Sorting an NSDictionary, descending. How do I send options with the `compare:options:` selector?

cocoa, ios, objective-c, xcode

Solution

Option 1: Use a comparator to invoke `-compare:` in reverse: (Thanks Dan Shelly!)

NSArray *blockSortedKeys = [dict keysSortedByValueUsingComparator: ^(id obj1, id obj2) {
     // Switching the order of the operands reverses the sort direction
     return [objc2 compare:obj1];
}];

Just reverse the descending and ascending return statements and you should get just what you want.

Option 2: Reverse the array you have:

See How can I reverse a NSArray in Objective-C?

Problem

I am attempting to sort an NSDictionary. From the Apple docs I see you can use `keysSortedByValueUsingSelector`: ``` NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys: [NSNumber numberWithInt:63], @"Mathematics", [NSNumber numberWithInt:72], @"English", [NSNumber numberWithInt:55], @"History", [NSNumber numberWithInt:49], @"Geography", nil]; NSArray *sortedKeysArray = [dict keysSortedByValueUsingSelector:@selector(compare:)]; ``` which gives: ``` // sortedKeysArray contains: Geography, History, Mathematics, English ``` but I want: ``` // sortedKeysArray contains: English, Mathematics, History, Geography ``` I have read that you can use `compare:options:`, and an `NSStringCompareOptions` to change the comparison to compare in the other direction. However I don't understand how you send `compare:options:` with an option in the selector. I want to do something like this: ``` NSArray *sortedKeysArray = [dict keysSortedByValueUsingSelector:@selector(compare:options:NSOrderDescending)]; ``` How should I switch the order of the comparison? Related: https://discussions.apple.com/thread/3411089?start=0&tstart=0

Original source

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