Question

I started Objective-C programming about a week ago and am stuck on creating a new string from an array of strings passed into the function. I’ve researched this thoroughly and can’t seem to figure out why I can use stringByAppendingString once (checked with NSLog) but not a second time without throwing this error:

Thanks for looking.

2012-06-29 20:46:35.761 Calculator[32883:f803] -[__NSCFNumber stringByAppendingString:]: unrecognized selector sent to instance 0x6e43d50

2012-06-29 20:46:35.763 Calculator[32883:f803] * Terminating app due to uncaught exception ‘NSInvalidArgumentException’, reason: ‘-[__NSCFNumber stringByAppendingString:]: unrecognized selector sent to instance 0x6e43d50’

+ (NSString *)descriptionOfTopOfStack:(NSMutableArray *)stack
{
    NSString *result;
    NSString *displayString = [[NSString alloc]init];
    id topOfStack = [stack lastObject];
    if (topOfStack) [stack removeLastObject];

    if ([self isNoOperandOperation:topOfStack]) {
        displayString = [displayString stringByAppendingString:topOfStack];
        result = displayString;
        }
    else if ([self isOperation:topOfStack]){
        id nextInStack = [stack lastObject];
        if (topOfStack) [stack removeLastObject];
        displayString = [displayString stringByAppendingString:nextInStack];
        displayString = [displayString stringByAppendingString:topOfStack];
        result = displayString;

Answer 1

stringByAppendingString is a method of NSString, you have to return your displayString as NSString,

+ (NSString *)descriptionOfTopOfStack:(NSMutableArray *)stack
{
NSLog(@"stack array :: %@",stack);
NSString *result;
NSString *displayString = [[NSString alloc]init];
id topOfStack = [stack lastObject];
if (topOfStack) [stack removeLastObject];

NSLog(@"topOfStack :: %@", (NSString*)topOfStack);

if ([self isNoOperandOperation:topOfStack]) {
    displayString = [[(NSNumber*)displayString stringValue] 
                                stringByAppendingString:(NSString*)topStack];
    result = displayString;
    }
else if ([self isOperation:topOfStack]){
    id nextInStack = [stack lastObject];
    if (topOfStack) [stack removeLastObject];
    displayString = [[(NSNumber*)displayString stringValue] 
                                stringByAppendingString:(NSString*)nextInStack];
    displayString = [[(NSNumber*) displayString stringValue] 
                                stringByAppendingString:(NSString*)topStack];
    result = displayString;

EDIT: stringByAppendingString method requires parameter type of NSString, your topOfStack is type ID, you need to type cast this to NSString assuming your topOfStack is string, same for nextInStack