Extract value from json string

Question

I am trying to learn how to use API's with Objective-C. I am using the data from here: https://btc-e.com/api/2/ltc_usd/ticker and I only want the 'last' value. I have tried extracting the value like this:

NSURL * url=[NSURL URLWithString:@"https://btc-e.com/api/2/ltc_usd/ticker"];
NSData * data=[NSData dataWithContentsOfURL:url];
NSError * error;

NSMutableDictionary  * json = [NSJSONSerialization JSONObjectWithData:data options: NSJSONReadingMutableContainers error: &error];
NSArray *keys = [json allKeys];
NSString *jsonStr = [json objectForKey:keys[0]];

NSArray *c1 = [jsonStr componentsSeparatedByString:@"last = \""];
NSArray *c2 = [[c1 objectAtIndex:1] componentsSeparatedByString:@"\";"];
NSString *result = [c2 objectAtIndex:0];

NSLog(@"%@", result);

However, this gives me the following errors:

2014-03-02 15:03:24.915 Litecoin Ticker[5727:303] -[__NSDictionaryM componentsSeparatedByString:]: unrecognized selector sent to instance 0x608000240690
2014-03-02 15:03:24.915 Litecoin Ticker[5727:303] -[__NSDictionaryM componentsSeparatedByString:]: unrecognized selector sent to instance 0x608000240690

I'm not entirely sure this is only way to extract values from API's, but i can't seem to find out how to do it. Can anyone help?

Go to json.org and learn the JSON syntax. It takes 5-10 minutes and knowing it makes everything easier to understand. — Hot Licks
– Hot Licks, Commented Mar 2, 2014 at 15:19
And there is no need to use componentsSeparatedByString. Just reference the NSDictionarys using objectForKey (or the new form using []). NSNumber* last = json[@"ticker"][@"last"]; — Hot Licks
– Hot Licks, Commented Mar 2, 2014 at 15:22
(And, especially when taking data off the web, always check the value back from NSJSONSerialization and, if nil, at least NSLog the error value.) — Hot Licks
– Hot Licks, Commented Mar 2, 2014 at 15:23

Gabriele Petronella · Accepted Answer · 2014-03-02 15:21:27Z

1

NSString *jsonStr = [json objectForKey:keys[0]];
^^^^^^^^^^^^^^^^^
// nope, it's a NSDictionary...

...and it has already been parsed!

And if you NSLog it you will see it's content. Here's how you access the last field after you have the JSON parsed by NSJSONSerialization:

NSMutableDictionary *json = [NSJSONSerialization JSONObjectWithData:data options: NSJSONReadingMutableContainers error: &error];
NSNumber *last = json[@"ticker"][@"last"];

And that's it.

As a side note

NSData * data = [NSData dataWithContentsOfURL:url];

is awful, since it's synchronous! Consider using an asynchronous approach (the nuclear - and probably best - option is to use AFNetworking). Here's a complete AFNetworking example:

AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
[manager GET:@"https://btc-e.com/api/2/ltc_usd/ticker" parameters:nil success:^(AFHTTPRequestOperation *operation, id JSON) {
    NSNumber *last = JSON[@"ticker"][@"last"];
    NSLog(@"last value: %@", last);
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
    NSLog(@"Error: %@", error);
}];

edited Mar 2, 2014 at 15:21

answered Mar 2, 2014 at 15:16

Gabriele Petronella

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6 Comments

user2397282 Over a year ago

Wow, I can't believe I was overcomplicating it that much! What does the AFNetworking part do?

Gabriele Petronella Over a year ago

It's a networking framework. It basically wraps the Objective-C networking API into a nicer block-based one.

Gabriele Petronella Over a year ago

@user2397282 w.r.t. your specific case it makes the call asynchronous, so that it doesn't block the thread you're performing it on (if it's the main thread you're going to block the UI, until the request is performed!)

Hot Licks Over a year ago

Note that the return value for last will be an NSNumber, not an NSString.

user2397282 Over a year ago

@GabrielePetronella I have tried using your asynchronous approach, but it gives me an error when I run, basically saying that the data in the link is in the wrong format: text/html, not json?

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