C's index[arr] notation

#1

Post by leegao » Tue, Apr 12, 2011, 6:02 AM UTC

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

Re: C's index[arr] notation

#2

Post by marcomonteiro » Tue, Apr 12, 2011, 9:03 AM UTC

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

Re: C's index[arr] notation

#3

Post by SamReidHughes » Tue, Apr 12, 2011, 9:47 AM UTC

marcomonteiro wrote: post #2

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

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".

Re: C's index[arr] notation

#4

Post by Jacquass12321 » Tue, Apr 12, 2011, 4:22 PM UTC

SamReidHughes wrote: post #3

marcomonteiro wrote: post #2

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

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".

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

Re: C's index[arr] notation

#5

Post by marcomonteiro » Wed, Apr 13, 2011, 12:30 AM UTC

SamReidHughes wrote: post #3

marcomonteiro wrote: post #2

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

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".

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.

Re: C's index[arr] notation

#6

Post by [deleted] » Wed, Apr 13, 2011, 12:33 AM UTC

marcomonteiro wrote: post #2

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

[deleted]

Re: C's index[arr] notation

#7

Post by [deleted] » Wed, Apr 13, 2011, 12:33 AM UTC

marcomonteiro wrote: post #2

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

[deleted]