Why variable does change to zero when it is compared?

Question

I have the code below. The problem is with variable r , which first holds a random number between 1-10. Later it is compared inside a loop to a user input number. The loop ends when the user guesses the randomly generated number.

I am using short type for the guess, and int for the randomly generated number r . The point is that after the first guess of the user, r becomes 0 and I don't know why. This problem does not happen if guess is an int .

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

int main(int argc, const char * argv[]) {
    srand(time(NULL));
    int r = (rand()%10)+1;
    short guess = -1;
    printf("%d\n", r);
    while ((guess)!= r){
        printf("%d\n", (int) guess);
        printf("%d\n", guess);
        printf("%d\n", r);
        printf("Give me a number from 1 to 10: ");
        scanf("%d",&guess);
        if (guess> r){
            printf("The number is lower\n");
        }
        else if (guess<r){
            printf("The number is higher\n");
        }
        else {
            printf("Corrent!\n");
        }
    }
    return 0;
}

An output example would be:

7
-1
-1
7
Give me a number from 1 to 10: 5
The number is lower
5
5
0
Give me a number from 1 to 10: 

Answer 1

The problem is here:

scanf("%d",&guess);

The variable guess is a short , but the %d format specifier to scanf expect a pointer to an int . Using the wrong format specifer to printf or scanf invokes undefined behavior .

What most likely happened is that an int on your system is larger than a short , resulting in writing to bytes in memory past where guess resides, overwriting nearby variables, in this case r . This behavior however is not guaranteed and could change with seemingly unrelated code changes.

Either use the correct format specifier:

scanf("%hd",&guess);

Or change the type of guess to int . Since there's no real reason to use a short here, I'd recommend the latter.