Objective-C sorting a NSMutableArray and returning the NSMutableArray (sorted)

Question

Maybe someone can enlighten me on this subject. I'm sorting a NSMutableArray declared as ids. When I'm done sorting I return it using return ids; but for some reason the returned array is not sorted. See this code:

NSMutableArray *ids = [[[NSMutableArray alloc] init] autorelease];

for (Career *career in careers) {
    [ids addObject:[career id]];
}

// Sort the array in an orderly fashion
[ids sortedArrayUsingSelector:@selector(caseInsensitiveCompare:)];

return ids;

This does not return a sorted array.

But if I return the array with the sorting method applied, it will be returned correctly sorted. Like this:

//Code is the same as above
return [ids sortedArrayUsingSelector:@selector(caseInsensitiveCompare:)];

Also while I'm at it, I mistakingly used two semi colons (;) when writing the return statement, but I didn't receive any error for doing so?

;; instead of ; is not a C (or Objective-C) syntax error. — user557219
– user557219, Commented Jul 6, 2011 at 9:53
To add to what Bavarious is saying above, an additional semicolon without an instruction is known as an empty statement. This is allowed so that certain loop/branching constructs can have an empty code block because all the work is in the loop/branching construct itself. Eg. "for (i=1 ; i<=5 ; i++, j+=i);" is a valid for-loop. All this loop really wants to do is add the numbers 1,2,3,4,5 to j so no need for a code-block after the for. — hktegner
– hktegner, Commented Jul 11, 2011 at 12:45

score 4 · Accepted Answer · 2011-07-06 09:56:38Z

4

-sortedArrayUsingSelector: returns a new, sorted array, and it doesn’t change the original array. This means that:

[ids sortedArrayUsingSelector:@selector(caseInsensitiveCompare:)];
return ids;

does the following:

Creates a new, sorted array based on ids and keeps ids intact;
Discards the method return value (i.e., the new, sorted array) since you’re not using it;
Returns the original ids array.

Whereas:

return [ids sortedArrayUsingSelector:@selector(caseInsensitiveCompare:)];

does the following:

Creates a new, sorted array based on ids and keeps ids intact;
Uses the method return value (i.e., the new, sorted array) as the return value.

If you don’t want a new array, use -sortUsingSelector: instead:

edited Jul 6, 2011 at 9:56

answered Jul 6, 2011 at 9:51

user557219

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1 Comment

Peter Warbo Over a year ago

Thanks! It wasn't that obvious but now when I checked the documentation it made perfectly sense.

gsempe · Accepted Answer · 2011-07-06 09:53:37Z

0

This is because the array sorted is not the array which receive the message but the return one. Take a look at sortedArrayUsingSelector:

answered Jul 6, 2011 at 9:53

gsempe

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