Stack and Queue extensions to NSMutableArray

2.21

Stack and Queue extensions to NSMutableArray

Code

This is all out there on the web, but I’m putting everything here all in one place so as not to forget. Just a simple Stack and Queue implementation as a category on NSMutableArray. It works and it doesn’t suck, and that’s all I’ll say about it.

Stack

A very basic Stack implementation with just push, pop, and peek.

@interface NSMutableArray (Stack)
 
- (void) push: (id)item;
- (id) pop;
- (id) peek;
 
@end

@implementation NSMutableArray (Stack)
 
- (void) push: (id)item {
    [self addObject:item];
}
 
- (id) pop {
    id item = nil;
    if ([self count] != 0) {
        item = [[[self lastObject] retain] autorelease];
        [self removeLastObject];
    }
    return item;
}
 
- (id) peek {
    id item = nil;
    if ([self count] != 0) {
        item = [[[self lastObject] retain] autorelease];
    }
    return item;
}
 
@end

No real magic above, just Objective-C’s annoying retain & autorelease dance to get an item out of NSMutableArray correctly.

Queue

Much like Stack above, here’s a very basic Queue implementation with just enqueue, dequeue, and peek.

@interface NSMutableArray (Queue)
 
- (void) enqueue: (id)item;
- (id) dequeue;
- (id) peek;
 
@end

@implementation NSMutableArray (Queue)
 
- (void) enqueue: (id)item {
    [self addObject:item];
}
 
- (id) dequeue {
    id item = nil;
    if ([self count] != 0) {
        item = [[[self objectAtIndex:0] retain] autorelease];
        [self removeObjectAtIndex:0];
    }
    return item;
}
 
- (id) peek {
    id item = nil;
    if ([self count] != 0) {
        item = [[[self objectAtIndex:0] retain] autorelease];
    }
    return item;
}
 
@end

Note that we need the same retain & autorelease dance as above.

Files

Comments

michelle

2.18.2012

1

Thanks for this! I’ve been trying to figure this out all morning.

Chet Harrison

4.25.2012

2

So how do you alloc and init the array?

Luca

5.6.2012

3

The dance retain autorelease is totally redundant and gives rise to an absurd code, like the one above.
[self lastObject] is already an autoreleased object.

Luca

5.6.2012

4

Sorry, it is justified in this case. removeObject decreases the retain count and cause an anticipated release of the object. It’s fine.

Luca

5.6.2012

5

But in peek method: item = [[[self objectAtIndex:0] retain] autorelease];
is wrong. it is sufficient:[self objectAtIndex:0]

Daniel Skinner

9.23.2012

6

Thanks, this is a nice tidy extension that was exactly what I needed.

Assad Khan

2.10.2013

7

Luca is right

There is no need for retain autorelease in peek.

Assad Khan

2.10.2013

8

And rather than writing a category on NSMutableArray create a class yourself and use composition rather than inheritance or categories.

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