Error in my program, help!

My program is supposed to take in an interger, and tell me if it's a prime number or not. If I enter something like "woof", it'll tell me that it is invalid and does not work.

My program is as follows:

Code:

#include<stdio.h>
#include<math.h>
#include<stdlib.h>

int is_prime(int n)
{
    int b;
    
    if(n<2)
        return 0;
    for(b = 2; b<=sqrt(n); b++)
    {
        if (n%b==0)
        {    
            return 0;
        }
    }
    return 1;    
}

int read_an_int(void)
{
    int a, ret, c;
    
    while(1)
    {    
        ret = scanf("%d", &a);
        if(ret == EOF)
            return -999;
        if(ret == 1)
            return a;
        printf("Invalid input, please try again: \n");
        while((c = getchar()) != '\n' && c != EOF)
            ;
            continue;
    }
}    

int main(void)
{

    int a, ret;

    while (1)
    {
        
        printf("Enter a number to test (negative number to stop): \n");
        ret = read_an_int();
        if(ret==-999)
            printf("End of file.");
            
        if(ret==a)
        {
            if (a<0)
            {
                printf("Bye!\n");
                break;
            }

            if(is_prime(a) == 1)
                printf("%d is prime\n", a);
            else
                printf("%d is not prime\n", a);

        }
    }
    return 0;
}

When I try to compile it, the printf statement works, but if I try a number, it won't tell me if it's prime or not. I tried to put in woof, and that does indeed tell me that what I entered is invalid and that I need to enter another number.
In other words, it scans in my function called read_an_int, but for some reason it doesn't read in the part where I have to say if it's a prime number or not.
like if(ret==a) and so on. Anyone know how I can fix it?

Code:

int a, ret;
.
.
.
if(ret==a)

What do you think the value of a is here?

"a" in main() and "a" in read_an_int() are two completely seperate and unrelated local variables.

Code:

int main(void)
{
 
    int a, ret; // uninitialized
 
    while (1)
    {
         
        printf("Enter a number to test (negative number to stop): \n");
        ret = read_an_int();
        if(ret==-999)
            printf("End of file.");
             
        if(ret==a)
        {

You are comparing ret to a, but what is a? It could be anything. Try this:

Code:

printf("main() a=%d\n", a);

Before and after you call read_an_int(). Notice it is a random number, and unchanged after the call.

I'm confused as to what I should declare "a" to be, because when I said in the first part of my function:

Code:

if(ret == EOF)            
return -999;

then at the main part of my function I used:

Code:

if(ret==-999)

Why doesn't it work for a, when I put at the top (in my read_an_int function):

Code:

if(ret == 1)
            return a;

If you are expecting to be able to enter "woof" and have it not wreck your program, then you need to change how you read input. Consider reading the entire line with fgets and then checking it with sscanf to see if it is valid or not. Cprogramming.com FAQ > How do I get a number from the user (C)


Quzah.