How to remove items from nested immutable collections

Question

I have an NSMutableDictionary that contains an array of items (NSDictionary type). I need to remove one particular key (and its associated object) stored in each item. What's the best way to go about this?

Example structure:

   NSDictionary *dict = @{ 
       @"v" : @[ 
          @{ @"key1": @"abc", @"key2" : @"def" },
          @{ @"key1": @"ghi", @"key2" : @"jkl" }, ...
        ]
    };

I want to eliminate all key1 from the nested dictionary element:

@{ 
   @"v" : @[ 
      @{ @"key2" : @"def" },
      @{ @"key2" : @"jkl" }, ...
    ]
}

You mean you have a Dictionary, where you have array objects, and each array has more dictionaries? — Merlevede, Feb 20 '14 at 04:06
Simon is right, in the sense that your inner dictionaries are not mutable. Can you confirm this? — Merlevede, Feb 20 '14 at 04:10
Immutable to start, but I can always create an NSMutableDictionary and initialize it with this dictionary. — Boon, Feb 20 '14 at 04:11
Updated my answer with code snippet. You just want to create NSMutabledictionary for each dictionary for removing the key/object pair, and then replace the corresponding entry in the array. — Simon, Feb 20 '14 at 04:13
This question is [being discussed on Meta Stack Overflow](http://meta.stackexchange.com/q/222162). — Aaron Brager, Feb 20 '14 at 04:55
@Emracool Not a duplicate, compare accepted answer with your post. — Boon, Feb 20 '14 at 05:04

Aaron Brager · Accepted Answer · 2014-02-20T04:47:43.287

2

This should do it:

NSMutableDictionary *dict = [@{
                       @"v" : @[
                               @{ @"key1": @"abc", @"key2" : @"def" },
                               @{ @"key1": @"ghi", @"key2" : @"jkl" }
                               ]
                       } mutableCopy];

NSArray *oldArray = [dict objectForKey:@"v"];
NSMutableArray *newArray = [NSMutableArray array];
[oldArray enumerateObjectsUsingBlock:^(NSDictionary *dictionary, NSUInteger idx, BOOL *stop) {
    NSMutableDictionary *mutableDictionary = [dictionary mutableCopy];
    [mutableDictionary removeObjectForKey:@"key1"];
    [newArray addObject:mutableDictionary];
}];
[dict setObject:newArray forKey:@"v"];

edited Feb 20 '14 at 04:47

answered Feb 20 '14 at 04:10

Aaron Brager

65,323
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why the `__block` when declaring `newArray`? – Sebastian Feb 20 '14 at 04:30
@Sebastian Normally blocks `copy` the variables inside them, so changes to the variables aren't available outside the block. See https://developer.apple.com/library/ios/documentation/cocoa/Conceptual/Blocks/Articles/bxVariables.html#//apple_ref/doc/uid/TP40007502-CH6-SW6 – Aaron Brager Feb 20 '14 at 04:32
(You wouldn't need `__block` if you used a simple for loop instead of `enumerateObjectsUsingBlock:`.) – Aaron Brager Feb 20 '14 at 04:33
2

But you're not changing the variable. (the variable being the pointer, not the object pointed to) – Sebastian Feb 20 '14 at 04:36
@AaronBrager There is no need to use __block with the array, I have confirmed. – Boon Feb 20 '14 at 04:42
Right. Updated to remove `__block`. – Aaron Brager Feb 20 '14 at 04:47

score 0 · Answer 2 · answered Feb 20 '14 at 04:17

For your main dictionary you can enumerate your objects using enumerateKeysAndObjectsUsingBlock: and for the arrays you can use enumerateObjectsUsingBlock

[self.myDictionary enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
    NSArray *array = (NSArray *)obj;
    [arr enumerateObjectsUsingBlock:^(id obj2, NSUInteger idx, BOOL *stop) 
    {
         NSDictionary *dict = (NSDictonary *)obj2;
         // remove item from dict
    }];
}];

I didn't test it but it should give you an idea.

Simon · Answer 3 · 2014-02-20T04:23:06.183

-1

Enumerate through the NSArray and call [dict removeObjectForKey:@"key1"]for each dictionary.

Edit: If they are NSDictionary instead of NSMutableDictionary, do the following:

NSMutableDictionary *mutableDict = [[NSMutableDictionary alloc] init];
[mutableDict setDictionary:originalDict];
[mutableDict removeObjectForKey:@"key1"];
NSDictionary *newDict = [[NSDictionary alloc] initWithDictionary:mutableDict];

Use newDict however you'd like, probably replace the original one in NSArray

Edit again with entire solution:

NSArray *a = [outerDict objectForKey:@"v"];
NSMutableArray *newArray = [[NSMutableArray alloc] init];
for (NSDictionary d in a) {
    NSMutableDictionary *mutableDict = [[NSMutableDictionary alloc] init];
    [mutableDict setDictionary:d];
    [mutableDict removeObjectForKey:@"key1"];
    NSDictionary *newDict = [[NSDictionary alloc] initWithDictionary:mutableDict];
    [newArray addObject:newDict];
}
[outerDict setObject:newArray ForKey:@"v"];

edited Feb 20 '14 at 04:23

answered Feb 20 '14 at 03:55

Simon

1,746
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How can you call removeObjectForKey when NSArray is storing NSDictionary? – Boon Feb 20 '14 at 03:56
If they are `NSDictionary`s then copy them into NSMutableDictionary then call remove. I'm afraid there is no quick and efficient way to do it as NSDictionary is immutable. – Simon Feb 20 '14 at 04:05
I don't think your solution will work because key1 is in dictionary inside the array. – Boon Feb 20 '14 at 04:14
I just didn't write out the entire solution. My code would fit in the for loop for the array, so you already have each of the inner dictionaries. – Simon Feb 20 '14 at 04:16
Your solution won't work. If this question is trivial, I would not ask it here. – Boon Feb 20 '14 at 04:26
You keep saying it won't work but don't explain why. If you know what you're talking about, you'd realize most of the proposed answers are effectively equivalent. – Simon Feb 20 '14 at 04:28
Because I am not a newbie, I can clearly see your solution won't work before you edited it. And I did explain why it won't but I can't keep explaining you know? – Boon Feb 20 '14 at 04:34

greymouser · Answer 4 · 2014-02-20T04:17:08.413

Assuming the inner dictionaries are actually mutable (as your stated) and not inline, you want something like this:

NSString *removeKey = @"key1";
for (NSArray *array in [outerDict allValues])
{
    for (NSMutableDictionary *dict in arrayOfDicts)
    {
        [dict removeObjectForKey:removeKey];
    }
}

... but you still need to have the inner dictionaries mutable. If you want to set it up inline, and have them be mutable, this will work:

NSDictionary *dict = @{
                       @"v" : @[
                               [@{ @"key1": @"abc", @"key2" : @"def" } mutableCopy],
                               [@{ @"key1": @"ghi", @"key2" : @"jkl" } mutableCopy]
                               ]
                       };

You don't need a key for the outermost. This is a dictionary literal. — Boon, Feb 20 '14 at 04:08

How to remove items from nested immutable collections

4 Answers4