Don't understand why I can't set my values

[Unicyclist]

Hi!

I'm pretty new to programming and I'm working through Programming in Objective-C by Stephen Kochan.

I've got a problem I can't seem to figure out.

Its an exercise in the book where we're supposed to create a program to work with rectangles. I've included the details below.

The problem is mainly in "Rectangle.m"
I don't understand why i can't seem to use the method "intersect" to set

 new.origin.x

and

 new.origin.y

I think its related to how I defined "setOrigin" but I don't understand how it all comes together.
I noticed that I can't simply use

 origin.x = value

and

 origin.y = value

and must instead use

 [origin setX: value andY: value]

.

I think they're related but I'm not sure. Thanks in advance!

this is the file XYPoint.h

#import <Foundation/Foundation.h>

@interface XYPoint : NSObject 
{
	float x;
	float y;
}

@property float x, y;

-(void) setX: (float) xVal andY: (float) yVal;
@end

this is the file XYPoint.m

#import "XYPoint.h"

@implementation XYPoint

@synthesize x, y;

-(void) setX: (float) xVal andY: (float) yVal;
{
	x = xVal;
	y = yVal;
}

@end

this is the file Rectangle.h

#import <Foundation/Foundation.h>
#import "XYPoint.h"

@interface Rectangle : GraphicObject
{
	float width;
	float height;
	XYPoint *origin;
}

@property float width, height;

-(XYPoint *) origin;
-(void) setOrigin: (XYPoint *) pt;
-(void) set: (float) w : (float) h;
-(float) area;
-(float) perimeter;
-(void) translate: (XYPoint *) trans;
-(Rectangle *) intersect: (Rectangle *) second;

@end

this is the file Rectangle.m

#import "Rectangle.h"

@implementation Rectangle

@synthesize width, height;

-(void) set: (float) w : (float) h
{
	width = w;
	height = h;
}

-(void) setOrigin: (XYPoint *) pt
{
	origin = [[XYPoint alloc] init];
	[origin setX: pt.x andY: pt.y];
}

-(float) area
{
	return width * height;
}

-(float) perimeter
{
	return (width + height) * 2;
}

-(XYPoint *) origin
{
	return origin;
}

-(void) translate: (XYPoint *) trans
{
	origin.x += trans.x;
	origin.y += trans.y;
}

-(Rectangle *) intersect: (Rectangle *) second
{	
	Rectangle * new = [[Rectangle alloc] init];
	
	new.width = (origin.x + width) - second.origin.x;
	new.height = (second.origin.y + second.height) - origin.y;
	
	new.origin.x = 4;
	new.origin.y = 5;
		
	return new;
}

[smasher]

Where's the init method for your Rectangle class? I'm gonna guess that you never set "origin" and it's still pointing to nil. Any message you send to nil will do nothing and return zero.

[Unicyclist]

Hi smasher!

Thank you for the prompt response... I'm not sure what you're referring to exactly but when I compile the program and let it run it does so fine except for the intersect method.

Also, would you happen to know why I can't seem to define origin.x using origin.x = value instead of resorting to [origin setX: value andY: value]?

Thanks!

[smasher]

You did not say what goes wrong when you set origin.x = value. Do you get a compiler error, or a crash, or does it fail to store the value and origin.x always returns zero?

If it's the last case -- origin.x always returns zero -- then it's because origin does not point to a valid XYPoint. If you never set it then it still points to nil, or zero. Checking nil.x or nil.y will always return zero.

You should point origin at a valid point, and the right place to do that is probably in the init method for your Rectangle class.

[Unicyclist]

Thanks Smasher!

I understand now. It makes a lot of sense =)

Cheers!
Unicyclist