Question

I have a NSArray made out of numbers 1..50, which represents a table with columns & rows.

I need to reverse only the order of the columns, while keeping the order of the rows.
So for example:

0,1,2,3,4,5,6
7,8,9,9,10,11,12

has to be

6,5,4,3,2,1,0
12,11,10,9,8,7

Right now, i use a huge IF statement for that:

for (dd *d in dates[i]) {
  if (tileNum==0) {
    reversedTileNum = 6;
  } else if (tileNum==1) {
    reversedTileNum = 5;
  }else if (tileNum==2) {
    reversedTileNum = 4;
  }else if (tileNum==3) {
    reversedTileNum = 3;
  }else if (tileNum==4) {
    reversedTileNum = 2;
  }else if (tileNum==5) {
    reversedTileNum = 1;
  } else if (tileNum==6) {
    reversedTileNum = 0;
  }
  ....
  .... 
}

Answer 1

Here’s a solution that should be easy to drop into any project. It involves two categories: one on NSMutableArray that provides a method to swap objects at two indices, and one on NSArray that provides the -arrayByReversingGroups: method. The idea is to swap the elements in pairs within a group, reversing the group. If number of elements in the array isn’t an even multiple of groupSize, the extras at the end are left untouched.

The code presented here is a complete program, so you can see an example of using -arrayByReversingGroups: in the main() function.

#import <Foundation/Foundation.h>

@interface NSArray(Reversible)

-(NSArray*)arrayByReversingGroups:(int)groupSize;

@end

@interface NSMutableArray(Swappable)

-(void)swapObjectAtIndex:(int)first withObjectAtIndex:(int)second;

@end

@implementation NSArray(Reversible)

-(NSArray*)arrayByReversingGroups:(int)groupSize
{
    NSMutableArray *newArray = [self mutableCopy];
    // Iterate over the array in chunks of groupSize elements. i will be first index in
    // the current chunk.
    for (int i = 0; (i + groupSize) < [newArray count]; i += groupSize) {
        // Iterate over the items in the current chunk, swapping the bth and
        // (groupsize-b-1)th elements until they meet at groupsize/2.
        for (int b = 0; b <= (groupSize / 2); b++) {
            int first = i + b;
            int second = i + groupSize - b - 1;
            [newArray swapObjectAtIndex:first withObjectAtIndex:second];
        }
    }
    return [newArray copy];
}

@end

@implementation NSMutableArray(Swappable)
-(void)swapObjectAtIndex:(int)first withObjectAtIndex:(int)second
{
    id temp = [[self objectAtIndex:second] retain];
    [self replaceObjectAtIndex:second withObject:[self objectAtIndex:first]];
    [self replaceObjectAtIndex:first withObject:temp];
    [temp release];
}

@end

int main (int argc, const char * argv[])
{
    @autoreleasepool {
        NSArray *array = [NSArray arrayWithObjects:
                          @"1", @"2", @"3", @"4", @"5", @"6", @"7", @"8", @"9", @"10", @"11", @"12", nil];
        NSLog(@"Original: %@", array);
        NSLog(@"Reversed: %@", [array arrayByReversingGroups:5]);
    }
    return 0;
}