help me to optimize prime numbers code

There are many algorithms that you can use to calculate primes. The question really is, what defines worthwhile optimisation? Are you trying to reduce the time taken to calculate the primes - which on only 100 values is going to be pretty darned fast using the implementation you have here. Are you looking for a more elegant solution? Really, the answer to your question depends entirely on what you are trying to achieve. For instance, if you used the Sieve of Erastothsenese, your code will look more complex but you will have implemented a solution that is capable of coping with larger primes more efficiently than this code.

You are checking every number from 2 to 100. But since 2 is the only even prime number, you can skip every even number after 2. This goes for both i and j. So start i and j at 3, and increment them by 2.

#include<iostream>

using namespace std;

int main() {
    cout<<"Prime numbers between 1 and 100 are:"<<endl;
    cout<<"2"<<"\t";
    for (int i=3; i<100;i+=2) {
        // This loop stops either when j*j>i or when i is divisible by j.
        // The first condition means prime, the second, not prime.
        int j=3;
        for(;j*j<=i && i%j!=0; j+=2); // No loop body

        if (j*j>i) cout << i << "\t";
    }
    cout<<endl;
    return 0;
}</iostream>

In addition to the trick mentioned above, I've added the condition j*j<=i which logically is the exact same as j<=sqrt(i). There's no need to compute the square root when you can do a simple multiplication.