Reverse an Array of String in Objective-C

Question

I would like to reverse an array of string in Objective-C.

I know there is a in built-method of doing it as follows:

NSArray* reversedArray = [[inputString reverseObjectEnumerator] allObjects];

but I want to implement same functionality with coding as follows and getting the following error.

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {

    NSMutableArray *inputString = [NSMutableArray arrayWithObjects:@"a", @"b", @"c", @"d", @"e", @"f", @"g", @"h", @"i", nil];

        int lenArray= (int)[inputString count];

        NSMutableArray *reverseString=[[NSMutableArray alloc]initWithCapacity:lenArray];

        for(int i=lenArray-1;i>0;i--)
        {
            [reverseString insertObject:inputString[i] atIndex:i];
        }
      return 0;
    }
  }

Terminating app due to uncaught exception 'NSRangeException', reason: ' -[__NSArrayM insertObject:atIndex:]: index 8 beyond bounds for empty array'

Have you tried to init your NSMutableArray without a capacity? It is not required and that may assist. — andrewbuilder
– andrewbuilder, Commented Apr 15, 2015 at 20:42
Why are you "insertObject" on reverseString? You're already going through the list backwards, you should just be addObject. — Travis M.
– Travis M., Commented Apr 15, 2015 at 20:43

mah · Accepted Answer · 2015-04-15 20:42:37Z

3

[reverseString insertObject:inputString[i] atIndex:i]; is the problem - specifically the atIndex:i part. You've created your mutable array with a large enough capacity, however it is still empty so specifying the index here is not what you meant to do.

Instead, just use addObject: [reverseString addObject:inputString[i]];

answered Apr 15, 2015 at 20:42

mah

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Johannes Fahrenkrug · Accepted Answer · 2015-04-15 20:42:29Z

2

Try this:

for (int i = lenArray-1; i >= 0; i--) {
    [reverseString addObject:inputString[i]];
}

Note: Just use addObject: and change your for-loop condition to i >= 0, otherwise the "a" will be missing.

Enjoy!

answered Apr 15, 2015 at 20:42

Johannes Fahrenkrug

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2 Comments

casillas Over a year ago

Hi Jonahnnes, system does not allow me add another question. Sorry for bothering. I wanted to print the reverseString and typed the following code` for(int i=0;i<=lenArray-1;i++){printf("%s\n",[reverseString objectAtIndex:i]);}return 0;}}` but it prints the weird characters.

Johannes Fahrenkrug Over a year ago

No problem. You are dealing with NSStrings. These are different from C strings. So you need to use different methods to print the string. Try either NSLog(@"%@", reverseString); or (in your for-loop) NSLog(@"%@\n", [reverseString objectAtIndex:i]);

casillas · Accepted Answer · 2015-04-15 21:13:43Z

Here is the complete solution:

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {

        NSMutableArray *inputString = [NSMutableArray arrayWithObjects:@"a", @"b", @"c", @"d", @"e", @"f", @"g", @"h", @"i", nil];
      //  NSArray* reversedArray = [[inputString reverseObjectEnumerator] allObjects];

            int lenArray= (int)[inputString count];

            NSMutableArray *reverseString=[[NSMutableArray alloc]initWithCapacity:lenArray];

                for (int i = lenArray-1; i >= 0; i--) {
                    [reverseString addObject:inputString[i]];
                }

               for(int i=0;i<=lenArray-1;i++)
                {
                    NSLog(@"%@\n", [reverseString objectAtIndex:i]);
                }
    return 0;
  }
}

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