Add values in NSMutableDictionary in iOS with Objective-C

Question

I'm starting objective-c development and I would like to ask the best way to implement a list of keys and values.

In Delphi there is the class TDictionary and I use it like this:

myDictionary : TDictionary<string, Integer>;

bool found = myDictionary.TryGetValue(myWord, currentValue);
if (found)
{
    myDictionary.AddOrSetValue(myWord, currentValue+1);
} 
else
{
    myDictionary.Add(myWord,1);
}

How can I do it in objective-c? Is there equivalent functions to the above mentioned AddOrSetValue() or TryGetValue()?

Thank you.

Cavanaugh

Answer 1

You'd want to implement your example along these lines:

EDIT:

//NSMutableDictionary myDictionary = [[NSMutableDictionary alloc] init];
NSMutableDictionary *myDictionary = [[NSMutableDictionary alloc] init];

NSNumber *value = [myDictionary objectForKey:myWord];

if (value)
{
    NSNumber *nextValue = [NSNumber numberWithInt:[value intValue] + 1];
    [myDictionary setObject:nextValue  forKey:myWord];
} 
else
{
    [myDictionary setObject:[NSNumber numberWithInt:1] forKey:myWord]
}

(Note: you can't store ints or other primitives directly in a NSMutableDictionary, hence the need to wrap them in an NSNumber object, and make sure you call [myDictionary release] when you've finished with the dictionary).

Muscatel

Answer 2

The other answers are correct, but there is more modern syntax for this now. Rather than:

[myDictionary setObject:nextValue  forKey:myWord];

You can simply say:

myDictionary[myWord] = nextValue;

Similarly, to get a value, you can use myDictionary[key] to get the value (or nil).

Sachi

The simplified form mentioned above, usually solves problems OCLint (metric static code analysis). Thanks @jesse-rusak :) – Godson

Answer 3

Yep:

- (id)objectForKey:(id)key;
- (void)setObject:(id)object forKey:(id)key;

setObject:forKey: overwrites any existing object with the same key; objectForKey: returns nil if the object doesn't exist.

Edit:

Example:

- (void)doStuff {
  NSMutableDictionary *dict = [NSMutableDictionary dictionary];

  [dict setObject:@"Foo" forKey:@"Key_1"]; // adds @"Foo"
  [dict setObject:@"Bar" forKey:@"Key_2"]; // adds @"Bar"

  [dict setObject:@"Qux" forKey:@"Key_2"]; // overwrites @"Bar"!

  NSString *aString = [dict objectForKey:@"Key_1"]; // @"Foo"
  NSString *anotherString = [dict objectForKey:@"Key_2"]; // @"Qux"
  NSString *yas = [dict objectForKey:@"Key_3"]; // nil
}

Reedit: For the specific example there exists a more compact approach:

[dict
  setObject:
    [NSNumber numberWithInteger:([[dict objectForKey:@"key"] integerValue] + 1)]
  forKey:
    @"key"
 ];

Crazy indentation for readability.

Augean

Hi are you able to append say @"Qux" onto @"Bar" forKey:@"Key_2"? How would you do this? – Rolfe

@GerardGrundy: That's really a completely different question. But check the docs for stringByAppendingString: – Augean