Access a nested element using NSIndexPath

Question

Is there really no API for using NSIndexPath to traverse/access a nested array construct in the iOS SDK? I've looked in the docs for both NSIndexPath and NSArray?

For example:

NSArray *nested = @[
    @[
        @[@1, @2], @[@10, @20]
    ],
    @[
        @[@11, @22], @[@110, @220]
    ],
    @[
        @[@111, @222], @[@1110, @2220]
    ]
];

If I wanted to model the access path to @222, I could assemble:

NSIndexPath *path = [[[NSIndexPatch indexPathWithIndex: 2] indexPathByAddingIndex: 0] indexPathByAddingIndex: 1];

So do I have to write my own recursive accessor?

id probe = nested;
for (NSInteger position = 0; position < path.length; position++) {
    probe = probe[path indexAtPosition: position];
}

It would amaze me that Apple went this far to actually model the construct, but then not provide an API to bridge the two together. I would expect a method on either NSArray or NSIndexPath which allowed one to do something like:

id value = [nested objectAtIndexPath: path];

Or maybe there's a more idiomatic twist I'm missing?

UPDATE:

I chose to follow the advice of @CrimsonChris and put together the following:

NSArray+NestedAccess.h

#import <Foundation/Foundation.h>
@interface NSObject (NestedAccess)
- (id) objectAtPath: (NSIndexPath*) path;
@end

NSArray+NestedAccess.c

#import "NSArray+NestedAccess.h"
@implementation NSArray (NestedAccess)
- (id) objectAtPath: (NSIndexPath*) path {
    id probe = self;
    for (NSInteger position = 0; position < path.length; position++) {
        probe = ((NSArray*)probe)[path indexAtPosition: position];
    }
    return probe;
}
@end

I'm really not sure what you're asking. Looking for `getIndexes:`? `indexPathWithIndexes:length:`? `indexAtPosition`? — David Berry, Apr 04 '14 at 18:32
if you aren't afraid of swizelling, you can do much more: http://kickingbear.com/blog/archives/9 — vikingosegundo, Apr 04 '14 at 18:54

score 2 · Accepted Answer · answered Apr 04 '14 at 18:44

2

Sounds like you want a category for NSIndexPath! NSIndexPath is not designed to "just work" with NSArrays. It is generic enough to be used in multiple kinds of data structures.

answered Apr 04 '14 at 18:44

CrimsonChris

4,651
2
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Wish there was an example of doing that. I've not experienced authoring my own category/extensions yet. If there was, I'd give this one the answer. – Travis Griggs May 07 '14 at 18:10
Apple provides categories for `NSIndexPath` for use in UITableViews and UICollectionViews. That's where `row`, `section`, and `item` come from. – CrimsonChris May 07 '14 at 18:22

zaph · Answer 2 · 2014-04-04T19:02:31.633

0

Just use indexing:

NSLog (@"nested 222: %@", nested[2][0][1]);

NSLog output:

nested 222: 222

But perhaps I don't understand what you mean by:

If I wanted to model the access path to @222, I could assemble:

If you want to use NSIndexPath why not create a subclass that performs array indexing. Or create a helper class or method.

edited Apr 04 '14 at 19:02

answered Apr 04 '14 at 18:25

zaph

111,848
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Access a nested element using NSIndexPath

UPDATE:

NSArray+NestedAccess.h

NSArray+NestedAccess.c

2 Answers2