Trying to sort an NSMutableDictionary by values using @selector in iOS

Question

I have an NSMutableDictionary that contains keys that are NSStrings, and values that are objects of type "Fraction". This is a custom object that I have created and is exactly what it is, Fractions that are composed of a numerator, and a denominator (each one is an int). What I would like to do is get an array of the values (in this case, "Fraction" objects) from the NSMutableDictionary, and then sort the array using @selector as follows:

NSArray *myFractions = [myDict allValues];
NSArray *sortedFractions = [myFractions sortedArrayUsingSelector:@selector(comparator)];

or

NSArray *myFractions = [myDict allValues];
NSArray *sortedFractions = [myFractions sortedArrayUsingFunction:comparatorFunction context:nil];

My question is, how do I create a comparatorFunction method that will sort the "Fraction" objects in order of biggest, to smallest? Please remember that each "Fraction" object is composed of two variables: a numerator of type int, and a denominator that is also of type int.

This question is different from others that have been posted because here I am asking about how, within the "sortedArrayUsingFunction" method I am to sort the "Fraction" objects from greatest to least. The other questions that were thought to be duplicates were looking at sorting based on NSDate objects, or based on NSString values, whereas mine is different. As I said, I am very new to this, and needed something much more specific.

Thanks in advance.

Anoop Vaidya · Answer 1 · 2013-03-10T10:19:01.213

0

In your class you need to implement a method similar to this :

-(NSComparisonResult)compareFirstName:(Fraction *)fraction{

      return ((float)self.numerator/self.denominator > (float)fraction.numerator/fraction.denominator);
}

*Not compiled and checked

edited Mar 10 '13 at 10:19

answered Mar 10 '13 at 10:11

Anoop Vaidya

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What are those calls to `objectAtIndex:` and `compare:` on a `float`? – Mar 10 '13 at 10:14
oops it should be directly > or < , i missed that this is a primitive :( – Anoop Vaidya Mar 10 '13 at 10:15
@AnoopVaidyia Still, what is `objectAtIndex:` doing there? – Mar 10 '13 at 10:17
@H2CO3: is it still wrong or makes some sense ? :( – Anoop Vaidya Mar 10 '13 at 10:19
It's good now. [15 chars] – Mar 10 '13 at 10:20
@H2CO3: now what is 15 chars? – Anoop Vaidya Mar 10 '13 at 10:21
1

A brainless restriction by SO on comments. "It's good now" can't be posted as a comment, because it's not at least 15 chars, so I had to write some garbage to make it long enough to make SO enable me to post it. – Mar 10 '13 at 10:22

score 0 · Accepted Answer · 2013-03-10T11:08:23.253

0

As a stylistic improvement, can I suggest you to use a closure instead of a function? To sort the objects in descending order, simply compare their actual numerical value, like this:

NSMutableDictionary *dict = // dictionary of NSString keys and Fraction values
NSArray *fractions = [dict allValues];

NSArray *sortedFractions = [fractions sortedArrayUsingComparator:^(id obj1, id obj2) {
    Fraction *f1 = obj1;
    Fraction *f2 = obj2;
    double diff = (double)(f1.num) / f1.denom - (double)(f2.num) / f2.denom;
    return diff < 0 ? NSOrderedAscending : diff > 0 ? NSOrderedDescending : NSOrderedSame;
}];

edited Mar 10 '13 at 11:08

answered Mar 10 '13 at 10:13

Thanks for your prompt reply. Shouldn't it be [dict allValues] instead of [dict allObjects]? – syedfa Mar 10 '13 at 11:06
@syedfa It certainly should. – Mar 10 '13 at 11:08
Thanks very much BiCarbonate, your solution worked! – syedfa Mar 10 '13 at 11:35
1

@syedfa, just a chemistry aside here. Bicarbonate is HCO3-, H2CO3 is carbonic acid. – rdelmar Mar 10 '13 at 16:47
@rdelmar Chemists to the rescue! – Mar 10 '13 at 16:48
@rdelmar: I stand corrected. – syedfa Mar 11 '13 at 03:38

Trying to sort an NSMutableDictionary by values using @selector in iOS

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