What means __NSSingleObjectArrayI in Objective C? when I try to read a Json

Question

I have this code Json format:

{
    "weather": [{
        "description": "clear sky",
        "icon": "01n"
    }],
    "base": "stations",
    "main": {
        "temp": 285.514
    },
    "clouds": {
        "all": 0
    },
    "dt": 1485792967,
    "id": 1907296
}

And I want to retrive icon string (01n)

And use this code:

@property(nonatomic,strong) NSString *cityImageName;

self.cityImageName = [[dataJson valueForKey:@"weather"] valueForKey:@"icon"];

And later when I check the variable print:

<__NSSingleObjectArrayI 0x604000009890>(
01n
)

Finally how can I get the string directly? not like a __NSSingleObjectArrayI

Answer 1

You got caught in the Key-Value Coding Trap.

valueForKey has a special behavior. Applied to an array it returns always an array of all values for the given key.

Never use valueForKey unless you intend to use KVC. The recommended syntax is objectForKey or – preferable – key subscription and in case of the array index subscription.

In this case you want to get the value for key icon of the first item in the array for key weather.

self.cityImageName = dataJson[@"weather"][0][@"icon"];

---

However I would add a check if the array is not empty to avoid an out-of-range exception

NSArray *weather = dataJson[@"weather"];
if (weather && weather.count > 0) {
    self.cityImageName = weather[0][@"icon"];
}

Answer 2

__NSSingleObjectArrayI is one of the implementations of the NSArray class cluster. It's not really important to this question other than knowing that it's an array.

The reason you're getting an array (of one element) instead of a string is because the JSON you're working with contains an array with one dictionary inside of it:

"weather": [{
    "description": "clear sky",
    "icon": "01n"
}],

Note: the square brackets surrounding the curly brackets.

So, when you call [dataJson valueForKey:@"weather"] you get back the object that represent this part of the JSON:

[ { "description": "clear sky", "icon": "01n" } ]

Which is in this case has been decoded as an NSArray containing one NSDictionary with two keys.

When you then call valueForKey: on that array it

Returns an array containing the results of invoking valueForKey: using key on each of the array's objects.

In other words, because [dataJson valueForKey:@"weather"] is an array of one dictionary, [[dataJson valueForKey:@"weather"] valueForKey:@"icon"] is an array of only the value for the "icon" key in that dictionary.

If the JSON you're working with always has this format, then you can get the firstObject from the array to get a hold of the first string (or nil if the array was empty).