How to turn an NSArray of strings into an array of unique strings, in the same order?

Question

If you have an NSArray of strings

{ @\"ONE\", @\"ONE\", @\"ONE\", \"TWO\", @\"THREE\", @\"THREE\" }

How would I turn that into

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Answer 1

Here's a nice category that defines a custom operator like @distinctUnionOfObjects, except it only works on strings and it will maintain their original order. Note: It does not sort the strings for you. It leaves intact only the first instance of whatever strings are repeated.

Usage:

#import "NSArray+orderedDistinctUnionOfStrings.h"
...
// if you feed it an array that has already been ordered, it will work as expected
NSArray *myArray = @[@"ONE", @"ONE", @"ONE", @"TWO", @"THREE", @"THREE"];
NSArray *myUniqueArray = [myArray valueForKeyPath:@"@orderedDistinctUnionOfStrings.self"];

Output:

myUniqueArray = ( "ONE", "TWO", "THREE" )

.h file:

#import 

@interface NSArray (orderedDistinctUnionOfStrings)

@end

.m file:

#import "NSArray+orderedDistinctUnionOfObjects.h"

@implementation NSArray (orderedDistinctUnionOfObjects)

- (id) _orderedDistinctUnionOfStringsForKeyPath:(NSString*)keyPath {
    NSMutableIndexSet *removeIndexes = [NSMutableIndexSet indexSet];

    for (NSUInteger i = 0, n = self.count; i < n; ++i) {
        if ([removeIndexes containsIndex:i]) {
            continue;
        }
        NSString *str1 = [[self objectAtIndex:i] valueForKeyPath:keyPath];

        for (NSUInteger j = i+1; j < n; ++j) {
            if ([removeIndexes containsIndex:j]) {
                continue;
            }
            id obj = [self objectAtIndex:j];
            NSString *str2 = [obj valueForKeyPath:keyPath];
            if ([str1 isEqualToString:str2]) {
                [removeIndexes addIndex:j];
            }
        }
    }

    NSMutableArray *myMutableCopy = [self mutableCopy];
    [myMutableCopy removeObjectsAtIndexes:removeIndexes];

    return [[NSArray arrayWithArray:myMutableCopy] valueForKeyPath:[NSString stringWithFormat:@"@unionOfObjects.%@", keyPath]];
}

@end

And here is an excellent read on how to generate your own operators, and demystifies (by a little bit) how this works: http://bou.io/KVCCustomOperators.html