How to move an item on NSMutableArray? - objective-c

Question

I want to move a string item to the top of the list.
NSMutableArray animal = "cat", "lion", "dog", "tiger";
How do I move dog to top of the list?

Answer 1

You would remove the item and insert it at the correct space:
id tmp=[[animals objectAtIndex:2] retain];
[animals removeObjectAtIndex:2];
[animals insertObject:tmp atIndex:0];
[tmp release];
You have to retain the object or when you tell the array to remove it, it will release the object.
If you don't know the index you could do something like this:
NSMutableArray* animals = [NSMutableArray arrayWithObjects:#"cat", #"lion", #"dog", #"tiger",nil];
for (NSString* obj in [[animals copy] autorelease]) {
if ([obj isEqualToString:#"dog"]) {
NSString* tmp = [obj retain];
[animals removeObject:tmp];
[animals insertObject:tmp atIndex:0];
break;
}
}
This method will go over all your list and search for "dog" and if it finds it will remove it from the original list and move it to index 0.

Answer 2

I would like to point out an inefficiency in your method. By removing/inserting an object from a NSMutableArray you potentially affect every row after the deletion/insertion. I say ‘potentially’ because it’s not clear what internal method Apple uses to maintain their mutable arrays. However, assuming it’s a simple c-array, then every row after that deletion/insertion index will be need to be moved down/up. In a very large array, this could be inefficient if the items moved are at the beginning. However, replacing items in an array are not inefficient at all. Thus the following is a category on NSMutableArray (note this code is under ARC, so no memory management):
- (void) moveObjectAtIndex:(NSUInteger)fromIndex toIndex:(NSUInteger)toIndex{
if (fromIndex == toIndex) return;
if (fromIndex >= self.count) return; //there is no object to move, return
if (toIndex >= self.count) toIndex = self.count - 1; //toIndex too large, assume a move to end
id movingObject = [self objectAtIndex:fromIndex];
if (fromIndex < toIndex){
for (int i = fromIndex; i <= toIndex; i++){
[self replaceObjectAtIndex:i withObject:(i == toIndex) ? movingObject : [self objectAtIndex:i + 1]];
}
} else {
id cObject;
id prevObject;
for (int i = toIndex; i <= fromIndex; i++){
cObject = [self objectAtIndex:i];
[self replaceObjectAtIndex:i withObject:(i == toIndex) ? movingObject : prevObject];
prevObject = cObject;
}
}
}
Also, a small bonus to further increase functionality, if you're performing operations on the items moved (like updating a db or something), the following code has been very useful to me:
- (void) moveObjectAtIndex:(NSUInteger)fromIndex toIndex:(NSUInteger)toIndex withBlock:(void (^)(id, NSUInteger))block{
if (fromIndex == toIndex) return;
if (fromIndex >= self.count) return; //there is no object to move, return
if (toIndex >= self.count) toIndex = self.count - 1; //toIndex too large, assume a move to end
id movingObject = [self objectAtIndex:fromIndex];
id replacementObject;
if (fromIndex < toIndex){
for (int i = fromIndex; i <= toIndex; i++){
replacementObject = (i == toIndex) ? movingObject : [self objectAtIndex:i + 1];
[self replaceObjectAtIndex:i withObject:replacementObject];
if (block) block(replacementObject, i);
}
} else {
id cObject;
id prevObject;
for (int i = toIndex; i <= fromIndex; i++){
cObject = [self objectAtIndex:i];
replacementObject = (i == toIndex) ? movingObject : prevObject;
[self replaceObjectAtIndex:i withObject:replacementObject];
prevObject = cObject;
if (block) block(replacementObject, i);
}
}
}

Answer 3

You can remove an existing element, e.g dog and then reinsert it at the beginning of the array.
NSMutableArray *animals = [NSMutableArray arrayWithObjects:#"cat", #"lion", #"dog", #"tiger",nil];
NSString *dog = #"dog";
// Check to see if dog is in animals
if ( [animals containsObject:dog] ) {
// Remove dog from animals and reinsert
// at the beginning of animals
[animals removeObject:dog];
[animals insertObject:dog atIndex:0];
}