无法对自定义对象数组进行排序

Question

So I have an array of custom "Element" objects (hey hold atomic number, chemical symbol, atomic mass, etc...) and I am having trouble sorting them by one of their properties;

Here is the code:

switch (sortDescriptor) {

        case 0: {
            //Sort the array by "ATOMIC NUMBER"
            NSArray *sortedArray = [self.elementsArray sortedArrayUsingComparator:^(id a, id b) {
                NSNumber *first = @([(SAMElement *)a atomicNumber]);
                NSNumber *second = @([(SAMElement *)b atomicNumber]);
                return [first compare:second];
            }];
            self.elementsArray = [sortedArray mutableCopy];
        }

        case 1: {
            //Sort the array by "ELEMENT NAME"
            NSArray *sortedArray = [self.elementsArray sortedArrayUsingComparator:^(id a, id b) {
                NSString *first = [(SAMElement *)a elementName];
                NSString *second = [(SAMElement *)b elementName];
                return [first compare:second];
            }];
            self.elementsArray = [sortedArray mutableCopy];
        }
        case 2:{
            NSLog(@"sorting by chemical symbol");
            //Sort the array by "CHEMICAL SYMBOL"
            NSArray *sortedArray = [self.elementsArray sortedArrayUsingComparator:^(id a, id b) {
                NSString *first = [(SAMElement *)a  chemichalSymbol];
                NSString *second = [(SAMElement *)b chemichalSymbol];
                return [first compare:second];
            }];
            self.elementsArray = [sortedArray mutableCopy];
        }

        case 3: {
            //Sort the array by "ATOMIC MASS"
            NSArray *sortedArray = [self.elementsArray sortedArrayUsingComparator:^(id a, id b) {
                NSNumber *first = [(SAMElement *)a atomicMass];
                NSNumber *second = [(SAMElement *)b atomicMass];
                return [first compare:second];
            }];
            self.elementsArray = [sortedArray mutableCopy];
        }
        default:
            break;
    }

When is sorts it returns a totally random list of elements. Am i doing something wrong?

Answer 1

The best way to sort an array of objects by some property of the object, its using NSSortDescriptor. In initWithKey , you can set the name of the property that you want to sort.

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"atomicNumber" ascending:NO];   
[self.elementsArray sortUsingDescriptors:@[sortDescriptor]];

In your case, just copy this code above in each case section of your switch statement, changing the key for @"elementName" and @"chemichalSymbol" .

You can change the ascending value from NO to YES , depending what type of order do you want.

Please, let me know if worked or not.

Answer 2

I'm not seeing the bug immediately, but you're reinventing the wheel here. The correct tool for this is sortedArrayUsingDescriptors:

[self.elementsArray sortedArrayUsingDescriptors:@[ 
                 [NSSortDescriptor alloc] initWithKey:@"atomicNumber"] ascending:YES]
                                                  ]];

Try that and see if it gets rid of your bug. If you're getting random orders, that usually suggests that your comparitor is inconsistent (sometimes A>B and sometimes B>A for the same A&B).