How can i use [NSMutableDictionary dictionaryWithDictionary] if the dictionary is empty?

Question

I get some data from the server and save it to NSDictionary then I want to initialise NSMutableDictionary with that NSDictionary.

NSDictionary  * const received = [self serverData];
NSMutableDictionary * const results = [NSMutableDictionary dictionaryWithDictionary:received];

And If I get empty JSON from the server and consequently my NSDictionary will be empty how can I use dictionaryWithDictionary for an empty one? I use this

NSDictionary * const received = [self serverData];
NSMutableDictionary * const results;
if (![received count]) {
    results = [NSMutableDictionary dictionary];
}
else
{
    results = [NSMutableDictionary dictionaryWithDictionary:received];
}

But it seems a code smell. May be there are more elegant solutions?

do you get a nsdictionary object with no key-vale pair or nil? — Abid Hussain, Dec 05 '12 at 15:38
I think that the problem is that when there is no data your dictionary is `nil` and you can't initialize a `NSMutableDictionary` from a `nil` object — tkanzakic, Dec 05 '12 at 15:38
If you use dictionarywithDictionary: and pass in nil or an empty dictionary, you get an empty dictionary. What are you expecting it to do? — James P, Dec 05 '12 at 15:39
if you recieve nil then you don't need to use dictionarywithdictionary. you just need to do results = [recieved mutablecopy]; if nil then it'll be ignored — Abid Hussain, Dec 05 '12 at 15:39
@Joe Why do you need to check if received is nil, won't it return you a dictionary either way? — James P, Dec 05 '12 at 15:46
That's what I'm not getting, surely `NSMutableDictionary* results = [NSMutableDictionary dictionaryWithDictionary:received];` is all that's needed. Unless i'm missing something. — James P, Dec 05 '12 at 15:50
@AbidHussain Can you please explain me a difference between these two states "object with no key-vale pair or nil"? — , Dec 05 '12 at 15:50
@JamesP Yes serverdata returns NSDictionary but further I need to use NSMutableDictionary — , Dec 05 '12 at 15:51
It will create you an empty NSMutableDictionary with which you can do what you want. — James P, Dec 05 '12 at 15:54

score 5 · Answer 1 · answered Dec 05 '12 at 16:02

You were fine in the first place.

NSDictionary* received = [self serverData];
NSMutableDictionary* results = [NSMutableDictionary dictionaryWithDictionary:received];

Will do what you want. Even if received is nil or empty, results will just be an empty NSMutableDictionary.

phoganuci · Accepted Answer · 2012-12-05T15:50:21.283

1

I would make a category on NSDictionary that provides for such a safe setting.

+ (NSMutableDictionary *)safeDictionaryWithDictionary:(NSDictionary *)dictionary
{
    return [NSMutableDictionary dictionaryWithDictionary:(dictionary ? dictionary : @{});
}

edited Dec 05 '12 at 15:50

answered Dec 05 '12 at 15:43

phoganuci

4,984
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What about overriding the existing method with category? – Anoop Vaidya Dec 05 '12 at 15:44
Categories do not have the ability to be polymorphed. If polymorphism is your desire create a new class with this as a function. – phoganuci Dec 05 '12 at 15:45
It can be, but has some limitations... :) – Anoop Vaidya Dec 05 '12 at 15:47
Could you elaborate, I'm interested. – phoganuci Dec 05 '12 at 15:48
http://stackoverflow.com/questions/1085479/override-a-method-via-objc-category-and-call-the-default-implementation – Anoop Vaidya Dec 05 '12 at 15:48
Thanks for the link. I love runtime magic, I'm sure it will be a fun read. – phoganuci Dec 05 '12 at 15:50
9

There is no point in doing this, just calling dictionaryWithDictionary will give you the same result. – James P Dec 10 '12 at 12:18

score 0 · Answer 3 · answered Dec 05 '12 at 15:39

0

You cant create an item in dictionary with "no key". As key must be a non-nil string.

If no items are there then no object and no key are set, this leads to setObject:nil forKey:nil

answered Dec 05 '12 at 15:39

Anoop Vaidya

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How can i use [NSMutableDictionary dictionaryWithDictionary] if the dictionary is empty?

3 Answers3