iOS Filling an NSMutableArray

[Olbert000]

I am having problems filling my an NSMutableArray one object at a time. Here is some sample code to illustrate my problem:

NSMutableArray* myArray = [[NSMutableArray alloc] init];
NSMutableString* myString = [[NSMutableString alloc] initWithString:@"a"];
for (int i=0;i<5;++i){
    [myArray addObject:myString];
    [myString appendString:@"a"];
}
for (int i=0;i<5;++i){
    NSLog(@"%@",[myArray objectAtIndex:i]);
}

Now this produces an output of:
aaaaa
aaaaa
aaaaa
aaaaa
aaaaa

Which is quite different to the output I expected:
a
aa
aaa
aaaa
aaaaa

I am a beginner to Objective C and am unsure why this is.

 

[robbieduncan]

You are adding the same string every time. So each element of the array is pointing to the exact same string. You need to create a new string and add it each iteration of the the loop.

 

[andrew086]

try this:

	NSMutableArray* myArray = [[NSMutableArray alloc] init];
	NSMutableString* myString = [[NSMutableString alloc] initWithString:@"a"];
	for (int i=0;i<5;++i){
		[myArray addObject:[myString copy]];
		[myString appendString:@"a"];
	}
	for (int i=0;i<5;++i){
		NSLog(@"%@",[myArray objectAtIndex:i]);
	}
	[myArray release];
	[myString release];

what's happening is what robbieduncan said. it adds the myString string 5 times, but it all 5 of them are pointing to the same location in memory.

 

[Comrade Yeti]

[myArray addObject:[myString copy]];

violates the memory guidelines... throw an autorelease on the end to avoid memory leaks

[myArray addObject:[[myString copy] autorelease]

See Apple's memory guidelines for a refresher:
"You only release or autorelease objects you own.
You take ownership of an object if you create it using a method whose name begins with “alloc” or “new” or contains “copy” (for example, alloc, newObject, or mutableCopy), or if you send it a retain message.
You use release or autorelease to relinquish ownership of an object."
http://developer.apple.com/library/ios/#documentation/cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html#//apple_ref/doc/uid/20000994-BAJHFBGH