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hELLO wORLD · Jun 18, 2002, 02:20 AM

Hi,

I have some questions to optimize code execution and memory :

First, what is the difference, for the cc compiler (and the coming gcc 3.1) between this :
#define kMyStringConstant @"Hello, I am a constant"

and this :
static NSString *kMyStringConstant = @"Hello, I am a constant";

What is the most efficient in a Cocoa project ?

Yet, another question :
What is the best, for an NSArray, between using an NSEnumerator, and this :
int loop, max;
max = [myArray count];
for (loop=0; loop<max; loop++)
{
do something with [myArray objectAtIndex:loop]
}

And if I use this instead :
for (loop=0; loop<[myArray count]; loop++)
Will the compiler be able to optimize the [myArray count] access, or will it go into myArray and extract count at each iteration ?

Thanks

Gul Banana · Jun 18, 2002, 09:35 AM

<blockquote>quote:<hr />Originally posted by hELLO wORLD:
Hi,

I have some questions to optimize code execution and memory :

First, what is the difference, for the cc compiler (and the coming gcc 3.1) between this :
#define kMyStringConstant @"Hello, I am a constant"

and this :
static NSString *kMyStringConstant = @"Hello, I am a constant";
<hr /></blockquote>The former is equivalent to [NSString stringWithCString:"Hello, I am a constant"], and is evaluated each time it is used. The latter is assigned only once, and so is more efficient in terms of cycles - however, if you're using it in a global context, it's going to take up more memory. More efficient, for NSStrings, is probably [b].

<blockquote>quote:<hr />
Yet, another question :
What is the best, for an NSArray, between using an NSEnumerator, and this :
int loop, max;
max = [myArray count];
for (loop=0; loop<max; loop++)
{
do something with [myArray objectAtIndex:loop]
}
<hr /></blockquote>An NSEnumerator is definitely better. Because an NSArray is actually a collection, not a true array, underneath, objectAtIndex can be quite expensive.

<blockquote>quote:<hr />
And if I use this instead :
for (loop=0; loop<[myArray count]; loop++)
Will the compiler be able to optimize the [myArray count] access, or will it go into myArray and extract count at each iteration ?

Thanks<hr /></blockquote>cc (which is gcc, by the way) does optimize such things, I _think_. However, the only way to be sure would be to have a look at the assembler it generates

Or read some documentation somewhere...

smeger · Jun 18, 2002, 03:32 PM

<blockquote>quote:<hr />Originally posted by Gul Banana:

<blockquote>quote:<hr />
And if I use this instead :
for (loop=0; loop<[myArray count]; loop++)
Will the compiler be able to optimize the [myArray count] access, or will it go into myArray and extract count at each iteration ?

Thanks<hr /></blockquote>cc (which is gcc, by the way) does optimize such things, I _think_. However, the only way to be sure would be to have a look at the assembler it generates

Or read some documentation somewhere...<hr /></blockquote>I don't think this is correct. If the number of elements in the array changes inside of the loop, the loop will still work correctly. So, it's calling [myArray count] every iteration.

More specifically, the second "phrase" in the for loop is evaluated completely every iteration. You can use "const" for optimization, but as written, it'll evaluate.

Ghoser777 · Jun 19, 2002, 11:13 AM

count is a method which I assume returns an instance variable of NSArray which doesn't change if we're talking about NSArray only, and does change only on creationg and when adding/removing objects if we're talking about NSMutableArray. But I'm just assuming. It would be highly inefficient if it recalculated the size every time. If possible, I think this would be better (if you're not going to use an enumerator):

<blockquote>code:<hr /><pre style="font-size:x-small; font-family: monospace;">int i;

for(i=[array count]-1; i>=0; i--)
{
//do your thang
}</pre><hr /></blockquote>But that's just me.

F-bacher