Can someone explain the result of CGRectGetMaxX and CGRectIntersection?

Question

Scenario

Assume two non-intersecting rectangles, both 10 points wide and 5 points high. The x/y values in the following schema indicate the coordinates of the corner points of the two rectangles.

x/y = 0/0 +------------------+ x/y = 9/0
          |                  |
          |                  |
          |                  |
          |                  |
x/y = 0/4 +------------------+ x/y = 9/4
x/y = 0/5 +------------------+ x/y = 9/5
          |                  |
          |                  |
          |                  |
          |                  |
x/y = 0/9 +------------------+ x/y = 9/9

Here's the code to set up these rectangles:

CGRect rect1 = CGRectMake(0.0f, 0.0f, 10.0f, 5.0f);
CGRect rect2 = CGRectMake(0.0f, 5.0f, 10.0f, 5.0f);

CGRectGetMaxX

The return value for the CGRectGetMaxX Core Graphics function is documented like this:

The largest value of the x-coordinate for the rectangle.

Here is a code snippet that uses the function:

CGFloat maxX = CGRectGetMaxX(rect1);

From the documentation, I would expect maxX to be 9. However, maxX is 10. Huh?

CGRectIntersection

The return value for the CGRectIntersection Core Graphics function is documented like this:

A rectangle that represents the intersection of the two specified rectangles. If the two rectangles do not intersect, returns the null rectangle. To check for this condition, use CGRectIsNull.

Here is a code snippet that uses the function:

CGRect intersectionRect = CGRectIntersection(rect1, rect2);
bool intersectionRectIsNull = CGRectIsNull(intersectionRect);

From the documentation, I would expect intersectionRectIsNull to be true since rect1 and rect2 are clearly not intersecting. However, intersectionRectIsNull is false. Huh???

If we examine intersectionRect in the debugger, we see this:

(CGRect) $0 = origin=(x=0, y=5) size=(width=10, height=0)

Yeah, so now it's clear why CGRectIsNull is returning false, but why on earth is intersectionRect not this?

(CGRect) $0 = origin=(x=+Inf, y=+Inf) size=(width=0, height=0)

Question

Can someone explain why the documentation of these functions is NOT wrong? I must be missing something, but for the life of me I cannot see what it is.

Answer 1

Your diagram is incorrect. It doesn't match your CGRectMake calls.

A rectangle that is 10 wide with an origin of 0 will have a max X of 10, just as you are getting.

The 9 in your diagram should be 10 and the 4 in your diagram should be 5.

Given the two calls to CGRectMake, the two rectangles share a common side. There is no gap between them.

Remember, 0 + 10 is 10, not 9 and 0 + 5 is 5, not 4.