NSSortDescriptor incorrectly sort date and strings, which are properties of custom class, but correctly sort integer

Question

I've created simple class Student with properties

@property (strong,nonatomic) NSString* firstname;
@property (strong,nonatomic) NSString* lastname;
@property (strong,nonatomic) NSDate* dateOfBirth;
@property (assign,nonatomic) NSInteger index;

After that, I created any objects of this class, gave it firstname, lastname, date and index, add all of them to self.InitialStudentsArray. And start sorting by dateOfBirth:

NSSortDescriptor* descriptorDate=[[NSSortDescriptor alloc]initWithKey:@"dateOfBirth" ascending:YES];
NSArray* descArray=[NSArray arrayWithObjects:descriptorDate, nil];
[self.InitialStudentsArray sortUsingDescriptors:descArray];

So, the result is strange. Array before sorting and after sorting have different items order, but not sorted at all by date.

before sort (
    "46 Alex  Dupel 25 10 1991",
    "236 Ivan  Evstegneev 20 09 1980",
    "179 Hugo  Igonin 03 02 1992",
    "189 Alexey  Boronenko 06 11 1978" ) 


after sort (
    "236 Ivan  Evstegneev 20 09 1980",
    "179 Hugo  Igonin 03 02 1992",
    "189 Alexey  Boronenko 06 11 1978",
    "46 Alex  Dupel 25 10 1991" )

The same thing with firstname and lastname (even if use selectors:

NSSortDescriptor* descriptorLastname=[[NSSortDescriptor alloc]initWithKey:@"lastname" ascending:YES selector:@selector(caseInsensitiveCompare:)];

It's only work for property "index",which is NSInteger.

What I've done else, I created array only with dates from student.dateOfBirth and sorted it with keyname self. And it works! Another experiment - I created array of dictionaries with key "dateOfBirth" and sorted it by key name @"dateOfBirth" and it also works!

I know about other methods live sortUsingComparator, but why this method doesn't work on date and strings in properties??

---

Upd Oh, guys, it's my mistake. I appreciate all of your answers, they pushed me to place, where to search mistake. And when I found it, I felt very ashamed. Absolutly stupid mistake. If you are interested in, I used this way to initialize student:

- (instancetype)init
{
    self = [super init];
    if (self) {
        self.firstname=[self getFirstname];
        self.lastname=[self getLastname];
        self.dateOfBirth=[self getDateOfBirth];
    }
    return self;
}
-(NSString*) getFirstname {//some code...arrayOfFirstames...name=arc4random()...}

I.e. I occasionally redefine getter for setting property. All three. So, when I was sorting array everytime new values was taken. Thank you.

Answer 1

You might have wrong another thing, SortDescriptors work fine with strings, here an example:

NSArray *people = @[@{@"name": @"Ivan"},@{@"name": @"Hugo"},@{@"name": @"Alexey"},@{@"name": @"Alex"}];
NSSortDescriptor *sort = [NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES];
NSArray *sorts = @[sort];
NSArray *orderedArray = [people sortedArrayUsingDescriptors:sorts];
NSLog(@"Show orderedArray: %@",[orderedArray description]);

Exit in console:

2014-11-14 20:19:45.377 pro[1366:60b] Show orderedArray: (
    {
    name = Alex;
},
    {
    name = Alexey;
},
    {
    name = Hugo;
},
    {
    name = Ivan;
}
)

I think it´s good idea to log your array before and after the sortedArrayUsingDescriptor methods.

Hi again. I´ve done the same again, with a Custom Class this is the header:

#import <Foundation/Foundation.h>

@interface CustomClass : NSObject

@property (nonatomic, strong) NSString *name;

@end

The implementation (.m) is blank, and again I´ve done:

CustomClass *ivan = [[CustomClass alloc] init];
ivan.name = @"Ivan";

CustomClass *hugo = [[CustomClass alloc] init];
hugo.name = @"Hugo";

CustomClass *alexey = [[CustomClass alloc] init];
alexey.name = @"Alexey";

CustomClass *alex = [[CustomClass alloc] init];
alex.name = @"Alex";


NSArray *people = @[ivan,hugo,alexey,alex];
NSSortDescriptor *sort = [NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES];
NSArray *sorts = @[sort];
NSArray *orderedArray = [people sortedArrayUsingDescriptors:sorts];
 NSLog(@"Show orderedArray:");
for (CustomClass *people in orderedArray) {
    NSLog(@"%@\n",people.name);
}

And again, the exit console is:

 2014-11-14 22:38:13.297 pro[1406:147113] Show orderedArray:
 2014-11-14 22:38:13.298 pro[1406:147113] Alex
 2014-11-14 22:38:13.298 pro[1406:147113] Alexey
 2014-11-14 22:38:13.298 pro[1406:147113] Hugo
 2014-11-14 22:38:13.298 pro[1406:147113] Ivan

There isn´t any problem with sortDescriptors and property. Best