Array of structures or structure of array in C

Question

Which one is the fastest/least memory consuming option of these two:

struct {
int index;
char string[10];
} a[10];

or

struct {
int index[10];
char string[10][10];
} a;

The first one is clearly easier to use and implement. I also have to mention I will dynamically allocate them. But which one will run faster or be least time consuming?

Thank you!

There may be minuscule differences which depend on information not provided in the question (e.g. what C implementation you use, including what compiler options, and more importantly what you actually do with the structure). Speed is not a property of C structs. The most common thing I can think of is that the former is going to be a bit bigger on a lot of implementations, perhaps 160 bytes vs 140. Good luck actually detecting any performance difference, though. — Steve Jessop
– Steve Jessop, Commented Dec 19, 2010 at 13:11
Why use an array of indexes in the second structure? If you want to address a particular element of string then you just need to know the range of the array. Would it not be simpler to store the size? — mmccomb
– mmccomb, Commented Dec 19, 2010 at 13:14

thkala · Accepted Answer · 2012-01-07 09:01:20Z

3

struct {
    int index;
    char string[10];
} a[10];

will introduce padding for each a[] item.

The second solution will introduce padding only once.

If you are going to allocate a large number of items in a[] then you will pay the price of having a larger domain to cover (not to mention the additional dereferencing).

edited Jan 7, 2012 at 9:01

thkala

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answered Dec 19, 2010 at 20:17

bert

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Comments

Milan · Accepted Answer · 2010-12-19 13:10:48Z

2

Don't bother with premature optimization. Use the one easier to understand/maintain. Since this is C, performance difference will barely be noticeable.

answered Dec 19, 2010 at 13:10

Milan

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1 Comment

Fred Foo Over a year ago

+1 for the first two sentences, but -1 for the last one. As long as the number of items is fixed at 10, this won't be noticeable in any language, but when this number grows big, locality-of-reference effects come into play so performance depends strongly on the task.

Tim Čas · Accepted Answer · 2010-12-19 13:12:19Z

1

The 2nd would probably be smaller memory-wise simply because sizeof struct above is 8, not 5 because of padding (provided that int is 32-bit).

As for which is faster, I'd say it would depend on what you're doing; the 2nd is a typical example of data-oriented design (not to be confused with data-driven design). Please see this article: http://gamesfromwithin.com/data-oriented-design

EDIT: That said, I agree with Milan here (at the other answer) -- don't bother optimizing prematurely, or at all. Both are fast enough; I didn't emphasize this earlier because I figured you might need it for an embedded system, where this could matter.

answered Dec 19, 2010 at 13:12

Tim Čas

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