Achieve NSArray merging more elegantly with enumerateObjectsUsingBlock:? - objective-c

Question

My challenge this week has been to come to terms with blocks in objective-c. There is something about the syntax that does my head in. Getting there.
I have the following code to achieve a merge of two arrays in a specific way (see comment in code below).
NSArray *keys = #[#"name", #"age"];
NSArray *data = #[
#[#"mark", #"36 years"],
#[#"matt", #"35 years"],
#[#"zoe", #"7 years"]
];
// desired outcome is
// # { #"name" : #[#"mark", #"matt", #"zoe"],
// #"age" : #[#"36 years", #"35 years", #"7 years"]
// }
NSMutableArray *mergedData = [[NSMutableArray alloc] initWithCapacity:keys.count];
for (NSString *key in keys) {
NSLog(#"key: %#", key);
NSInteger keyIndex = [keys indexOfObject:key];
NSMutableArray *dataItemsForKey = [[NSMutableArray alloc] initWithCapacity:data.count];
for (NSArray *row in data) {
// double check the array count for row equals the expected count for keys - otherwise we have a 'match up' issue
if (row.count == keys.count) {
[dataItemsForKey addObject:[row objectAtIndex:keyIndex]];
}
}
[mergedData addObject:[NSDictionary dictionaryWithObject:dataItemsForKey forKey:key]];
}
NSLog (#"mergedData: %#", mergedData);
While this code works fine, in the interest of my challenge and learning, I was wondering if there is a more 'elegant' (aka less code, easier to read) way to do this using enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) ??
I can't quite see a way to make it work, but in the interests of self-education, wonder if those more learned in blocks and arrays may have a more elegant solution.

Answer 1

The first issue that I notice is that you are asking for the index of the current object while enumerating the array. This is a waste of operations, because at every loop iteration you have to look over all array elements (potentially O(N)) to find where the object is.
You could instead do this:
for(NSUInteger i=0; i<keys.count; i++)
{
NSString* key= keys[i];
<Rest of the code>
}
Or just keep track of the index manually incrementing it:
NSUInteger i=0;
for (NSString *key in keys)
{
<Your code>
i++;
}
Or like you wanted, with enumerateObjectsUsingBlock:, which is IMO the most elegant way to do it in this case. Here is an example:
NSMutableDictionary* dict=[NSMutableDictionary new];
[keys enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop)
{
NSMutableArray* fields=[NSMutableArray new];
for(NSArray* array in data)
{
[fields addObject: array[idx]];
}
[dict setObject: fields forKey: obj];
}];
In the case you haven't understood how it works, here is a further explanation:
This way at every execution of the block you can know which is the current object (obj) and it's index (idx). stop is just used to stop enumerating the array, but you don't need it in this case (say that you want to stop the enumeration, you set *stop=YES). In my code I just took every element at the index idx of data, and build an array which is the value that I put into the dictionary, that has obj (what you called key in your code) as key. For any further doubt feel free to ask any clarification through a comment.

Answer 2

The first thing to say is your code does not produce the desired output. You get an array with two dictionaries each with one key.
One way to solve the problem is like this:
NSMutableDictionary* mergedData = [[NSMutableDictionary alloc] init];
[keys enumerateObjectsUsingBlock: ^(id key, NSUInteger keyIndex, BOOL *stop)
{
NSMutableArray* keyValues = [[NSMutableArray alloc] init];
for (NSArray* row in data)
{
[keyValues addObject: [row objectAtIndex: keyIndex]];
}
[mergedData setObject: keyValues forKey: key];
}];
The above will throw an exception if a row doesn't have enough objects in it. You could either check it beforehand or allow the program to crash, it's up to you.