author: Weidi 

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this is a demo to show what is random function

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random (min , max);

Return a random floating-point number between min number with max number you set.

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random(max); ----- this equals to  random(0, max);

this function will return a random float num between 0 and maximum

if you min number is 0, then you can write just with maximum number for your range - random(max) 

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function setup() {

  createCanvas(400, 400);

  //change the framerate to 5 frames per second

  frameRate(5);

}

​

function draw() {

  background(220);

  //a white circle pop up in random position of canvas

  fill('white');

  noStroke();

  circle(random(width),random(height),50);

  //a red rectangle in random size with different width and height both from 20 to 90 pixels 

  fill('red');

  rectMode(CENTER);

  rect(width/2,height/2-40,random(20,90),random(20,90));

  //a random color square below the red one

  fill(random(255),random(100,200),random(80,100));

  rect(width/2,height/2+40,80,80);

}