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Have students recursively draw circles to build their recursion skills and understanding.

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  • Activity:
    • Write a simple method to draw a circle.
    • Add an if-statement and a recursive call to draw circle, which draws a smaller circle.
    • Have students draw a picture of what they think it will look like.
    • Modify the program to draw the circle toward the left.
    • Modify the program to make two recursive calls. They often think that they can predict what will happen, but rarely expect the actual result.
  • Below is some sample code for this assignment using the Processing Language:
    void setup(){
        background(0);
        size(800,600);
        noFill();
        stroke(255);
        drawCircle(400,300,200);
    }

    //top code never changes - drawCircle evolves.

    //evolution 1 - basic method that draws a circle
    void drawCircle(int x, int y, int sz){
        ellipse(x,y,sz,sz);
    }

    //evolution 2 - 1st recursion - have students draw on paper what they think the code will
    //produce
    void drawCircle(int x, int y, int sz){
        ellipse(x,y,sz,sz);
        if(sz > 10)
            drawCircle(x,y,sz/2);
    }

    //evolution 3 - move the circles to the right - have students draw on paper before running code
    void drawCircle(int x, int y, int sz){
        ellipse(x,y,sz,sz);
        if(sz > 10)
            drawCircle(x+sz/2,y,sz/2);
    }

    //evolution 4 - add second recursion - this is where the surprise happens
    void drawCircle(int x, int y, int sz){
        ellipse(x,y,sz,sz);
        if(sz > 10) {
            drawCircle(x+sz/2,y,sz/2);
            drawCircle(x-sz/2,y,sz/2);
        }
    }

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