Populate a mutableArray with objects from dictionaries in a dictionary

Question

I am having trouble populating an array of objects (employee names) from a group of NSDictionaries (each one representing an individual employee) which are inside another NSDictionary *dictionary (personnel). The key for each sub Dictionary is an employee ID.

I might well be coming at this from a completely wrong direction, i am very much a newbie but from what i have read it should be something like this , i have managed to get as far as a pName (employee name) from each sub Dictionary, all be it fleetingly. I just can't get the pNames added into MutableArray *names.

any help or guidance would be greatly appreciated ...

  int i, count;
  id key, value;

  keys = [dictionary allKeys];
  count = [keys count];
  for (i = 0; i < count; i++)
  {
   key = [keys objectAtIndex: i];
   value = [dictionary objectForKey: key];
   NSLog (@"value:%@", value); 

   NSString *pName = [value objectForKey:@"personName"];
   NSLog (@"pName:%@", pName);// the debugger shows the correct pName with each loop

   [names addObject:pName];// this is wrong and i don't know why
  }
   NSLog (@"names:%@", names);// in the debugger names (null)

Welcome to Stack Overflow. In future please take the time to format the code in your question appropriately (using the "{}" editor control). — John Parker
– John Parker, Commented Jan 23, 2011 at 11:38
Any chance you could update your question with a slightly more fleshed out code sample? (One that shows how the names NSMutableArray is initialised, etc.) — John Parker
– John Parker, Commented Jan 23, 2011 at 11:41

outis · Accepted Answer · 2012-07-20 02:09:10Z

You can also do this using collection operators, specifically the @unionOfObjects operator:

NSMutableArray *names = [[[personnel allValues] 
                            valueForKeyPath:@"@unionOfObjects.personName"] 
                            mutableCopy];

Note that names will be owned by the method (its retain count is 1), but the interim arrays ([personnel allValues] and the one created by valueForKeyPath:) are autoreleased and not owned by the method.

For some reason (possibly because key names beginning with "@" are special in valueForKey:, and thus valueForKeyPath: wouldn't be equivalent to a sequence of calls to valueForKey:), NSDictionary doesn't have a valueForKeyPath: method, so you first need to convert it to an array using allValues. Alternatively, you could implement valueForKeyPath: in a category, perhaps starting from GnuStep's or cocotron's (which doesn't actually support @unionOfObjects, but allows for easy extension by defining a _kvo_operator_unionOfObjects: method).

grahamparks · Accepted Answer · 2011-01-23 11:47:19Z

1

It looks like you aren't creating an array object to put your names in. There needs to be code like this:

names = [NSMutableArray array];

... before the start of the loop. That creates an empty mutable array.

answered Jan 23, 2011 at 11:47

grahamparks

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2 Comments

barneyb Over a year ago

thank you very much .. i appreciate you solving such an elementary issue.

John Parker Over a year ago

+1 Damn. There was me thinking he'd simply omitted that part of the code sample. :-)

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