Question

I have an NSArray containing n elements at indices 0, 1 ... n-1. I want to populate an NSDictionary with the contents of my array.

Specifically the dictionary should contain key-value pairs where the key is the hash of the ith element in the array and the value is the index into the array.

For example: array = [123, 101, 199] then the dictionary will contain three key-value pairs:

• ([123 hash], 0)
• ([199 hash], 2)
• ([101 hash], 1)

I’ve done this with a for loop over the array. What’s a more concise way to do this? Perhaps something from NSKeyValueCoding?

More info: I’m thinking of something like this:

NSArray *keys = [myArray valueForKey:@"hash"];
NSArray *values = [myArray valueForKey:@"index"]; // @"index" needs to change
NSDictionary *dictionary = [NSDictionary dictionaryWithObjects:values 
                                                       forKeys:keys];

Answer 1

I would probably use something like David’s for loop solution myself, but just for kicks, and because I’m still trying to wrap my head completely around them, I came up with a solution using blocks:

NSMutableDictionary *dict = [NSMutableDictionary dictionaryWithCapacity:[array count]];

[array enumerateObjectsUsingBlock:^ (id obj, NSUInteger idx, BOOL *stop) {
    [dict setObject:[NSNumber numberWithUnsignedLong:idx] forKey:hashFor(obj)]; } ];

I’m assuming the existence of a function hashFor that generates the hash. You can replace that part with a message to obj or whatever you do to generate the hash.