bit and set to zero */
inline bool btr (volatile void * mem, size_t offset) {
	bool result;
	__asm__ (
		"btr %2, %1; setc %0;"
		: "=r" (result), "+m" (* (volatile long *) mem)
		: "r" (offset)
		: "cc");
	return result;
}

/* Read bit and set to one */
inline bool bts (volatile void * mem, size_t offset) {
	bool result;
	__asm__ (
		"bts %2, %1; setc %0;"
		: "=r" (result), "+m" (* (volatile long *) mem)
		: "r" (offset)
		: "cc");
	return result;
}

/* Bit value */
inline bool bittest (volatile void * mem, size_t offset) {
	bool result;
	__asm__ (
		"bt %1, %2; setc %0;"
		: "=r" (result)
		: "r" (offset), "m" (* (volatile long *) mem)
		: "cc");
	return result;
}

/* Set bit to one */
inline void bitset1 (volatile void * mem, size_t offset) {
	__asm__ ("bts %1, %0;" : "+m" (* (volatile long *) mem) : "r" (offset) : "cc");
}

/* Set bit to zero */
inline void bitset0 (volatile void * mem, size_t offset) {
	__asm__ ("btr %1, %0;" : "+m" (* (volatile long *) mem) : "r" (offset) : "cc");
}
  • »
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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Why do you need volatile everywhere?

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      3 years ago, # ^ |
      Rev. 2   Vote: I like it 0 Vote: I do not like it

      Just to make sure that value is actually changed. It gives information to the compiler that memory is changed indirectly (inside asm block), to avoid unexpected optimizations. Modern compilers have aggressive optimizations. If you used some value from memory, compiler probably saved it to intermediate register. Let's imagine, that you then called bitset on that memory and used value again. Compiler may decide: "Ok, he didn't even touched that mem variable, I'll use the old value". But it's wrong. You changed it inside asm block. Everything inside asm — direct instructions to processor, compiler doesn't know what you are doing there.

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        3 years ago, # ^ |
          Vote: I like it +11 Vote: I do not like it

        Yes, GCC does not know what is inside the asm block. However, GCC does know which variables are used and modified — you specified this yourself in the asm block input/output operands! In particular, "+m" should tell GCC that this variable/location in memory is read and modified.

        You can see that GCC indeed reloads the value as it should here: http://goo.gl/Jz8SYH. If GCC thought the variable was unmodified, it would do

        movl    $31, %eax

        instead (comment out the btr() call to see this).

        Bottom line: volatile is not needed in correct code. The only valid uses for volatile I can think of are signal handler flags and hardware registers that are mapped in memory.

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          3 years ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          Well, it seems like volatile is indeed redundant in this case. Clobber "+m" should take care of all things. I put it there just in case. Because redundant information isn't a problem, but lack of information is. volatile also comes in handy in multithreaded programs, when you are messing up with custom synchronization/locking technique. Actually anything that involves shared memory involves volatile somehow. In regular programs volatile rarely used, because everything is already written (like synchronization primitives/threadsafe data structures...) and program uses high-level functions for this.

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            3 years ago, # ^ |
              Vote: I like it +8 Vote: I do not like it

            I'm sorry for being a nerd, but volatile can't be used to implement thread synchronization primitives too. Even volatile sig_atomic_t won't do. You are confusing volatile with atomic operations, which are two different things.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Please note that regex is part of the standard but it is not part of g++(at least prior to 4.9). Have a look here. I'm not 100% sure but I think code with regex will not compile on codeforces.

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    actually, regex's compile fine on g++4.6 or 4.7 (I don't remember) but they just worked incorrectly.

