The type `size_t` causes an issue for printing data [duplicate]

Question

I am asking a user to enter five integers. The iteration index i is declared as size_t. However, I have noticed that the behavior of the second loop is abnormal. When I declare the index as int, there are no issues, and the second loop prints all the data correctly. I need to understand what is happening here so that I can avoid this problem in the future. I am using the C compiler in Windows. The mininal working example is

#include <stdio.h>

int main(void)
{
#define N 5
    size_t i; // <-- if replace 'size_t' with 'int' no issue
    int a[N];

    printf("Enter 5 numbers: ");
    for (i = 0; i < N; ++i)
        scanf("%d", &a[i]);

    printf("\nIn reverse order: ");
    for (i = N - 1; i >= 0; --i) // <-- problem here
        printf(" %d ", a[i]);

    return 0;
}

Answer 1

You cannot write a loop with decreasing index value with the i >= 0 comparison when the index variable is unsigned because an unsigned value is always greater or equal to zero. When i is zero, decrementing it gives it the value SIZE_MAX which is positive and huge, causing a[i] to invoke undefined behavior. It may not crash immediately but most certainly will at some point when a[i] will refer to an invalid address.

The correct idiom for a decreasing loop, that works for both signed and unsigned index variables is this:

for (size_t i = N; i-- > 0;) 
    printf(" %d ", a[i]);

The loop iterates for all values between N - 1 and 0 inclusively and stops when i reaches 0, which is exactly what you want.

Note that this comparison could be written i --> 0 with exactly the same effect (--> is just 2 tokens -- and > glued in an unusual but meaningful way :) Some people call it the downto operator and it is the subject of one the most active questions on Stack Overflow: What is the '-->' operator in C/C++?

Here is a modified version with a compact idiomatic alternative:

#include <stdio.h>

int main(void)
{
#define N 5
    int a[N];
    size_t i;

    printf("Enter up to 5 numbers: ");
    for (size_t i = 0; i < N; ++i) {
        if (scanf("%d", &a[i]) != 1)
            break;
    }

    printf("\nIn reverse order: ");
    while (i--) {
        printf(" %d ", a[i]);
    }
    printf("\n");
    return 0;
}