8.8. Rectangles

Now let’s say that we want to create a structure to represent a rectangle. The question is, what information do I have to provide in order to specify a rectangle? To keep things simple let’s assume that the rectangle will be oriented vertically or horizontally, never at an angle.

There are a few possibilities: I could specify the center of the rectangle (two coordinates) and its size (width and height), or I could specify one of the corners and the size, or I could specify two opposing corners.

The most common choice in existing programs is to specify the upper left corner of the rectangle and the size. To do that in C++, we will define a structure that contains a point and two doubles.

struct rectangle {
  point corner;
  double width, height;
};

Notice that one structure can contain another. In fact, this sort of thing is quite common. Of course, this means that in order to create a rectangle, we have to create a point first:

point corner = { 0.0, 0.0 };
rectangle box = { corner, 100.0, 200.0 };

This code creates a new rectangle structure and initializes the instance variables. The figure shows the effect of this assignment.

We can access the width and height in the usual way:

box.width += 50.0;
cout << box.height;

In order to access the instance variables of corner, we can use a temporary variable:

point temp = box.corner;
double x = temp.x;

Alternatively, we can compose the two statements:

double x = box.corner.x;

It makes the most sense to read this statement from right to left: “Extract x from the corner of the box, and assign it to the local variable x.”

While we are on the subject of composition, I should point out that you can, in fact, create the point and the rectangle at the same time:

rectangle box = { { 0.0, 0.0 }, 100.0, 200.0 };

The innermost curly braces are the coordinates of the corner point; together they make up the first of the three values that go into the new rectangle. This statement is an example of a nested structure.

The active code below uses the rectangle structure. Feel free to modify the code and experiment around!

#include <iostream> struct point { double x, y; }; struct rectangle { point corner; double width, height; }; int main() { using std::cout; using std::endl; rectangle box = { { 0.0, 0.0 }, 100.0, 200.0 }; box.width += 50.0; cout << box.height << endl; cout << box.width << endl; }

Q-2: How can you combine these two statements into one?

point temp = box.corner;
double x = temp.x;

• double x = corner.box.x;
• Try again.
• double x = box.corner.x;
• Correct!
• double x = corner.x;
• Try again.
• double x = box.x;
• Try again.

Q-3: Click on the legal ways to create a point and rectangle structure, assuming that the point and rectangle structures are declared above the main function in the same way as in the active code above.Re-read the text above and try again.

def main() {
    point corner = { 0.0, 0.0 );
    rectangle box = { ( 0.0, 0.0 ), 100.0, 200.0 }
    rectangle box = { { 0.0, 0.0 }, 100.0, 200.0 };
    point corner = { 0.0, 0.0 };
    rectangle box = { corner, 100.0, 200.0 };
}

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