How to sort NSMutableArray using sortedArrayUsingDescriptors?

Question

I have a question about sorting NSMutableArray. I can use sortedArrayUsingDescriptors: method to sort an array with objects.

For example I have an NSMutableArray of places where I have an attribute frequency (int value) and I want to sort descending on frequency but I don't know how to use it correctly.

What do I put as a key in initWithKey?

My object place contains:

NSString * name;
NSString * address;
NSString * frequency;
NSString * type;

---

NSMutableArray * places;

... populate array with objects ...

NSSortDescriptor * sortByFrequency =
   [[[NSSortDescriptor alloc] initWithKey:@"????????" ascending:NO] autorelease];

NSArray * descriptors = [NSArray arrayWithObject:sortByFrequency];
NSArray * sorted = [x sortedArrayUsingDescriptors:descriptors];

Answer 1

To sort your array of objects you:

1. setup NSSortDescriptor - use names of your variables as keys to setup descriptor for sorting plus the selector to be executed on those keys
2. get the array of descriptors using NSSortDescriptor that you've setup
3. sort your array based on those descriptors

Here are two examples, one using NSDictionary and NSString/NSNumber values sorting on NSNumber, the other one using custom class with sorting on two NSString fields.

Follow Sorting and Filtering NSArray Objects in Cocoa programming topics to see more examples and explanation.

Example:

This was done on GNUStep it should work the same on Cocoa - the code is exactly the same - I'll try when I sit in front of my Mac:

First example using NSString and NSNumber values with sorting on NSNumber value:

NSString * NAME      = @"name";
NSString * ADDRESS   = @"address";
NSString * FREQUENCY = @"frequency";
NSString * TYPE      = @"type";

NSMutableArray * array = [NSMutableArray array];

NSDictionary * dict;

dict = [NSDictionary dictionaryWithObjectsAndKeys:
            @"Alehandro", NAME, @"Sydney", ADDRESS,
            [NSNumber numberWithInt:100], FREQUENCY,
            @"T", TYPE, nil];
[array addObject:dict];

dict = [NSDictionary dictionaryWithObjectsAndKeys:
            @"Xentro", NAME, @"Melbourne", ADDRESS,
            [NSNumber numberWithInt:50], FREQUENCY,
            @"X", TYPE, nil];
[array addObject:dict];

dict = [NSDictionary dictionaryWithObjectsAndKeys:
            @"John", NAME, @"Perth", ADDRESS,
            [NSNumber numberWithInt:75],
            FREQUENCY, @"A", TYPE, nil];
[array addObject:dict];

dict = [NSDictionary dictionaryWithObjectsAndKeys:
            @"Fjord", NAME, @"Brisbane", ADDRESS,
            [NSNumber numberWithInt:20], FREQUENCY,
            @"B", TYPE, nil];
[array addObject:dict];

---

Sorting part using descriptors with the Frequency field which is NSNumber:

NSSortDescriptor * frequencyDescriptor =
    [[[NSSortDescriptor alloc] initWithKey:FREQUENCY
                                 ascending:YES] autorelease];

id obj;
NSEnumerator * enumerator = [array objectEnumerator];
while ((obj = [enumerator nextObject])) NSLog(@"%@", obj);

NSArray * descriptors =
    [NSArray arrayWithObjects:frequencyDescriptor, nil];
NSArray * sortedArray =
    [array sortedArrayUsingDescriptors:descriptors];

NSLog(@"\nSorted ...");

enumerator = [sortedArray objectEnumerator];
while ((obj = [enumerator nextObject])) NSLog(@"%@", obj);

---

OUTPUT - sorted by Frequency field:

2009-12-04 x[1] {address = Sydney; frequency = 100; name = Alehandro; type = T; }
2009-12-04 x[1] {address = Melbourne; frequency = 50; name = Xentro; type = X; }
2009-12-04 x[1] {address = Perth; frequency = 75; name = John; type = A; }
2009-12-04 x[1] {address = Brisbane; frequency = 20; name = Fjord; type = B; }
2009-12-04 x[1]
Sorted ...
2009-12-04 x[1] {address = Brisbane; frequency = 20; name = Fjord; type = B; }
2009-12-04 x[1] {address = Melbourne; frequency = 50; name = Xentro; type = X; }
2009-12-04 x[1] {address = Perth; frequency = 75; name = John; type = A; }
2009-12-04 x[1] {address = Sydney; frequency = 100; name = Alehandro; type = T; }

---

---

Second example with custom class and sorting on two NSString variables.

