Value of loop variable after "for in" loop in Objective C

Question

Is the value of the loop variable in a for - in loop guaranteed to be nil after the loop executes, as long as the loop does not contain a break statement? For example, I'd like to write code like the following:

NSArray *items;
NSObject *item;
for (item in items) {
   if (item matches some criterion) {
      break;
   }
}
if (item) {
   // matching item found. Process item.
}
else {
   // No matching item found.
}

But this code depends on item being set to nil when the for loop runs all the way through with no break.

David Rönnqvist · Answer 1 · 2013-09-24T11:02:22.760

2

Another solution would be to check the array objects passing a specific test instead. You seem to break on the first occurrence so I only get one index back as well. If you want all matches you could use indexesOfObjectsPassingTest: instead.

NSUInteger index = [items indexOfObjectPassingTest:^BOOL(id obj, NSUInteger idx, BOOL *stop) {
    if (/* check the item*/) {
        *stop = YES;
    }
}];

if (index == NSNotFound) {
    // no match was found
} else {
    // at least one item matches the check
    id match = items[index];
}

edited Sep 24 '13 at 11:02

answered Sep 24 '13 at 10:48

David Rönnqvist

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with arc all is nilled automatically – Daij-Djan Sep 24 '13 at 10:53
This looks good, thanks, but it looks like you have to set the value of matchingItem when breaking, as in bneely's answer. – Will Nelson Sep 24 '13 at 10:56
@Daij-Djan You are right that it's probably safe to assume ARC nowadays but I still feel that it's good practice. Still good to point out that are will set it to nil for you. – David Rönnqvist Sep 24 '13 at 11:00
@David Rönnqvist My comment referred to an earlier version of your answer in which the variable "matchingItem" appeared. It seems you've edited it out, so my comment no longer applies. – Will Nelson Sep 24 '13 at 11:10
@WillNelson Oh, sorry. I didn't realise that. – David Rönnqvist Sep 24 '13 at 11:11

bneely · Accepted Answer · 2013-09-24T10:55:08.373

1

You should instead use this:

id correctItem = nil;
for (id item in items) {
    if (item matches some criteria) {
        correctItem = item;
        break;
    }
}

edited Sep 24 '13 at 10:55

answered Sep 24 '13 at 10:48

bneely

9,083
4
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with arc all is nilled automatically – Daij-Djan Sep 24 '13 at 10:53

score 1 · Answer 3 · edited May 23 '17 at 12:08

With ARC enabled, your Objective-C object pointer variables will be set to nil regardless of where you create them. _[Source]

If you are not using ARC, you can assign the pointer to nil explicitly:

NSArray *items;
NSObject *item = nil;  //<- Explicitly set to nil
for (item in items) {
   if (item matches some criterion) {
      break;
   }
}
if (item) {
   // matching item found. Process item.
}
else {
   // No matching item found.
}

I don't know the exact case, but couldn't you just do this, doing the work inside the for loop:

NSArray *items;
NSObject *item; //Doesn't really matter about item being set to nil here.
for (item in items) {
   if (item matches some criterion) 
   {
       // matching item found. Process item.
      break;
   }
}
//Don't have to worry about item after this

Why would you want/need/care about setting `item = nil`, with or without arc, when you're about to reassign it with `for (item in items)` ? — mah, Sep 24 '13 at 11:26

Value of loop variable after "for in" loop in Objective C

3 Answers3

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