C programming: array not have any bound [duplicate]

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No, an array of size 2 is not meant to store 3 elements, it can only store 2 elements. C does not prevent you from trying to access arrays beyond their boundaries, but the code then invokes undefined behavior.

The compiler should be smart enough to detect your erroneously accessing a[2], a[3] and a[4]. Maybe you did not give it enough of a warning level or maybe changing the optimization settings can do it. In any case, your code invokes undefined behavior, so anything can happen.

Here is a corrected version:

#include <stdio.h>
int main() {
    int a[5];

    a[0] = 1;
    a[1] = 2;
    a[2] = 3;
    a[3] = 9;
    a[4] = 99;
    printf("%d\n", a[0]);    
    printf("%d\n", a[1]);
    printf("%d\n", a[2]);
    printf("%d\n", a[3]);
    printf("%d\n", a[4]);
    return 0;
}

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