Question

I have several arrays that need to be sorted side by side.

For example, the first array has names: @[@"Joe", @"Anna", @"Michael", @"Kim"], and
and the other array holds addresses: @[@"Hollywood bld", @"Some street 3", @"That other street", @"country road"], where the arrays’ indexes go together. “Joe” lives at “Hollywood bld” and so on.

I would like to sort the names array alphabetically, and then have the address array sorted alongside so they still go together, with “Hollywood bld” having same index as “Joe”. I know how to sort one array alphabetical with

NSSortDescriptor *sort=[NSSortDescriptor sortDescriptorWithKey:@"name" ascending:NO];
[myArray sortUsingDescriptors:[NSArray arrayWithObject:sort]];

But is there any easy way of getting the second array sorted using the appropriate order?

Answer 1

1. Create a permutation array, initially set to p[i]=i
2. Sort the permutation according to the name key of the first array
3. Use the permutation to re-order both arrays

Example: let’s say the first array is {"quick", "brown", "fox"}. The permutation starts as {0, 1, 2}, and becomes {1, 2, 0} after the sort. Now you can go through the permutation array, and re-order the original array and the second array as needed.

NSArray *first = [NSArray arrayWithObjects: @"quick", @"brown", @"fox", @"jumps", nil];
NSArray *second = [NSArray arrayWithObjects: @"jack", @"loves", @"my", @"sphinx", nil];
NSMutableArray *p = [NSMutableArray arrayWithCapacity:first.count];
for (NSUInteger i = 0 ; i != first.count ; i++) {
    [p addObject:[NSNumber numberWithInteger:i]];
}
[p sortWithOptions:0 usingComparator:^NSComparisonResult(id obj1, id obj2) {
    // Modify this to use [first objectAtIndex:[obj1 intValue]].name property
    NSString *lhs = [first objectAtIndex:[obj1 intValue]];
    // Same goes for the next line: use the name
    NSString *rhs = [first objectAtIndex:[obj2 intValue]];
    return [lhs compare:rhs];
}];
NSMutableArray *sortedFirst = [NSMutableArray arrayWithCapacity:first.count];
NSMutableArray *sortedSecond = [NSMutableArray arrayWithCapacity:first.count];
[p enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
    NSUInteger pos = [obj intValue];
    [sortedFirst addObject:[first objectAtIndex:pos]];
    [sortedSecond addObject:[second objectAtIndex:pos]];
}];
NSLog(@"%@", sortedFirst);
NSLog(@"%@", sortedSecond);