Question

So my problem is basically this, I’m parsing through a mutable array of numbers and strings to try and find variables. I have a private helper method, which is working just fine, and differentiaties between operations (@”+”, @”-“, etc) and variables (@”x”, @”y”, etc). Problem is, the code in my else block below isn’t working. I get inside the else statement just fine with @”x” or @”y”, but the NSSet i’m trying to create isn’t working. I just keep NSLog(ging) “return variables is empty”. Any ideas?

+ (NSSet *)variablesUsedInProgram:(id)program
{

    NSMutableArray *stack;

    if([program isKindOfClass:[NSArray class]]){
        stack = [program mutableCopy];
    }

    NSSet *returnVariables = nil;

    for (int i=0; i<stack.count; i++) {
        if ([[stack objectAtIndex:i] isKindOfClass:[NSString class]]) {
            if ([self isOperation:[stack objectAtIndex:i]]) {
                continue;
            } else {
                returnVariables = [returnVariables setByAddingObject:[stack objectAtIndex:i]];
                if (returnVariables.count == 0) {
                    NSLog(@"returnVariables is empty");
                }
            }
        } 
    }

    return returnVariables;
}

Answer 1

This is because you’re not initializing returnVariables. Change this line:

NSSet *returnVariables = nil;

to this:

NSSet *returnVariables = [NSSet set];

The -[NSSet setByAddingObject:] is an instance method, meaning it is to be called on an instance of NSSet. It isn’t meant to create a new set from scratch, rather it takes the existing set you call it on and creates a new set which is a copy of that set with an additional object added to it, and returns that new set.

Even better, use an NSMutableSet:

NSMutableSet returnVariables = [NSMutableSet mutableSet];

// Inside your loop:
[returnVariables addObject:[stack objectAtIndex:i]];