iOS/Objective-C: Adding NSString object to NSMutableArray, NSMutableArray is (null)

Question

- (void)webViewDidFinishLoad:(UIWebView *)webView {
if (urlIndex == maxIndex) { 
    maxIndex = maxIndex + 1;

    NSString* sURL = webpage.request.URL.absoluteString;

    [urlHistory addObject:sURL];
    NSLog(@"%d: %@", urlIndex, [urlHistory objectAtIndex:urlIndex]);
    NSLog(@"%d: %@", urlIndex, webpage.request.URL.absoluteString);


    [sURL release];

    urlIndex = urlIndex + 1;
}
else {
    [urlHistory insertObject:webView.request.URL.absoluteString atIndex:(urlIndex - 1)];
}
}

This line

    NSLog(@"%d: %@", urlIndex, [urlHistory objectAtIndex:urlIndex]);

prints (null), while as this line

    NSLog(@"%d: %@", urlIndex, webpage.request.URL.absoluteString);

prints the actual URL.

On my initWithNibName I have:

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
self = [super initWithNibName:@"Tab1ViewController" bundle:nil];

if (self) {
    urlHistory = [[NSMutableArray alloc] init];
    urlIndex = 0;
    maxIndex = 0;
}

return self;
}

But I still keep getting (null) when I access my array. Why is that?

Is that a typo in your question, you are adding objects to urlIndex (presumably an integer) and not urlHistory? — jrturton
– jrturton, Commented Oct 31, 2011 at 22:03
Yeah fixed that thanks, but that was on an irrelevant line tho... — Gibson
– Gibson, Commented Oct 31, 2011 at 22:07

Lily Ballard · Accepted Answer · 2011-11-01 18:27:13Z

6

It sounds like your urlHistory object is nil. Calling -objectAtIndex: on a nil object will return nil. You probably forgot to initialize urlHistory in your -init, or you're not actually calling the -init method you think you are (e.g. if your VC is loaded from a nib it will be using -initWithCoder: instead of -initWithNibName:bundle:).

For the record, if -objectAtIndex: on an NSArray ever returns nil, it means the array itself is nil. Since NSArray cannot store nil, -objectAtIndex: with a valid index will never return nil, and -objectAtIndex: with an invalid index will throw an exception. So the only way for -objectAtIndex: to return nil is if the method itself is never actually called.

edited Nov 1, 2011 at 18:27

answered Oct 31, 2011 at 21:14

Lily Ballard

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7 Comments

Lily Ballard Over a year ago

@Gibson: I can't really answer that without access to all of your code, but you can verify that this is what's going on by simply logging out NSLog(@"urlHistory: %@", urlHistory). You should see urlHistory: (null) logged out.

jrturton Over a year ago

@gibson, are you definitely creating the controller using initWithNibName? Have you logged in that method?

Lily Ballard Over a year ago

@Gibson: If your view controller is being created inside a nib (e.g. the VC itself is in a nib) then it uses -initWithCoder: instead of -initWithNibName:bundle:.

Lily Ballard Over a year ago

@jrturton: Yeah, I saw his comment saying that too, which is why I didn't suggest that he had the wrong init method.

jrturton Over a year ago

@KevinBallard - just update your answer, it's way better than mine and an extra one would just be noise.

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