integer type long and division

Question

Hi all,

I just write a procedure to decompose an unsigned integer to prime numbers. it will work normally if I define the data type as "int", if I change it to "long", result will be wrong. I don't know why.

BTW, I used Win-TC as my compiler.

Code as below:

#include "stdio.h"
#define True    0xff
#define False   0x00
char DividerIsPrime(unsigned long data);
void CheckIfDataCanBeExtracted(unsigned long data);
main()
{
    unsigned long data;
    printf("please input data:");
    scanf("%d",&data);
    printf("\n%d=",data);
    CheckIfDataCanBeExtracted(data);
//    printf("%d",sizeof(short));
    getch();
}

void CheckIfDataCanBeExtracted(unsigned long data)
{
    unsigned long divider,temp,data1;
    data1=data;
    for(divider=2;divider<=data;divider++)
    {
         temp=data1%divider;
         if(temp) {continue;  }
         if(DividerIsPrime(divider)) {
        data1 = data1/divider;
        printf("%d",divider);
        if(data1==1) break;

        else {printf("*");  divider--;}


      }
    }
    return;

}

/* Check if this number is a prime number */
char DividerIsPrime(unsigned long data)
{
    unsigned long divider;
    char    status=True;
    for(divider=2;divider<data;divider++)
    {
        if(data%divider) status=True;
        else status=False;
    }
    return status;
}

Thanks for Paul's help, I know where is wrong. %d should be replaced by %ld.

Answer 1

Your function DividerIsPrime,as currently written, has the defect that logically it must always return True.

The reason for this is that status is changed at each iteration. Even if status=False is reached (the number is composite because the modulus came out zero for a divider), then the iterations will continue and in every case, status=True will be reached on the final iteration when divider == (data - 1).

You can change this as follows:

/* Check if this number is a prime number */
char DividerIsPrime(unsigned long data)
{
    unsigned long divider;
    for(divider=2;divider<data;divider++)
    {
        if (0==(data % divider))
            return False;
    }

    return True;
}

You would have found this with some "unit test" such as:

assert(DividerIsPrime(5));
assert(!DividerIsPrime(6));  /* This test would fail without corrected code. */

Obviously there are much more efficient algorithms for "primality testing".