How to solve uninitialized local variable used?

Question

#include <stdio.h>

int main() {
    int x;  int counter;
    while (scanf_s("%d", &x) != 0)
    {
        puts("Enter the a signed number : ");
        if (x % 2 == 0) { counter=counter++; }
    }
    printf(" %d pair numbers", counter);
}

I get

uninitialized local variable counter used.

The program is supposed to keep asking for numbers until it gets zero and tells us the amount of pairs given.

Answer 1

You need to initialize counter to 0 before trying to increment it. Also counter = counter++; is redundant (and undefined!). Just use counter++

Answer 2

The problem you are facing is cause by your use of the variable counter without initializing it. This is exactly what the compiler is telling you.

When you try to execute counter=counter++; for the first time, counter has no definied value. You might think by int counter; it gets initialized with 0, but this is wrong in C.

The next problem is the line counter=counter++; itself. If you want to increment counter, just use counter++. Depending on the compile you are using, the use of counter=counter++ should give you at least a warning. On my machine using Apple LLVM version 8.1.0 (clang-802.0.42) I get

warning: multiple unsequenced modifications to 'counter' [-Wunsequenced]

Then you try to loop until you read 0. But scanf() (use this instead of the Microsoft specific scanf_s()) does not return what it has read from stdin but the number of input items assigned. It returns 0 in the event of a matching failure.

So here is what to do:

1. Initialize counter
2. Replace counter=counter++ with counter++
3. Use another loop breaking condition than the return value of scanf()

One approach could be the following:

#include <stdio.h>

int main() {
    int x=0;
    int counter=0;

    do{
        printf("Enter the a signed number: ");
        scanf("%d", &x);
        if (!(x % 2)){
            counter++;
        }
    } while(x);

    printf(" %d pair numbers\n", counter);
}