EARTH 801
Computation and Visualization in the Earth Sciences

Lesson 4: The Random Function

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syntax introduced: random()

The random function is a fun way to produce unpredictable results in a drawing. It works two ways. The first way is that you specify one number inside the parentheses, such as random(100), and a float variable will be created between zero and the number you wrote, in this case 100. The second way is that you specify two numbers inside the parentheses, and a float variable will be created between the two numbers. For example, random(2,5) will output a float between 2 and 5.

Example 4.3: Unpredictable placement of shapes

Test out the code below to draw circles on the display window, for example:

// draw several circles to the screen in unpredictable places

void setup() {
   size(400, 400);
}

void draw() {
   float x   = random(width);  // A number between 0 and width
   float y   = random(height); // A number between 0 and height
   float rad = random(20);     // A number between 0 and 20
  
   ellipse(x, y, rad, rad);    // Draw the ellipse using the created numbers.
                               // On the next loop through draw, new numbers 
                               // will be created.
}
still from running program in which white circles are drawn to the display window in random locations
Screenshot from running program in which white circles are drawn to the display window in random locations and at a random size between 0 and 20.
E. Richardson

The code above continuously draws circles to the screen in a variety of places because each time it loops through draw(), the x and y values are reset to a random value in between 0 and the number specified, which is width for x and height for y. The radius of each circle is a randomly generated number between 0 and 20.

Example 4.4: Unpredictable placement of shapes in unpredictable colors

We can modify that code in a fun and more visually interesting way by also choosing the color of each circle with the random() function, like this:

// Draw several circles to the screen in unpredictable places.
// Make their colors unpredictable, too.

void setup() {
   size(400, 400);
}

void draw() {
   float x   = random(width);  // A number between 0 and width
   float y   = random(height); // A number between 0 and height
   float rad = random(20);     // A number between 0 and 20

   fill(random(255), random(255), random(255)); //random color
   ellipse(x, y, rad, rad); //Draw the ellipse using the created numbers.
                            //On the next loop through draw, new numbers 
                            //will be created.
}
screenshot from the running program in which randomly colored circles are drawn to the display window.
Screenshot of the program running in which the fill value of the circles is set with a random number generator.
E. Richardson

The only difference between the two programs is that I added a line that sets the fill() in the second program. Now every time the program loops through draw() each value of red, green, and blue is randomized, too. The default is for random() to produce a float, but you can make it an int instead by enclosing the call to random() inside int(). For example, int x = int(random(4)) will initialize the variable x and set it to an integer between 0 and 4.

cartoon eliza alerting you that an explanation followsBelow is a screencast of me walking you through the randomly colored circles program.

 

 

Screencast walking through the randomly colored circles program.
E. Richardson

Exercises

4.7: Modify the program above so that the aspect ratio of each ellipse is also randomized

4.8: Modify the program above so that the top half of the screen gets filled with randomly-placed rectangles and the bottom half gets filled with randomly-placed circles

Turning in your work

Submit exercise 4.8 to its assignment dropbox in Canvas. Name your file lastname4.8.pde

What I am looking for

In Exercise 4.8 I want to see correct use of the random() function.