C's index[arr] notation

5 pointsleegao15y ago4 comments

I had always assumed that it was merely some strange artifact of the language, until I finally thought about it, and then finally understood:

arr[index] is equivalent to (arr + index), which by the commutative property of addition is equivalent to (index + arr) = index[arr].

Mind = blown

4 comments

4 comments · 1 top-level

marcomonteiro15y ago· 3 in thread

It's not the same as (index + arr). arr is a pointer to the beginning address of a series of particular "types". These types have a specific size in memory. Index increments the pointer by (n * size_of(type)).

arr[index] == arr + (n * size_of(type)) == (n * size_of(type)) + *arr

SamReidHughes15y ago

He said it's the same as

    *(index + arr)

which is correct.

C pointer arithmetic already accounts for object sizes.

And it's sizeof, not size_of. And

    arr + x

is completely different from

    x + *arr

And it's a series of "values".

marcomonteiro15y ago

I don't know what I was on. Proof positive that sleep depravation does not give me more time to be "productive". Thanks for the correction and apologies for my mistake.

Jacquass1232115y ago

Always time for a refresher :) http://augustcouncil.com/~tgibson/tutorial/arr.html

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