Question

This c++ code prints out the following prime numbers: 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97.

But I don’t think that’s the way my book wants it to be written. It mentions something about square root of a number. So I did try changing my 2nd loop to for (int j=2; j<sqrt(i); j++) but it did not give me the result I needed.

How would I need to change this code to the way my book wants it to be?

int main () 
{
    for (int i=2; i<100; i++) 
        for (int j=2; j<i; j++)
        {
            if (i % j == 0) 
                break;
            else if (i == j+1)
                cout << i << " ";

        }   
    return 0;
}

A prime integer number is one that has
exactly two different divisors,
namely 1 and the number itself. Write,
run, and test a C++ program that
finds and prints all the prime numbers
less than 100. (Hint: 1 is a prime
number. For each number from 2 to 100,
find Remainder = Number % n, where n
ranges from 2 to sqrt(number). \ If n
is greater than sqrt(number), the
number is not equally divisible by n.
Why? If any Remainder equals 0, the
number is no a prime number.)

Answer 1

Three ways:

1.

int main () 
{
    for (int i=2; i<100; i++) 
        for (int j=2; j*j<=i; j++)
        {
            if (i % j == 0) 
                break;
            else if (j+1 > sqrt(i)) {
                cout << i << " ";

            }

        }   

    return 0;
}

2.

int main () 
{
    for (int i=2; i<100; i++) 
    {
        bool prime=true;
        for (int j=2; j*j<=i; j++)
        {
            if (i % j == 0) 
            {
                prime=false;
                break;    
            }
        }   
        if(prime) cout << i << " ";
    }
    return 0;
}

3.

#include <vector>
int main()
{
    std::vector<int> primes;
    primes.push_back(2);
    for(int i=3; i < 100; i++)
    {
        bool prime=true;
        for(int j=0;j<primes.size() && primes[j]*primes[j] <= i;j++)
        {
            if(i % primes[j] == 0)
            {
                prime=false;
                break;
            }
        }
        if(prime) 
        {
            primes.push_back(i);
            cout << i << " ";
        }
    }

    return 0;
}

Edit: In the third example, we keep track of all of our previously calculated primes. If a number is divisible by a non-prime number, there is also some prime <= that divisor which it is also divisble by. This reduces computation by a factor of primes_in_range/total_range.