Difference between two ways of Iteration in Objective C?

Question

i have some problem with two ways of iteration.

NSArray *array=[[NSArray alloc] initWithObjects:@"A",@"B",@"C",@"D",nil];

NSMutableArray *mutArray=[[NSMutableArray alloc] initWithArray:array];

when i do like this then it's working correct

for (int i=0;[mutArray count]!=0;) {
    [mutArray removeObjectAtIndex:i];
}
NSLog(@"%d,",[mutArray count]);

But when i do like this, it's crashing... why?

for(id obj in mutArray)
{
    [mutArray removeObject:obj]
}
NSLog(@"%d,",[mutArray count]);

Please give me the solution for second case.

Hey, check out this post: http://stackoverflow.com/questions/5826336/remove-items-in-a-for-loop-without-side-effects - why you can't remove items from an array using the fast for loop. — Jon Boydell, Feb 27 '12 at 10:28
@joerick's answer is spot on but just FYI this exists : `[mutArray removeAllObjects];` ;) — deanWombourne, Feb 27 '12 at 10:51

justin · Answer 1 · 2012-02-27T10:30:44.273

2

You must not mutate a collection while it is being fast-enumerated.

Fast enumeration works by requesting a group of elements from the collection. It does this for performance and correctness. Since you are enumerating over a slice of elements, the program does not need to round trip to the container all the time. But if you mutate the collection behind the enumerator's back, then all guarantees of correctness and object lifetimes are lost.

You'll see an error in the console in this case.

See also: -removeAllObjects.

edited Feb 27 '12 at 10:30

answered Feb 27 '12 at 10:25

justin

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can you tell me what is the reason behind this? – Mudit Bajpai Feb 27 '12 at 10:27
1

One more link for your ref: https://developer.apple.com/library/ios/#documentation/cocoa/conceptual/objectivec/Chapters/ocFastEnumeration.html – Hanon Feb 27 '12 at 10:31

score 2 · Accepted Answer · answered Feb 27 '12 at 10:29

The second case is called fast enumeration. You can't edit an array while it is being fast-enumerated, due to an implementation detail of fast enumeration.

I often get around this problem by doing the following:

for(id obj in [[mutArray copy] autorelease])
{
    [mutArray removeObject:obj]
}
NSLog(@"%d,",[mutArray count]);

This way you're doing the iterating over a temporary copy of the array, but you can modify the original as you go through.

p.s. you can't edit during fast enumeration because fast enumeration works by asking an object for a C array of the contents at the start of the enumeration. Once that has been obtained, the iteration is quick, because there are no Objective-C message calls between the enumerations. However, if you modify the array, the C array will no longer be a valid representation of the contents, so an exception is raised.

score 0 · Answer 3 · answered Feb 27 '12 at 10:33

0

My suggestion:

NSMutableArray *temps = [NSMutableArray array];
for(id obj in mutArray)
{
    // checking condition here
    [temps addObject:obj];
}
[mutArray removeObjectsInArray:temps];

answered Feb 27 '12 at 10:33

Tommy

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score 0 · Answer 4 · answered Feb 27 '12 at 10:38

0

Mutating the collection invalidates the enumerator. The enumerator 'detects' the change and triggers an error. In a fast enumeration you can mark what needs to be removed or use indexing in the 1st place.

answered Feb 27 '12 at 10:38

hburde

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Difference between two ways of Iteration in Objective C?

4 Answers4