Question

I’m new in Obj C programming, so i will start from beginning
I have an array with some NSNumber and NSString elements, i want to replace NSString elems to NSNumber elems. I decide to make this with for in construction get to new NSMutableArray all elements value and to NSMutableIndexSet their indexes, but when i made this i faced with trouble, here it is, i show you my code and log.

NSArray *oneMoreStack = [program copy];
NSLog(@" stack=%@",oneMoreStack);
NSLog(@" show me index ind=%@",[oneMoreStack indexOfObject:@"x"]);
NSMutableIndexSet *myVarIndexesSet;
NSMutableArray *myVarsArray;


for (id myVarConst in oneMoreStack) {
    if([myVarConst isKindOfClass:[NSString class]])
    {
        if([myVarConst isEqualToString:@"x"]){
            [myVarsArray addObject:myVarConst];
            NSLog(@"obj x=%@",myVarConst);
            [myVarIndexesSet addIndex:[oneMoreStack indexOfObject:myVarConst]];
            NSLog(@"ind x=%@",[oneMoreStack indexOfObject:myVarConst]);

        } 
}

When i run it log show me this:

Calculator[6306:f803]  stack=(
    x,
    2,
    "+"
)
Calculator[6306:f803]  show me index ind=(null)
Calculator[6306:f803] obj x=x
Calculator[6306:f803] ind x=(null)
then crash [NSMutableArray replaceObjectsAtIndexes:withObjects:]: index set cannot be nil

So, my question is why does [oneMoreStack indexOfObject:myVarConst] return null? How can i fix it? Thanks.
I don’t mention this code before because think problem is in getting indexes.

after previous part of code goes this part:

[stack replaceObjectsAtIndexes:[myVarIndexesSet copy] withObjects:[myVarsArray copy]]; 

after that crash with error report i mention previously. Thanks.

Answer 1

[oneMoreStack indexOfObject:myVarConst] returns an NSUInteger, a primitive type, not an object.

In your NSLog, you are using %@, which says “Hey, take this object I’m passing as an argument, call -description on it, and print it out”. Luckily, the NSUInteger you are passing in happens to be 0, NSLog thinks that it is a null object, and you aren’t crashing. If the call to -indexOfObject: returns non-0, you will likely crash.

Try this instead:

// For iOS
NSLog(@" show me index ind=%u", [oneMoreStack indexOfObject:@"x"]);

or

// For 64-bit and 32-bit Mac OS X
NSLog(@" show me index ind=%lu", (unsigned long)[oneMoreStack indexOfObject:@"x"]);

You can learn more about string formats at:
Formatting String Objects