Question

It’s a calculator. I have a display where I can put digits and variables (x, y etc.). When I push Enter button it sends what is on display to array with all operand.

As on display can be NSString (variables) or NSNumber (digits) I thought to use “id” as method argument.

- (IBAction)enterPressed 
{
   [self.brain pushOperand:self.display.text];
}

/////////////////////

- (void) pushOperand:(id)operand
{
    ////// So if operand is digit I need to transform it into NSNumber.

    NSNumber *digitToStack = [NSNumber numberWithDouble:operand];
    /////// Here is problem - "Sending '___strong id' to parameter of incompatible type 'double'

    NSNumber *digitToStack = [operand doubleValue];
    //////// If i do like this, i have warning - "Initializing 'NSNumber *__strong' with an expression of incompatible type 'double'

    [self.programStack addObject:operand];
}

I don’t understand what this warnings are all about.

So the question is can I somehow put in Array NSNumber and NSString using id method, or how should I do it?

Can i ‘transform’ argument from ‘id’ method into NSNumber?

Answer 1

Yes you can “transform” your argument of operand, but you’d need to do a cast.

Also, the line:

NSNumber *digitToStack = [NSNumber numberWithDouble:operand];

fails because “operand” is an Objective C object while that function is expecting a C-style double type (which is NOT an Objective C object).

Here’s some code I wrote off the top of my head:

// Let's make operand always be a NSString object
// since that's what is being passed in from the label
- (void) pushOperand:(NSString *)operand
{
    double doubleValueFromOperand = [operand doubleValue];
    if(fabs(doubleValueFromOperand) != HUGE_VAL)
    {
        NSNumber *digitToStack = [NSNumber numberWithDouble:doubleValueFromOperand];
        if(doubleValueFromOperand != 0.0)
        {
            [self.programStack addObject:digitToStack];
            return;
        } else {
            // because NSString's doubleValue also returns 0.0 for a 
            // non-numerical string, let's make sure the input from the label
            // is not 0.0
            if([operand compare: @"0.0" options: NSCaseInsensitiveSearch range: NSMakeRange(0, 3)] == NSOrderedSame)
            {
                // the operand string *is* a 0.0 input, so let's add it to your stack and return
                [self.programStack addObject: digitToStack];
                return;
            }
        }
    }

    // if we get to this point, we probably have a non-numerical string object
    [self.programStack addObject: operand];
}

This code hasn’t been tested, has no warranties, and could certainly use a further cleaning up and optimization (e.g. the check for “0.0” isn’t what I would put into production code, myself).

But hopefully this is enough to get you further along, Sasha!