Recursive methods Java

Question

Recursion always has been something I have a hard time with. I have a test tomorrow and he said there will be some Recursion on the test so I want to be prepared.

The problem I am trying to do says this:

Given a class Rectangle with instance variables width and height, provide a recursive getArea() method. Construct a rectangle whose width is one less than the original and call its getArea method.

So I know that in recursion you end up calling the method inside itself with a simplified rendition. Like, I know somewhere in getArea(int n) I will have to call getArea(n - 1). I am just not sure what to do with width and height.

So I have this:

public int getArea()
{
   if (width == 1) {
      // Base case here. Not sure what to do for this.
      return 1;   // Maybe? I'm not sure.
   } else {
      Rectangle smallerRect = new Rectangle (width - 1);
      int smallerArea = smallerRect.getArea();
      return smallerArea + height + width;
   }
}

Can anyone help me better understand recursion or maybe how to go about thinking through a recursive function? Thanks.

So what should be the output? If you decrease the width by on and call the method again it will continue until width is 0? — Ya Wang
– Ya Wang, Commented May 5, 2015 at 0:02
Note that when you construct a new Rectangle the constructor will require two parameters to initialize the height and width instance variables, not just width. — Simon
– Simon, Commented May 5, 2015 at 0:06

Aasmund Eldhuset · Accepted Answer · 2015-05-05 00:13:42Z

3

You've got the recursion itself right, with a base case and a recursive case, and a correct reduction of the parameter in the recursive call (except that, as the commenters have noted, you also need to specify the height of the new rectangle). It's only the geometry that needs fixing: height doesn't change during the recursion; what is the area of the base case rectangle, which has got width 1 and height height? And if you are told the area of the rectangle with width width - 1 and height height, how much extra area do you get by adding a strip of width 1 and height height?

For later use: while mathematically correct, this is a terrible way to compute the area of a rectangle, so please don't do this outside of exam/homework situations :-)

edited May 5, 2015 at 0:13

answered May 5, 2015 at 0:04

Aasmund Eldhuset

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Comments

Bruce Hamilton · Accepted Answer · 2015-05-05 00:12:14Z

1

Something like this perhaps? It's basically just multiplying width by height with recursion...

public int getArea() {
    return getArea(width);
}

private int getArea(int x) {
    return x == 0 ? 0 : height + getArea(x-1);
}

answered May 5, 2015 at 0:12

Bruce Hamilton

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1 Comment

user4580220 Over a year ago

This is best answer but do not confuse the hell out of poster, he is new to programming.

KM11 · Accepted Answer · 2015-05-05 00:06:33Z

0

public int getArea()
{
   if (width == 1) {
      // Base case
      return height;   // Area = width(1)*height
   } else {
      Rectangle smallerRect = new Rectangle (width - 1, height);
      int smallerArea = smallerRect.getArea();
      return smallerArea + height;
   }
}

answered May 5, 2015 at 0:06

KM11

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