JSON request error [closed] - objective-c

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I'm experimenting with JSON for the first time in Objective-C.
Here's the code for my Terminal Application:
#import <Foundation/Foundation.h>
int main(int argc, const char * argv[])
{
#autoreleasepool
{
NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:#"https://itunes.apple.com/search?term=harry&country=us"]];
NSData *response = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];
NSError *jsonParsingError = nil;
NSArray *publicTimeline = [NSJSONSerialization JSONObjectWithData:response options:0 error:&jsonParsingError];
NSDictionary *list;
for(int i=0; i<[publicTimeline count];i++)
{
list = [publicTimeline objectAtIndex:i];
NSLog(#"Statuses: %#", [list objectForKey:#"trackName"]);
}
}
return 0;
}
Here's the error I'm getting:
2013-09-11 20:58:55.524 Tweets[7291:303] -[__NSCFDictionary objectAtIndex:]: unrecognized selector sent to instance 0x100391d60
2013-09-11 20:58:55.526 Tweets[7291:303] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFDictionary objectAtIndex:]: unrecognized selector sent to instance 0x100391d60'
*** First throw call stack:
(
0 CoreFoundation 0x00007fff91688b06 __exceptionPreprocess + 198
1 libobjc.A.dylib 0x00007fff8bf8a3f0 objc_exception_throw + 43
2 CoreFoundation 0x00007fff9171f40a -[NSObject(NSObject) doesNotRecognizeSelector:] + 186
3 CoreFoundation 0x00007fff9167702e ___forwarding___ + 414
4 CoreFoundation 0x00007fff91676e18 _CF_forwarding_prep_0 + 232
5 Tweets 0x0000000100000d1a main + 378
6 libdyld.dylib 0x00007fff908e67e1 start + 0
)
libc++abi.dylib: terminate called throwing an exception
What's wrong? I have followed a tutorial perfectly but I am the only one getting the error.

Answer 1

I ran your code. My results didn't agree with yours. I didn't crash, but I did receive (null) from the final NSLog call.
Yes, you should be checking the result of +[NSJSONSerialization JSONObjectWithData:options:error]. As #HotLicks has pointed out to you quite rightfully, if the result you get is nil, examining the error object can reveal important clues as to what's gone wrong.
But in this case, it's academic because the result (again, in my case) wasn't nil. The real problem, however, is that you don't appear to have an understanding of your data's schema. jsonObject is a dictionary, but it has only two keys, results and resultCount, and that's the reason you're getting nil from [jsonObject objectForKey:#"trackName"]; trackName isn't one of the keys.
Those two keys that you do get, however, should tell you plenty. It told me enough to try this:
NSArray *results = [jsonObject objectForKey:#"results"];
NSDictionary *dictionary = [results objectAtIndex:0];
id latestLoans = [dictionary objectForKey:#"trackName"];
and got a result:
Harry Potter and the Deathly Hallows - Part 1
In other words, I simply got the first dictionary in the array for results, and asked it for the object it had for trackName.
Since you were originally asking for an array, I'm guessing that you wanted all the values for that particular key, so if you use this:
NSArray *latestLoans = [jsonObject valueForKeyPath:#"results.trackName"];
it provides a list of 50 values corresponding to the trackName key in the dictionaries under the results key.
EDIT:
One other thing you really need to do is to protect yourself against the possibility of nil being returned from your call to -[NSData initWithContentsOfURL:] by checking for it before calling anything else. This alone won't fix your inability to get a response, but it will keep you from crashing. And if you do receive nil, be sure to check the domain and code properties of the error object you get back to get an idea of what's going wrong.