For loop in Objective-C

Question

Where should I use the for loop and where should I use the for in loop?

I would like to know the difference between them.

Solution 1

The traditional for loop in Objective-C is inherited from standard C and takes the following form:

for (/* Instantiate local variables*/ ; /* Condition to keep looping. */ ; /* End of loop expressions */)
{
    // Do something.
}

For example, to print the numbers from 1 to 10, you could use the for loop:

for (int i = 1; i <= 10; i++)
{
    NSLog(@"%d", i);
}

On the other hand, the for in loop was introduced in Objective-C 2.0, and is used to loop through objects in a collection, such as an NSArray instance. For example, to loop through a collection of NSString objects in an NSArray and print them all out, you could use the following format.

for (NSString* currentString in myArrayOfStrings)
{
    NSLog(@"%@", currentString);
}

This is logically equivilant to the following traditional for loop:

for (int i = 0; i < [myArrayOfStrings count]; i++)
{
    NSLog(@"%@", [myArrayOfStrings objectAtIndex:i]);
}

The advantage of using the for in loop is firstly that it's a lot cleaner code to look at. Secondly, the Objective-C compiler can optimize the for in loop so as the code runs faster than doing the same thing with a traditional for loop.

Hope this helps.

Solution 2

You mean fast enumeration? You question is very unclear.

A normal for loop would look a bit like this:

unsigned int i, cnt = [someArray count];
for(i = 0; i < cnt; i++)
{ 
   // do loop stuff
   id someObject = [someArray objectAtIndex:i];
}

And a loop with fast enumeration, which is optimized by the compiler, would look like this:

for(id someObject in someArray)
{
   // do stuff with object
}

Keep in mind that you cannot change the array you are using in fast enumeration, thus no deleting nor adding when using fast enumeration

Comments

[Stefan Steinegger]

@koregan: Add a comment to ask James to change the first sentence when you rephrased the question. You could also add a comment to be more tolerant to non english speaking people...

[JeremyP]

In Objective-C, it's for (element in collection) not for (element : collection). Also, collection can be any object that implements NSFastEnumeration.

[James Bedford]

Ah yes sorry - edited. Yes I also agree with the NSFastEnumeration comment. Thanks. :)

[Scott Lahteine]

The one exception to this rule would be to go ahead and make one change within the body of the loop (for example, to delete the item you were looking for), but immediately "break" to exit the loop.

[Antwan van Houdt]

@ScottLahteine I actually doubt that, as soon as you try to mutate the array an exception will be thrown.

[Scott Lahteine]

You can enumerate mutable arrays. For an example of this, see the _windowClosed: method of ATApplicationController.m in the TableViewPlayground sample code from ADC.

[Antwan van Houdt]

@ScottLahteine Has nothing to do with it being a mutable array or not, however I did try to mutate the array and break the loop immediately after that and it seems to work just fine -- so your first comment is true. edit I did not look at the code from ATAApplicationController, wrote my own sample thing

[vivek241]

Is there any notable performance difference between the following NSArray *myArray = [self generateArray]; for (NSString *currentString in myArray) and for (NSString *currentString in [self generateArray])

[James Bedford]

No, you're just doing an extra declaration (which isn't expensive) and the compiler could easily optimize this.