The Other Array Access Notation in C/C++

C/C++ is sometimes considered the mid-level programming language because if offers the powerful pointers. We know that, like many other programming languages, if you have a array named arr to access the fifth element, the notation is arr[4] (zero index). This is very straightforward and easy to understand. In C/C++ you can write it the other way, which is 4[arr].

Don’t believe this? Try the following simple C program and it will work.

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#include <stdio.h>
#include <string.h>
 
int main()
{
    int a[10];
    const int N = 10;
    for (int i = 0; i < N; i ++) i[a] = i;
    for (int i = 0; i < N; i ++)
        printf("%d", i[a]);
 
    char s[100];
    strcpy(s, "Hello, World!");
    for (int i = 0; i < strlen(s); i ++)
        printf("%c", i[s]);
    return 0;
}

#include <stdio.h>
#include <string.h>

int main()
{
    int a[10];
    const int N = 10;
    for (int i = 0; i < N; i ++) i[a] = i;
    for (int i = 0; i < N; i ++)
        printf("%d", i[a]);

	char s[100];
	strcpy(s, "Hello, World!");
	for (int i = 0; i < strlen(s); i ++)
        printf("%c", i[s]);
	return 0;
}

This prints the following as expected.

0123456789Hello, World!

So, the question is why this works? Well, in fact, the notation a[i] is translated to *(a + i) which is the same as *(i + a). The latter is the i[a]. The array accesses are translated into pointers. The address of the element to access will be calculated.

However, this kind of notation just serves to understand the array/pointer better, I would not recommend this syntax as the first one is much more straightforward.

–EOF (The Ultimate Computing & Technology Blog) —