shyam81295's blog

By shyam81295, history, 6 years ago, In English

Hello, guys,

I am still learning algorithms and implementing them. But, Sometimes I get stuck when asked about generating prime numbers in O(N) time complexity. Although, I have implemented Sieve of Eratosthenes, but I have read somewhere that, it requires O(N*log(logN)) or something like that. I find this algorithm as good one, but in some practice problems, we need to implement in O(N) time.

It would be nice if someone of you will guide me to generate prime numbers in O(N) time.

 
 
 
 
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6 years ago, # |
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6 years ago, # |
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I don't think that you will find many (or any) problems in which regular sieve is not enough.

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6 years ago, # |
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It is well known that log(log(N))<=5 :)

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6 years ago, # |
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One optimization in sieve

for (int i = 4; i <= n; i += 2) {
  bad[i] = true;
}
for (int i = 3; i * i <= n; i += 2) 
  for (int j = i * i; j <= n; j += i + i) 
    bad[j] = true;

UPD. Fixed

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6 years ago, # |
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so if you need to get all prime numbers <=n;do this

for(i=10;i<=n;i++) { if(i%2!=0 && i%3!=0 && i%5!=0 &&i%7!=0) { then number is prime; }} keep in mind hat we are starting from 10...as we already know primes less than 10....we could write our code with if---else ......hope this helps.....ask if you have any doubt..

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6 years ago, # |
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I think if you study Number Theory,you won't have any problems in these questions in future :)

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5 years ago, # |
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