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Sieve of Eratosthenes is a simple algorithm to find prime numbers. Sieve of Eratosthenes is a great example of the sieve approach. This post is A way to find out prime numbers lesser than a very big number !! Screenshots/Sample Output: Code for It: #include<stdio.h> int isprime[10000001]={0}; main() { long long int i,j; isprime[0]=1; isprime[1]=1; for(i=2;i*i<10000001;i++) { if(isprime[i]==1) { continue; } for(j=i*2;j<10000001;j+=i) { isprime[j]=1; } } printf("enter the no. till which the small primes are to be found : "); scanf("%lld",&j); printf("the prime numbers till %lld are: \n",j); for(i=2;i<=j;i++) { if(isprime[i]==0) printf("%lld\t",i); } } Efficiency of the code : The normal algorithm we follow has its complexity defined by  O(n). This Sieve algorithm reduces it to O(n*loglog(n))/O(1).