Question

I have a method which is passed a dict (whose values themselves consist of nested dicts), and I am using recursion to traverse this dict, and I want to store only the information I need in another dict which is also passed to the method as the second argument.

So, the method (ignoring some optional arguments that’s needed to aid recursion) looks like:

-(NSMutableDictionary*) extractData: (NSDictionary*)complicatedDict intoSimpleDict:(NSMutableDictionary*)simpleDict {
    // do recursive traversal of complicatedDict and return simpleDict
}

I am facing two problems:

a. I need to pass an empty dict initially when I first call the method. How do I initialize an empty NSMutableDictionary to pass as the simpleDict parameter?

b. I am trying to group values having common key in the simpleDict. In JS (which is my most familiar language), I will implement it like:

if( !simpleDict.hasOwnProperty('some_key') ) {
    simpleDict['some_key'] = [];
}
simpleDict['some_key'].push("value");

The above code would initially an array in each index of the dict, and push the value to it. How do I do this in objective-c?

Answer 1

a. Create an empty mutable dictionary simply like this

NSMutableDictionary *empty = [NSMutableDictionary dictionary];

b. This is equivalent to your JS example

if(![[simpleDict allKeys] containsObject:@"some_key"])
    [simpleDict setObject:[NSMutableArray array] forKey:@"some_key"];
[[simpleDict objectForKey:@"some_key"] addObject:@"value"];