Question

I was having some confusion when the book started talking about alphabetizing elements in the array using the sort and compareNames: method. How is the argument for compareNames obtained when the method sort is called? And how do the elements actually get alphabetize when all that’s returned is a type of NSComparisonResult?

#import <Foundation/Foundation.h>

@interface AddressCard : NSObject
@property (copy, nonatomic) NSString *name, *email;

-(NSComparisonResult) compareNames: (id) element;

@end

@implementation AddressCard 
@synthesize name, email;

-(NSComparisonResult) compareNames:(id)element {
    return [name compare: [element name]];
}
@end

---

#import "AddressCard.h"
#import <Foundation/Foundation.h>

@interface AddressBook : NSObject
@property (nonatomic, copy) NSString *bookName;
@property (nonatomic, strong) NSMutableArray *book;

-(void) addCard: (AddressCard *) myCard;
-(void) sort;

@end

@implementation AddressBook
@synthesize book, bookName;

-(void) sort {
    [book sortUsingSelector: @selector(compareNames:)];
}

Answer 1

A NSComparisonResult is just one of NSOrderedAscending, NSOrderedSame, NSOrderedDescending.

The method compareNames is responsible of the true comparison. In this case, since you are comparing strings, the method just rely on already-implemented compare method of NSString. The result gives information about the comparison between two strings according to the alphabetical order.

Method sortUsingSelector of NSMutableArray is a useful method to sort custom types of data (you can appreciate better if you try to sort custom objects according to custom criteria). It accepts a selector, which means that every time it needs to compare two object, the method specified by the selector is called.

For what I remember you are not allowed to know the method sortUsingSelector internally uses (quick sort / heap sort / bubble sort…), what you need to know is that object are ordered using the criteria specified by the implementation of the method you pass with the selector.