Sort NSArray based on derived value… with a hitch?

Question

I want to be able to sort a NSArray of objects based on its tag value. The hitch is if the tag is 0, I want it pushed to the end of the array.

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"tag" ascending:TRUE];
NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];

NSArray *sortedArray = [self.array sortedArrayUsingDescriptors:sortDescriptors];

This returns with all the objects with tags in the front of the index (like it should).

I want a tag value > 0 to be at the front, and all the tag values of 0 at the end.

I though blocks would work, something similar to:

NSArray *sortedArray;
sortedArray = [drinkDetails sortedArrayUsingComparator:^NSComparisonResult(id a, id b) {
    NSDate *first = [(Person*)a birthDate];
    NSDate *second = [(Person*)b birthDate];
    return [first compare:second];
}];

that I found in another SO question, but unsure how to add the condition

if (obj.tag == 0) {
    // push to end of array
}

Any suggestions?

Jim Puls · Accepted Answer · 2013-04-11T16:21:13.287

4

You've found the idea of a comparator block; all you need to do in that block is say that an object with tag 0 sorts after anything else.

sortedArray = [self.array sortedArrayUsingComparator:^NSComparisonResult(id a, id b) {
    NSInteger first = [a tag];
    NSInteger second = [b tag];
    if (first == 0) {
        return (second == 0 ? NSOrderedSame : NSOrderedDescending);
    }
    if (second == 0) {
        return NSOrderedAscending;
    }
    if (first < second) {
        return NSOrderedAscending;
    }
    if (first > second) {
        return NSOrderedDescending;
    }
    return NSOrderedSame;
}];

edited Apr 11 '13 at 16:21

answered Apr 11 '13 at 15:19

Jim Puls

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2

I seems to me that the case first > 0, second == 0 is not handled correctly (should return NSOrderedAscending). – Martin R Apr 11 '13 at 15:27
Martin R is correct, tested with a larger sample and I get tag's with 0 still at the front. – Padin215 Apr 11 '13 at 15:30
1

Just add `if (second == 0) return NSOrderedAscending;` after the first if-block. – Martin R Apr 11 '13 at 15:36

score 0 · Answer 2 · answered Apr 11 '13 at 19:15

// sort the part without tag = 0
NSPredicate* p = [NSPredicate predicateWithFormat: @"tag != 0"];
NSSortDescriptor* sort = [NSSortDescriptor sortDescriptorWithKey: @"tag" ascending: YES];
NSMutableArray* result = [[[videos filteredArrayUsingPredicate: p]
                           sortedArrayUsingDescriptors: @[ sort ]] mutableCopy];

// append the part with tag = 0;
p = [NSPredicate predicateWithFormat: @"tag = 0"];
[result addObjectsFromArray: [videos filteredArrayUsingPredicate: p]];

Another approach, splice the original array to two parts, sort part without the 0s, and then append the part with 0s to the end.

Sort NSArray based on derived value… with a hitch?

2 Answers2