NSMutableArray of NSMutableDictionary return always nil - ios

Question

I have a NSMutableArray of NSMutableDictionary but when i access the values of the array they are always nil. Here is the code i'm using
NSMutableDictionary *tmpDictionary = [[NSMutableDictionary alloc] init];
self.arrayOfData = [[NSMutableArray alloc] init];
[tmpDictionary setObject:#aaa forKey:Key1];
[tmpDictionary setObject:#1 forKey:Key2];
[tmpDictionary setObject:#1 forKey:Key3];
[tmpDictionary setObject:#10.0f forKey:Key4];
[tmpDictionary setObject:#10.0f forKey:Key5];
[self.arrayOfData addObject:tmpDictionary];
[tmpDictionary removeAllObjects];
[tmpDictionary setObject:#bbb forKey:Key1];
[tmpDictionary setObject:#1 forKey:Key2];
[tmpDictionary setObject:#2 forKey:Key3];
[tmpDictionary setObject:#50.0f forKey:Key4];
[tmpDictionary setObject:#50.0f forKey:Key5];
[self.arrayOfData addObject:tmpDictionary];
[tmpDictionary removeAllObjects];
And i access the Array in this way
for (int i = 0; i < self.arrayOfData.count; ++i) {
NSLog(#"%#", [self.arrayOfData[i] objectForKey:Key3]);
}
But the log always returns nil.
What's my mistake?
Thanks in advance

Answer 1

There is only one dictionary, and it is mutable. When [tmpDictionary removeAllObjects]; gets called, all the objects are removed, so it is empty afterwards. To fix this, create multiple mutable dictionaries, and don't empty them.

Answer 2

The issue is due to reusing tmpArray over and over and removing all of the objects from the array. That won't work. Do it this way:
self.arrayOfData = [[NSMutableArray alloc] init];
NSMutableDictionary *tmpDictionary = [[NSMutableDictionary alloc] init];
[tmpDictionary setObject:#aaa forKey:Key1];
[tmpDictionary setObject:#1 forKey:Key2];
[tmpDictionary setObject:#1 forKey:Key3];
[tmpDictionary setObject:#10.0f forKey:Key4];
[tmpDictionary setObject:#10.0f forKey:Key5];
[self.arrayOfData addObject:tmpDictionary];
tmpDictionary = [[NSMutableDictionary alloc] init];
[tmpDictionary setObject:#bbb forKey:Key1];
[tmpDictionary setObject:#1 forKey:Key2];
[tmpDictionary setObject:#2 forKey:Key3];
[tmpDictionary setObject:#50.0f forKey:Key4];
[tmpDictionary setObject:#50.0f forKey:Key5];
[self.arrayOfData addObject:tmpDictionary];
Even better would be use modern syntax:
self.arrayOfData = [[NSMutableArray alloc] init];
NSDictionary *tmpDictionary = #{
Key1 : #aaa",
Key2 : #1,
Key3 : #1,
Key4 : #10.0f,
Key5 : #10.0f
};
[self.arrayOfData addObject:tmpDictionary];
tmpDictionary = #{
Key1 : #bbb",
Key2 : #1,
Key3 : #2,
Key4 : #50.0f,
Key5 : #50.0f
};
[self.arrayOfData addObject:tmpDictionary];

Answer 3

#rmaddy is right.
Just as small addition.
You probably think that -addObject: method copies dictionary object to array (creates a copy of object). It doesn't
.
It just stores reference to the tmpDictionary in dictionary. You basically save pointer to the memory part which has your dictionary.
What you do in your code:
Create a NSMutableDictionary instance
Fill this instance with some data
Add pointer to this instance to the array
Remove oll objects from dictionary (the one in the array will be empty at this point)
Create another instance of NSMutableDictionary
Repeat 2-4
To illustrate that - put this NSLog after adding dictionary first time to the array. This NSLog will print memory addresses of the tmpDictionary and array item with index 0. You'll see that it is exactly same object
NSLog(#"tmpDictionary: %p, Array Item 0: %p", tmpDictionary, self.arrayOfData[0]);
Hope I explained it well...