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      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      As is mentioned in the bug I relate to, some of the functionality is not working as expected and some of not implemented at all. As per the comments in the bug I think this is fixed in 4.9. However I think codeforces uses an earlier version.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

array<int, 4> a; a = {5, 8, 9, 2};

This code fails on c++11 compilation with error error: no match for ‘operator=’ in ‘a' no known conversion for argument 1 from ‘’ to ‘const std::array<int, 4ul>&’

Need additional braces a = {{5, 8, 9, 2}};

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3 years ago, # |
Rev. 3   Vote: I like it -19 Vote: I do not like it

I use some tricks too, for example:

Input in vector n elements:

for ( int i = 0 ; i < n ; cin >> vec [ i++ ] );

Or analog of:

for(i = 1; i <= n; i++) {
    for(j = 1; j <= m; j++)
        cout << a[i][j] << " ";
   cout << "\n";
}

//

for(i = 1; i <= n; i++ , cout << endl)
    for(j = 1; j <= m; j++)
        cout << a[i][j] << " ";
  • »
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    3 years ago, # ^ |
      Vote: I like it +14 Vote: I do not like it

    I would call it not a C++ trick, but a creative way to use for in C++. It's indeed shorter (just a little), but the code is unreadable IMHO.

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3 years ago, # |
  Vote: I like it +11 Vote: I do not like it

This is really priceless!

Just another two tricks that might help.

std::string to_string( int value ); // Converts a numeric value to std::string.

int stoi( const std::string& str, std::size_t* pos = 0, int base = 10 ); // Interprets a signed integer value in the string str.

For more information, review std::to_string and std::stoi.

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3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

Thanks, very interesting. Let's do blogs like this often!

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3 years ago, # |
Rev. 2   Vote: I like it 0 Vote: I do not like it

Can someone tell what I am doing wrong with trick __builtin_popcount where it's written function with suffix 'l' gets a unsigned long argument and with suffix 'll' gets a unsigned long long argument in this problem

485C - Bits

Solution 9506498 gives WA because of overflow.

  • »
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    3 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    1ll<<i

    • »
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      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Thanks man!! and after that contest I cursed __builtin_popcount for making me lose points :P .

      I wonder then what is the difference between __builtin_popcount and __builtin_popcountll as both solution give AC. I thought __builtin_popcount should give wrong result if I send long long as an argument.

      9506854 --> __builtin_popcountll

      and 9506856 __builtin_popcount

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

please show us some tricks in swift language :D :D

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

One of the best quick C++/STL tutorials,I have ever read. Congratulations to people who helped for this tut.

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3 years ago, # |
Rev. 2   Vote: I like it +11 Vote: I do not like it

It is not part of c++11(only one of this), but useful cpp functions

    vector<int> a(n), b(n), c(n);
    iota(a.begin(), a.end(), 1); //c++11
// a = 1..10
    random_shuffle(a.begin(), a.end());
// a = random permutation of a
    partial_sum(a.begin(), a.end(), b.begin());
// b[i] = sum(a[j], j <= i)
    adjacent_difference(a.begin(), a.end(), c.begin());
// c[i] = a[i] - (i == 0 ? 0 : a[i - 1])
    cout << accumulate(a.begin(), a.end(), 123) << "\n";
// x = 123 + sum(a[i])
    cout << inner_product(a.begin(), a.end(), b.begin(), 234) << "\n";
// x = 234 + sum(a[i] * b[i])

All functions have two iterators as input, some of them have outputIterators and init values. All operators, used in these functions can be user-defined or standard:

    cout << accumulate(a.begin(), a.end(), 1, multiplies<int>()) << "\n";
// x = product(a[i])
// foldl in functional languages
    adjacent_difference(a.begin(), a.end(), c.begin(), [](int a, int b){return a * b;});
// c[i] = a[i] * (i == 0 ? 1 : a[i - 1])

These functions are defined in <numeric>

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3 years ago, # |
Rev. 3   Vote: I like it +3 Vote: I do not like it

Swift ,I think you forgot a semicolon in your perfect tutorial,right here:

""""" auto f = [] (int a, int b) -> int { return a + b; } ..HERE.. cout << f(1, 2); // prints "3" """"

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3 years ago, # |
  Vote: I like it +11 Vote: I do not like it

Using complex, p.real() = x or cin >> p.real() don't work in C++11 but they do in C++98.

