Question

I make some method, that takes digits and operands from stack and display it in a more user friendly style. The problem is with descriptionString variable, it return null in part when topOfStack is “+”. I show a log below.

+(NSString *)descriptionOfTopOfStack:(NSMutableArray *)stack
{

    NSString *descriptionString;
    id topOfStack = [stack lastObject];
    NSString *secondInStack;
    NSString *thirdInStack;
    if (topOfStack)
    [stack removeLastObject];    
    if ([topOfStack isKindOfClass:[NSNumber class]]) {
        descriptionString = [topOfStack stringValue];
    }
    else if([topOfStack isKindOfClass:[NSString class]]){
        if(([topOfStack isEqualToString:@"+"]) || ([topOfStack isEqualToString:@"—"])){
            secondInStack = [self descriptionOfTopOfStack:stack];
            thirdInStack = [self descriptionOfTopOfStack:stack];
           descriptionString = [descriptionString stringByAppendingFormat:@"%@ %@ %@",thirdInStack,topOfStack,secondInStack]; 
          NSLog(@"description is %@",descriptionString);

        }

    }
    return descriptionString;
}

i made example with 2 + 6, this is log:

• 2012-02-21 22:09:39.983 Calculator[12536:f803] stack = (
  2,
  6,
  “+”
  )
• 2012-02-21 22:09:39.983 Calculator[12536:f803] description is (null)

Why descriptionString is null? Where i made a mistake? Thanks

Answer 1

In the line:

descriptionString = [descriptionString stringByAppendingFormat:@"%@ %@ %@",thirdInStack,topOfStack,secondInStack];

The variable descriptionString is nil. Replace that line with the following.

descriptionString = [NSString stringWithFormat:@"%@ %@ %@",thirdInStack,topOfStack,secondInStack];