Question

A friend asked me to write a function in C to return the 100th element of an array. I’m not very familiar with C, so I wasn’t sure how to make a generic function that could do this with any type of array, so I cheated and assumed that it was an array of integers and wrote this function:

int GetHundredthElement(int *array) {
  return array[100 - 1];
}

(the - 1 is there because arrays are zero-indexed)

I asked him how to make a function that would work for any type of array. He told me there was a simple way to do it:

int GetHundredthElement = 100 - 1;

and that this “function” could be called like this:

GetHundredthElement[array];

I tried it, and it worked, but doesn’t look like a function to me because it uses bracket notation, which isn’t how function calls are written in C. I don’t really understand exactly what this code is doing or how it’s doing it. What’s going on here?

Answer 1

You are right about the fact that GetHundredthElement is not a function– it is, as you would expect, an integer.

However, this illustrates a surprising ability in C where you can reverse the order of your array access!

assert(a[5] == 5[a]);

This is because an array access can be implemented in pointer arithmetic:

assert(a[5] == *(a+5));
assert(*(a+5) == *(5+a));
assert(*(5+a) == 5[a]);