Objective C Object to Method

Question

Happy 2013.

I have a bit of a problem with an intersection method for 2 rectangles, but I tripped with an issue that it won't allow me advance on this topic.

I'm using 4 objects, 2 points and 2 rectangles. Point1 is the origin of rectangle1 and point2 is the origin for rectangle2.

Then I call the intersect method for rectangle1 and send the rectangle2 object to see if there is an intersection or not.

The problem here, is that inside the method, the origin points of both rectangles wind up being the same coordinate, thus, I will not get good information if I do not have different origins.

Here is my .h file:

@interface XYPoint : NSObject @property int x,y; -(void) setX:(int)xVal andY: (int) yVal; @end @interface Rectangle: graphicObject @property float width, height; - (void) setWidth:(float)w andHeight: (float) h; - (void) setOrigin: (XYPoint *) pt; - (bool) containsPoint: (XYPoint *) aPoint; - (void) intersect: (Rectangle *) rect; @end

Here is my .m file:

@implementation XYPoint @synthesize x,y; -(void) setX:(int)xVal andY:(int)yVal; { x = xVal; y = yVal; } @end @implementation Rectangle @synthesize width, height; XYPoint *origin; - (void) setWidth:(float) w andHeight: (float) h; { width = w; height = h; } - (float) area { return width * height; } - (float) perimeter { return (2*width) + (2*height); } -(void) setOrigin: (XYPoint *) pt { if (! origin) { origin = [[XYPoint alloc]init]; } origin.x = pt.x; origin.y = pt.y; } - (XYPoint *) origin { return origin; } -(void) intersect: (Rectangle *) rect // { int pointCount; pointCount = 0; /* R1o R2o R1F R2F */ /* origin.x rect.origin.x origin.x+w rect.origin.x+w */ if ( (origin.x <= rect.origin.x) && (rect.origin.x <= (origin.x + width) ) ) pointCount = pointCount +1; NSLog(@"width = %g height = %g origin = (%d,%d)", width, height, origin.x, origin.y); NSLog(@"rect.width = %g rect.height = %g rect.origin (%d,%d)", rect.width, rect.height, rect.origin.x, rect.origin.y); if ( (rect.origin.x <= ( origin.x + width ) ) && (origin.x + width <= rect.origin.x + rect.width) ) pointCount = pointCount + 1; if ( (origin.y <= rect.origin.y) && (rect.origin.y <= (origin.y + height) ) ) pointCount = pointCount + 1; if ( (rect.origin.y <= (origin.y + height) && ( origin.y + height <= rect.origin.y + rect.height)) ) pointCount = pointCount +1; if (pointCount == 4) NSLog (@"the rectangles intersect!"); else NSLog (@"The rectangles don't intersect."); } @end

This is my main file:

int main(int argc, const char * argv[]) { @autoreleasepool { Rectangle * myRectangle1 = [[Rectangle alloc] init]; XYPoint * myPoint1 = [[XYPoint alloc]init]; [myPoint1 setX: 0 andY: 0]; [myRectangle1 setWidth: 3 andHeight: 3]; [myRectangle1 setOrigin : myPoint1]; Rectangle * myRectangle2 = [[Rectangle alloc] init]; XYPoint * myPoint2 = [[XYPoint alloc]init]; [myPoint2 setX: 2 andY: 2]; [myRectangle2 setWidth: 4 andHeight: 4]; [myRectangle2 setOrigin : myPoint2]; [myRectangle1 intersect: myRectangle2]; } return 0; }

The last time I ran It, i've got this :

width = 3 height = 3 origin = (2,2)

rect.width = 4 rect.height = 4 rect.origin (2,2)

the rectangles intersect!

See? both rectangles have the same origin, even though they were both set different.

Thanks a lot!

Is this just an intellectual exercise, and you are deliberately not using any of the geometry structures or functions that are already available? — jrturton
– jrturton, Commented Jan 1, 2013 at 19:51
jrTurton: Yes it is only an intellectual example, I'm a newbie trying to learn objective-C on my own just for fun! Thanks. — Rodrigo Rodriguez
– Rodrigo Rodriguez, Commented Jan 2, 2013 at 6:25

CRD · Accepted Answer · 2013-01-01 19:52:15Z

You currently have:

@implementation Rectangle @synthesize width, height; XYPoint *origin;

This declares origin as a global variable and not an instance variable; so rather than there being one variable per rectangle all rectangles share a single variable. Change these lines to:

@implementation Rectangle { XYPoint *origin; } @synthesize width, height;

This makes origin an instance variable.

[Note: older compilers required you to declare instance variables in the @interface but with current compilers you can declare them in the @implementation. This is much better from a design point of view.]

CRD, Thanks a lot, it did correct my problem, I've been going over it for a couple of days. Thanks a lot again!

Levi · Accepted Answer · 2013-01-01 20:00:03Z

You declared origin in the rectangle class .m file as a file level global variable, which means every Rectangle instance will use the same variable instance for origin. You need to declare it as a member variable in the interface inside the .h file instead.

Updated with the correction from @Chuck. (thanks)

Actually, he declared it as a global variable. There's no static keyword.
Hi Chuck, I'm sorry I'm a newbie, I had to read CRD's answer in order to understand yours, now I get it, thanks a lot for answering!

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