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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    You can use p.real(x) in C++11. I don't know any way to cin real.

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Here is a trick that might interest you. In C++, a class can inherit from a template instantiation of itself. So you can write class X: vector<X> {...}; for example. Class X inherits the members of vector and you can use this trick to implement multidimensional arrays, tries, and other useful data structure without using pointers. More here.

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3 years ago, # |
Rev. 2   Vote: I like it -11 Vote: I do not like it

C++11 Tricks or Traps?

One should not use this:

    vector<int> s(5);
    for(int i=0;i<5;i++) s[i]=(101*i)%37;
    for(int z:s) cout<<s[z]<<' ';

instead of this:

    vector<int> s(5);
    for(int i=0;i<5;i++) s[i]=(101*i)%37;
    for(int z=0;z<s.size();z++) cout<<s[z]<<' ';

or, am I missing something?

  • »
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    3 years ago, # ^ |
    Rev. 2   Vote: I like it +8 Vote: I do not like it
    for(int z:s) cout<<s[z]<<' ';
    

    should be

    for(int z:s) cout<< z <<' ';
    
    • »
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      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Oh I see, misunderstood that, thanks.

  • »
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    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    You trapped in your own mistake!

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

for(auto& e: ...) will cause compile error on vector<bool>. use universal reference instead: for(auto&& e: ...)

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3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

There is a tiny typo in the section 6, dijkstra's part:tie(dist, ode, prev) = q.top(); q.pop();

should be:tie(dist, node, prev) = q.top(); q.pop();

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21 month(s) ago, # |
  Vote: I like it +46 Vote: I do not like it

Here's another trick:

For max/min functions, these functions don't need to take two parameters, they can take more :)

Instead of writing,

int a = 5, b = 6, c = 2, d = 10;
cout << max(a,max(b,max(c,d))) << endl;

You can just use "{ }" braces around your parameters and insert a list into the max function (works the same for min function) like below:

int a = 5, b = 6, c = 2, d = 10;
cout << max( {a,b,c,d} ) << endl;

Here's a source code for reference: http://ideone.com/lllqIK

  • »
    »
    21 month(s) ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Hey is there a shortcut to Something like:

    a = max(a , Something being computed);

    I always wanted something like: a+=Something being computed for max too. Although a function with variable parameters can be defined in a template but I don't like working with templates! :)

    • »
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      21 month(s) ago, # ^ |
      Rev. 2   Vote: I like it 0 Vote: I do not like it

      What's wrong with templates? This would work just fine:

          template<class T>
          maxx(T &l, T r) {
              if (l < r) l = r;
          }
      
      • »
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        21 month(s) ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        Probably I fear them! Can you suggest some source to read more about templates and classes and stuff!

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21 month(s) ago, # |
  Vote: I like it +5 Vote: I do not like it

Here's another trick:

You can write return 14 / 88 instead of return 0

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17 months ago, # |
  Vote: I like it -8 Vote: I do not like it

Can I write a void which like

void read(T &a,Args... args) {
    cin << a;
    read(args...);
}

and got the result a=1, b=2, c=3, d=4 if I have input 4 numbers 1, 2, 3, 4 when run read(a,b,c,d)?

  • »
    »
    17 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Yes. Why do you ask? You can simply test it by doing so!

