Question

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In C arrays why is this true? a[5] == 5[a]

Can anyone explain me why for any array the expression c a[7] == 7[a] is true.

is there any case that it can be false

I have seen in wiki that x[i] is a syntactic sugar for *(x+i) and this is equal to *(i+x) and this is i[x], but I can not understand this properly.

Answer 1

• a[7] is equivalent to *(a + 7).
• Similarly 7[a] is equivalent to *(7 + a).

This gives the same result because adding an integer to a pointer is a commutative operation.

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is there any case that it can be false

The short answer is no. For simple pointer arithmetic it will always be true.

The long answer is that to find a counterexample you would have to use a hack. One way you could do it is to use a macro:

#include <stdio.h>

#define a 1+s

int main(void) {
    char s[] = "1q2w3e4r5t6y";
    int x = a[7];
    int y = 7[a];
    printf("%c %c\n", x, y);
    return 0;
}

Result:

s 5

The result is different because after the macro expansion the expressions become different: 1+(s[7]) versus 7[1+s].

See it online: ideone