Optimizing prime numbers code?

Question

I wrote this code to show the primes between 1 and 100. The only condition is to do not use functions, whole code should be inline. I would ask if I can improve (optimize) it mu

Answer 1

Two simple optimizations you could do:

cout << 2 << '\t';
for (int i = 3; i <= 100; ++i) {
    for (int j = 3, l = (int)sqrt(i); j <= l; j += 2) {
        if (i % j == 0) {
            cout << i << '\t';
            break;
        } 
} 

What I did:

Math:

• Stop when j > sqrt(i), there's no need to go further than that. Note however that sqrt is an expensive function; for your small sample (from 1 to 100), it might (read, will surely) cost you more to use it.
• Only check odd numbers; do j += 2 instead of incrementing j one by one

Micro-optimizations:

• Use ++i instead of i++; the latter has a temporary variable in which it stores the original value of i; the former does not.
• Print '\t' as a character not as a string "\t".

(These micro-optimizations are probably made automatically by the compiler anyway, but there's no harm in knowing about them.)