    • »
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      17 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      I got this error

      /home/tunc/Documents/try_C++11.cpp: In instantiation of ‘void read(T&, Args ...) [with T = int; Args = {int, int, int}]’:
      /home/tunc/Documents/try_C++11.cpp:36:14:   required from here
      /home/tunc/Documents/try_C++11.cpp:14:9: error: no match for ‘operator<<’ (operand types are ‘std::istream {aka std::basic_istream<char>}’ and ‘int’)
           cin << A;
               ^
      /home/tunc/Documents/try_C++11.cpp:14:9: note: candidates are:
      In file included from /usr/include/c++/4.8/bitset:1578:0,
                       from /usr/include/x86_64-linux-gnu/c++/4.8/bits/stdc++.h:65,
                       from /home/tunc/Documents/try_C++11.cpp:1:
      /usr/include/c++/4.8/debug/bitset:405:5: note: template<class _CharT, class _Traits, long unsigned int _Nb> std::basic_ostream<_CharT, _Traits>& std::__debug::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::__debug::bitset<_Nb>&)
           operator<<(std::basic_ostream<_CharT, _Traits>& __os,
           ^
      etc.
      

      when I ran that code. How to fix it?

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        17 months ago, # ^ |
        Rev. 2   Vote: I like it +1 Vote: I do not like it

        lol, change

        cin << a
        

        to

        cin >> a;
        
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          17 months ago, # ^ |
          Rev. 3   Vote: I like it 0 Vote: I do not like it

          I changed it, but when i ran with 1 2 3 4 the result was 1 0 0 0. How to fix it?

          p/s: haha, I learnt to code for a while but now I still get that mistake =)) so ashame =))

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17 months ago, # |
  Vote: I like it 0 Vote: I do not like it

The Dijkstra code that uses emplace_back + tie has a little typo: node is spelt as ode

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15 months ago, # |
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Thanks a lot! I am beginning to love C++ <3

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15 months ago, # |
  Vote: I like it 0 Vote: I do not like it

How do I define the "rep" macro if i want to include the end indexes too ?

Like -> rep(i,1,10) prints 1...10 rep(i,10,1) prints 10....1 .

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    15 months ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    An ugly way, but it works. link

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      15 months ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      The link you mentioned isn't working . Can you post it on ideone ?

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      15 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it
      #define ftoa(i, x, y, a) for(int i = (x); i != (((x) < (y)) ? (((y)-(x))/a+1)*a+(x) : (x)-(((x)-(y))/a+1)*a); i += ((x) < (y)) ? (a) : -(a))
      

      I have use this code and try 1000 test cases to make sure that it is correct.

      Here is 3 codes:

      By ftoa

      By normal for

      Make test case

      Note: to make the test cases you download these 3 codes and then run the third one. It will automatically run.

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12 months ago, # |
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Thanks for the great tips; but are all of them usable without C++14?

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12 months ago, # |
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Why would you use

array<int, 4> a;

instead of

int a[4];

?

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    12 months ago, # ^ |
      Vote: I like it +5 Vote: I do not like it

    To use it as elements of vector for example.

    vector<array<int, 4>> v

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      12 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      What are the advantages of vector<array<int, 4>> v; over vector<vector<int>> v;?

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    12 months ago, # ^ |
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    Because you can compare arrays, access elements with bound-checking or get iterators support.

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12 months ago, # |
Rev. 5   Vote: I like it -13 Vote: I do not like it

Why the downvotes I didn't say anything wrong did I ???

Here's a submission by me using what I described (the check function)23252012(I got WA because the idea is wrong not the implementation)

My life now is a lot easier...Thank you Swift.

:)

I'm not sure if this is well known but in C++ you can give a default value to a function for example:

void DFS(int node, int par = -1){

...

}

int main(){

// input a graph

DFS(1);

// rest of the code

}

the DFS function works as a normal function but when you don't provide a second parameter it will take the default value you have given it as its value...hope this helps.

:)

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11 months ago, # |
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thanks Swift

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8 months ago, # |
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Great Work Man

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4 months ago, # |
  Vote: I like it +10 Vote: I do not like it

Old post, but one important mistake: there should be no std::move() call at the end of your split() function. std::move() should never be used to move automatic objects out of functions.

Source

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3 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Auto comment: topic has been updated by Swift (previous revision, new revision, compare).