objective c - iOS How to check if valueForKey exist or not? -


i have question checking existence of valueforkey. here's code:

id _jsonid = [nsjsonserialization jsonobjectwithdata:[_json datausingencoding:nsutf8stringencoding] options:0 error:&_error]; monday = [[nsmutablearray alloc] init]; if ([_jsonid valueforkey:@"mon"]!= nil) {     _keymon = [_jsonid valueforkey:@"mon"];     [monday addobject:@"monday"];     (int i=0; i<5; i++) {         if ([_keymon objectatindex:i]!=[nsnull null]) {              [monday addobject:[_keymon objectatindex:i]];         }     }  } else {     [_keytues addobject:@"no class today"];     [monday addobject:[_keymon objectatindex:1]]; }

the idea is, if _jsonid valueforkey came nothing, can add nsstringaddobject:@"no class today incase xcode tells me that: *** terminating app due uncaught exception 'nsinvalidargumentexception', reason: '*** -[__nsarraym insertobject:atindex:]: object cannot nil'

like has been telling me now. have solution? tried if ([_jsonid valueforkey:@"mon"]!= nil) didn't work. tried if ([_jsonid valueforkey:@"mon"]!= nil) didn't work. tried if ([_jsonid valueforkey:@"mon"]!= (id)[nsnull null]) didn't work. tried if ([_jsonid valueforkey:@"mon"]!= [nsnull null) didn't work.

i know _jsonid valueforkey came pointer, have no idea how check if pointer null(or nil in objective-c if i'm correct). please me.

update: have tried using objectforkey , here code(_jsonid objectforkey nil, know try testing code)

if (_jsonid[@"sun"]== nil) {     [_keysun addobject:@"no class today"];     [monday addobject:[_keysun objectatindex:1]];             } else{    _keysun = [_jsonid objectforkey:@"sun"];    [monday addobject:@"sunday"];    (int i=0; i<5; i++) {        if ([_keysun objectatindex:i]!=nil) {             [monday addobject:[_keysun objectatindex:i]];        } } }

still, xcode telling me i'm add nil nsarray.

*** terminating app due uncaught exception 'nsinvalidargumentexception', reason: '*** -[__nsarraym insertobject:atindex:]: object cannot nil'

this driving me crazy...

i think figured out. let's @ code: 

if (_jsonid[@"sun"]== nil) {     [_keysun addobject:@"no class today"];     [monday addobject:[_keysun objectatindex:1]]; } else {    _keysun = [_jsonid objectforkey:@"sun"];     ...

if there object key "sun" store _keysun. if there no object, store nothing _keysun. _keysun nil. send addobject message _keysun - message sent nil object nothing. call [_keysun objectatindex:1]. again, sending message nil object nothing, , returns nil, why add nil monday object , crash.

if had set _keysun empty nsmutablearray, have crashed reason: try add one object. can read 1 object objectatindex:0. read objectatindex:1 need two objects in array.

btw. [nsnull null] json parser if json document reading contains explicit null. if json contains "sun": null objectforkey:@"sun" give [nsnull null]. if json doesn't contain key "sun" @ all, objectforkey:@"sun" give nil.


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