Array to sort (see class A at the bottom):

NSMutableArray * array = [NSMutableArray array];
[array addObject:[[A alloc] initWithFirstName:@"Alehandro"
                                     lastName:@"Xentro"
                                          age:[NSNumber numberWithInt:40]]];
[array addObject:[[A alloc] initWithFirstName:@"John"
                                     lastName:@"Smith"
                                          age:[NSNumber numberWithInt:30]]];
[array addObject:[[A alloc] initWithFirstName:@"John"
                                     lastName:@"Smyth"
                                          age:[NSNumber numberWithInt:25]]];
[array addObject:[[A alloc] initWithFirstName:@"Torro"
                                     lastName:@"Ola"
                                          age:[NSNumber numberWithInt:45]]];
[array addObject:[[A alloc] initWithFirstName:@"Alehandro"
                                     lastName:@"Bento"
                                          age:[NSNumber numberWithInt:41]]];
[array addObject:[[A alloc] initWithFirstName:@"Alehandro"
                                     lastName:@"Axel"
                                          age:[NSNumber numberWithInt:41]]];

---

The sorting part, sort on lastName then firstName:

NSString * LASTNAME = @"lastName";
NSString * FIRSTNAME = @"firstName";

NSSortDescriptor *lastDescriptor =
    [[[NSSortDescriptor alloc]
        initWithKey:LASTNAME
          ascending:YES
           selector:@selector(localizedCaseInsensitiveCompare:)] autorelease];

NSSortDescriptor *firstDescriptor =
    [[[NSSortDescriptor alloc]
        initWithKey:FIRSTNAME
          ascending:YES
           selector:@selector(localizedCaseInsensitiveCompare:)] autorelease];

NSArray * descriptors =
   [NSArray arrayWithObjects:lastDescriptor, firstDescriptor, nil];
NSArray * sortedArray =
   [array sortedArrayUsingDescriptors:descriptors];

---

Print the result:

NSLog(@"\nSorted ...");

enumerator = [sortedArray objectEnumerator];
while ((obj = [enumerator nextObject])) NSLog(@"%@", obj);

---

Result (before and after sorting):

2009-12-04 00:52:16.637 x[11375] Alehandro, Xentro, age:40
2009-12-04 00:52:16.644 x[11375] John, Smith, age:30
2009-12-04 00:52:16.644 x[11375] John, Smyth, age:25
2009-12-04 00:52:16.644 x[11375] Torro, Ola, age:45
2009-12-04 00:52:16.645 x[11375] Alehandro, Bento, age:41
2009-12-04 00:52:16.645 x[11375] Alehandro, Axel, age:41
2009-12-04 00:52:16.645 x[11375]
Sorted ...
2009-12-04 00:52:16.645 x[11375] Alehandro, Axel, age:41
2009-12-04 00:52:16.645 x[11375] Alehandro, Bento, age:41
2009-12-04 00:52:16.645 x[11375] Torro, Ola, age:45
2009-12-04 00:52:16.645 x[11375] John, Smith, age:30
2009-12-04 00:52:16.645 x[11375] John, Smyth, age:25
2009-12-04 00:52:16.645 x[11375] Alehandro, Xentro, age:40

Class A extends NSObject - nothing special here:

#import <Foundation/Foundation.h>

@interface A : NSObject
{
    NSString * firstName;
    NSString * lastName;
    NSNumber * age;
}

- (id)initWithFirstName:(NSString*)aFirstName
               lastName:(NSString*)aLastName
                    age:(NSNumber*)anAge;

-(NSString* )description;
+(NSString*)action;

@end

Implementation:

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

@implementation A

- (id)init
{
    return [self initWithFirstName:@"N/A"
                          lastName:@"N/A"
                               age:0];
}

- (id)initWithFirstName:(NSString*)aFirstName
               lastName:(NSString*)aLastName
                    age:(NSNumber*)anAge
{
    self = [super init];
    if (!self) return nil;

    firstName = [aFirstName copy];
    lastName = [aLastName copy];
    age = [anAge copy];

    return self;
}

- (void)dealloc
{
    [firstName release];
    [lastName release];
    [age release];
    [super release];
}

---

- (NSString *) description
{
    return [NSString stringWithFormat: @"%@, %@, age:%@",
                                       firstName, lastName, age];
}
@end