descriptionOfTopOfStack class method

gistfile1.m

+ (NSString *)descriptionOfTopOfStack:(NSMutableArray *)stack
{
    // get the last object in the program stack
    id topObject = [stack lastObject];
    if (topObject) [stack removeLastObject];
    // if this is just a number just return it
    if ([topObject isKindOfClass:[NSNumber class]]) {
        return [topObject description];
    } 
    // otherwise it's an operation or variable
    else if ([topObject isKindOfClass:[NSString class]]) {
        // is this a one operand operation or two operand operation
        if ([CalculatorBrain isOneOperandOperation:topObject] || [CalculatorBrain isTwoOperandOperation:topObject]) {
            if ([CalculatorBrain isTwoOperandOperation:topObject]) {
                // setup array with two operands for this operation by recursion
                NSArray *ops = [NSArray arrayWithObjects:[CalculatorBrain descriptionOfTopOfStack:stack],[CalculatorBrain descriptionOfTopOfStack:stack],nil];
                
                // setup NSMutableArray needed to hold the checked operands
                NSMutableArray *checkedOps = [NSMutableArray arrayWithCapacity:2];
                
                // alloc init the number formatter needed to check if the operand returned is actually an operation
                NSNumberFormatter *f = [[NSNumberFormatter alloc] init];
                        
                // for-in loop to go through the two operands
                for (NSString *op in ops) {
                    // check if operand returned is an operation and put it in parentheses
                    if ([f numberFromString:op] == NULL && ![op isEqualToString:@"π"]) {
                        [checkedOps addObject:[NSString stringWithFormat:@"(%@)", op]];
                    } else { // otherwise just keep it the same
                        [checkedOps addObject:op];
                    }
                }
                            
                // retrun a string with the two oeprands and the operation
                return [NSString stringWithFormat:@"%@ %@ %@", [checkedOps objectAtIndex:1], topObject, [checkedOps objectAtIndex:0]];
            } else {
                // this is a one operand operation, return it like a function call
                return [NSString stringWithFormat:@"%@(%@)", topObject, [CalculatorBrain descriptionOfTopOfStack:stack]];
            }
        } 
        // this is a no-operand operator or varible, just return it
        else {
            return topObject;
        }
    }
    // return nil if the stack was empty or object popped of had the wrong class
    return topObject;
}