**UPD:** Tricks to make `unordered_map`

faster added.

Hi!

##### What is `unordred_map`

?

It is a data structure like `map`

but it is more than 4 times faster than `map`

.you can use it in C++11 with including `#include<unordered_map>`

.for example:

```
#include<unordered_map>
using namespace std;
int main(){
unordered_map<int,int>mp;
mp[5]=12;
mp[4]=14;
cout<<mp[5]<<' '<<mp[4]<<endl;//prints: 12 14
}
```

Lets explain it more.

##### How it works?

Focus on `unordered_set`

for simplify.You can imagine that it has vector of vector like `vector<vector<type> > mp`

. Every time you insert value `V`

in that, it calculate its `hash`

(I will explain how it works), let `hash(V)=K`

; it inserts `V`

into `mp[K]`

(formally it calls `mp[K].push_back(V)`

).When you call `mp.count(V)`

it searchs for `V`

in `mp[K]`

.

`map`

VS `unordered_map`

(and `set`

VS `unordered_set`

)

**1- unordered_map is more than 4 times faster**

Focus on problem 527E - Data Center Drama, it seems that it is good to use `unordered_map`

instead of `map`

.

My submission with `map`

: 14269000 Time:484 MS.

My submission with `unordered_map`

: 14269381 Time:358 MS.

Another good sample to show how fast is `unordered_map`

is problem 178C3 - Smart Beaver and Resolving Collisions:

My submission with `map`

: 15781810 Time limit exceeded on test 36 .

My submission with `unordered_map`

: 15774685 Accepted (Time:966 MS).

It seems that there isn't big gap between them, but in fact we know that `unordered map`

is more that 4 times faster than `map`

, I will explain it more later.

**2- unordered_set (and unordered_map) is not sorted**

Please note that `set`

(and `map`

) is sorted, for example *(s.begin()) is always smallest number in the set; but in `unordered_set`

it isn't. similarly there isn't `lower_bound`

and `upper_bound`

in `unordered_set`

(and `unordered_map`

similarly).

##### Creating hash function for structs

Now, **one other problem** remains, you can try to compile this code:

```
unordered_map<pair<int,int>,int>mp;
```

You will get Compilation Error! Why? See this page for `unordered_map`

supported types. For unsupported types, you have to create your own hash function for use. For example lets see how we can create a hash function for `pair<int,int>`

.

As you know any `int`

value is between -2^31+1 to 2^31-1.so if we create function that for every `pair<int,int>`

returns distinct value in type `size_t`

(alias of `unsigned int`

), it will be done. It is pretty easy: `x.first^(x.second<<32)`

is good. but be careful about BUG(OVERFLOW) (It's different from my friend bug-overflow :D );for having good hash function we use `hash<long long>`

.The code is looks like this:

```
struct HASH{
size_t operator()(const pair<int,int>&x)const{
return hash<long long>()(((long long)x.first)^(((long long)x.second)<<32));
}
};
unordered_map<pair<int,int>,int,HASH>mp;
```

Now you have a `unordered_map`

of `pair<int,int>`

(it isn't problem what is second member, in example it was `int`

).Creating hash function for other structs is same.

##### How to test `unordered_map`

is faster than `map`

?

You can test that for N=10^6, `unordered_set`

(`unordered_map`

) is more than 4 times faster than `set`

(`map`

) ;with this code:(compile code with command: g++ file.cpp -std=c++11 **-O2**)

```
#include <bits/stdc++.h>
using namespace std;
unordered_set<int>s;//replace it with set for test.
int main(){
auto T=clock();
for(int i=0;i<10000000;i++)
s.insert(i);
cout<<double(clock()-T)/CLOCKS_PER_SEC<<'\n';
}
```

**Note1:** Let your hash function `H(V)`

, it is better that H(V) returns distinct value for every distinct `V`

, it makes `unordered_map`

faster; but if you can't do that **it doesn't problem**. The only problem is that `unordered_map`

becomes slower(however it is better than `map`

).

**Note2:** Please be careful about your hash function time complexly! it is fun to use this hash function:

```
struct HASH{
size_t operator()(const pair<int,int>&x)const{
size_t ans=0;
for(int i=0;i<x.first;i++)
ans+=x.second;
return ans;
}
};
```

**UPD** I will explain `hash<type>`

in my next post.

**UPD** It seems that sometimes `unordered_map`

becames so slow.but it can improve with this two lines of code:

```
unordered_map<int,int>mp;
mp.reserve(1024);
mp.max_load_factor(0.25);
```

With this two lines `unordered_map`

become about 10 times faster. You can replace `1024`

with another suitable power of two.(it depends on number of `insert`

s you will do).