unrecognized selector sent to instance

Question

I'm attempting to initialize an array and add a chapter object to an array of book objects, but it crashes with the error:

2012-07-25 21:41:01.503 Project1[2364:f803] -[__NSCFString arrayOfChapters]: unrecognized selector sent to instance 0x6ad80b0 2012-07-25 21:41:01.505 Project1[2364:f803] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFString arrayOfChapters]: unrecognized selector sent to instance 0x6ad80b0'

My code:

Chapter *myChapter = [[Chapter alloc]init];
myChapter.pageCount = self.TextField2.text;
myChapter.chapterTitle = self.TextField1.text;
if(!currentBook.arrayOfChapters)
{
    currentBook.arrayOfChapters = [[NSMutableArray alloc]init];
}
currentBook = [books objectAtIndex:segControl.selectedSegmentIndex];
[currentBook.arrayOfChapters addObject:myChapter];

I think the code is correct, is there something set up wrong with my project? I believe it's the initialization which is causing the actual crash, but there isn't anything non-standard there.

In the class Book: @property(nonatomic, strong) NSMutableArray *arrayOfChapters; — Tasonir, Jul 26 '12 at 01:49
Do you initialize `currentBook` anywhere before the line `currentBook = [books objectAtIndex:segControl.selectedSegmentIndex];`? — Ethan Holshouser, Jul 26 '12 at 01:53
currentBook is a local variable which is initialized in the line of code on screen - which is just after I check if it's null. I've moved that line of code above the if statement, but it still crashes with the same error. — Tasonir, Jul 26 '12 at 01:53
You can't refer to a property (`arrayOfChapters`) of an item that's not, itself, initialized. You want to check `currentBook` before you start using it. — Rob, Jul 26 '12 at 01:54
You aren't checking if `currentBook` is nil, you're checking if `currentBook.arrayOfChapters` returns nil, which is not the same thing. If moving that line above the if statement doesn't fix the problem, then there is likely something wrong with `books`. What is `books` (I'm assuming it's an `NSArray`?) and where is it initialized? — Ethan Holshouser, Jul 26 '12 at 01:59
Books is an NSMutableArray which holds chapters, and chapters contain a title and page count. I added an if(!currentBook) check, and currentBook is non-null. Is it possible to get in trouble if I'm accidentally double allocing it? — Tasonir, Jul 26 '12 at 02:03
Possible duplicate of [How can I debug 'unrecognized selector sent to instance' error](https://stackoverflow.com/questions/25853947/how-can-i-debug-unrecognized-selector-sent-to-instance-error) — Cœur, Jul 08 '19 at 05:55

score 1 · Answer 1 · answered Jul 26 '12 at 01:55

1

It looks like currentBook is a string or not initialized.

answered Jul 26 '12 at 01:55

Dmorneault

199
1
8

Turns out it was a string - I had originally had a more simple data structure before I added more variables to chapter/book and hadn't gone back and actually created a book where I thought I had one... – Tasonir Jul 26 '12 at 02:26

score 1 · Accepted Answer · answered Jul 26 '12 at 06:24

you can make a breakpoint on the line "if(!currentBook.arrayOfChapters)" to check whether the currentBook is nil;

make a breakpoint on the line"currentBook = [books objectAtIndex:segControl.selectedSegmentIndex]; " to check whether the segControl.selectedSegmentIndex >= [books count]

score 0 · Answer 3 · answered Jul 26 '12 at 02:30

I may be misreading this but...

    if(!currentBook.arrayOfChapters)
    {
        // creating array for current book
        currentBook.arrayOfChapters = [[NSMutableArray alloc]init]; 
    }

    // reassigning a new book to current book. Are you sure this new one has array of chapters?
    currentBook = [books objectAtIndex:segControl.selectedSegmentIndex]; 

    // trying to access array of chapters
    [currentBook.arrayOfChapters addObject:myChapter];

Shouldn't you be assigning first and then creating array if it's not already there?

Yes, I should, and I did make that change, although that wasn't what caused the crash. CurrentBook was completely lying, it was actually a string which I set entirely incorrectly elsewhere. — Tasonir, Jul 26 '12 at 02:38

unrecognized selector sent to instance

3 Answers3

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