C plus plus program to find prime number?

Answer 1

Here's an example. I've included analysis.

#include<stdio.h>

#include<stdlib.h>

#include<math.h>

int main(void)

{

/* User inputs high and low ends of range to search.

/ This could easily be done as function inputs, if

/ if so desired. */

int testprime,testdivisor, minprime, maxprime;

printf("Search for primes between:\nLow end: ");

scanf("%d",minprime);

printf("Top end: ");

scanf("%d",maxprime);

int isprime; // 0 indicates a number is not prime..

for(testprime=minprime;testprime<=maxprime;testprime++)

{

isprime = 1;

for(testdivisor=2;testdivisor<sqrt(testprime);testdivisor++) // Primes divide by 1!

{

isprime = testprime%testdivisor; // % finds remainders, so 7%3 returns 1.

// If 0 is returned, a divisor has been found.

if(isprime == 0) break; // Hence, not prime. "break;" exits the loop.

}

if(isprime != 0) printf("%d is prime.\n",testprime);

}

system("pause");

return 0;

}