How do you test a number and tell if it’s prime or not?

Question

I have a small problem, after the first time the code runs it only shows "NOT PRIME" and no longer shows "PRIME", where "NOT PRIME", would be for non-prime numbers, and "PRIME" for prime numbers.

  #include <stdio.h>
  main()
  {

    int i, x=1; 
    int div = 0;

    while(x!=0)
    {
      do
      {
        printf("\n\t\tEntre com numero inteiro e positivo: \n\n");
        scanf("%d", &x);
      } while (x <= 0);   
      for (i = 1; i <= x; i++)
       {
        if (x % i == 0)
        { 
         div++;
        }
      }     
      if (div == 2)
      {
        printf("\t\t\nPRIME\n", x);
      }
      else
      {
        printf("\t\t\nNOT PRIME\n", x);
      }
    }
  }

Answer 1

You can use this function to check if the number is prime:

#include <stdio.h>

/* Retorna 1 para numeros primo ou 0 para numeros que nao sao primos.*/
int IsPrime(unsigned int number) {
    if (number <= 1) return 0; // se o numero for menor ou igual a 1 então nao é primo.
    unsigned int i;

    for (i = 2;  i * i <= number; i++) {
        if (number % i == 0) return 0;
    }

    return 1;
}

int main(void) {
    printf("0 é primo %s\n", IsPrime(0) ? "sim" : "nao");
    printf("1 é primo %s\n", IsPrime(1) ? "sim" : "nao");
    printf("3 é primo %s\n", IsPrime(3) ? "sim" : "nao");
    printf("2 é primo %s\n", IsPrime(2) ? "sim" : "nao");
    printf("4 é primo %s\n", IsPrime(4) ? "sim" : "nao");
    printf("5 é primo %s\n", IsPrime(5) ? "sim" : "nao");
    printf("7 é primo %s\n", IsPrime(7) ? "sim" : "nao");
    printf("9 é primo %s\n", IsPrime(9) ? "sim" : "nao");

    return 0;
}

Exit:

0 is not cousin
1 is not cousin
3 is cousin yes
2 is cousin yes
4 is not cousin
5 is cousin yes
7 is cousin yes
9 is not cousin

This function IsPrime() can be easily used in a loop, and will not have a high maintenance cost, see:

int i, v;

for(i = 0; i < 5; i++) {
    printf("Informe um numero: ");
    scanf("%i", &v);

    printf("%i é primo %s\n",v , IsPrime(v) ? "sim" : "nao");
}

It is not necessary to do the number check calculation within the same loop that reads the values typed by the user, it is always a good way to create separate functions to perform certain tasks, this way you can reuse it and use it elsewhere.

And we also have to ensure the treatment of the numbers typed by the user, so the use of unsigned for unsigned numbers, and validation of the number at the beginning, however, it is necessary to do other validations of the data before sending to the function IsPrime() verify the number.

See working on Ideone.

Source.

Answer 2

It was enough to equal the div to zero whenever the while in addition to the second paragraph while I changed the condition to x < 0 since zero would be the number for the program to close.

#include <stdio.h>

int main(void) {

    int i, x; 
    int div;

    do {

        do {
            printf("\n\t\tEntre com numero inteiro e positivo: \n\n");
            scanf("%d", &x);
        } while (x < 0);   

        for (i = 1; i <= x; i++)
            if (x % i == 0)
                div++; 

        if(x != 0) {
            if (div == 2)
                printf("\t\t\nPRIME\n", x);
            else
                printf("\t\t\nNOT PRIME\n", x);
        } 

        div = 0;

    }while(x != 0);

    return 0;
}

No further corrected details such as indentation, return, etc.