let slider;

function setup() {

  createCanvas(400, 400);

  slider = createSlider(1,20,4,0.01);

  stroke(255);

}

​

function draw() {

  translate(200,200);

  background(0);

  noFill();

  k = slider.value();

  beginShape();

  for(let a = 0; a < TWO_PI*10; a+= 0.001){

    let r = 200*cos(k* a);

    var x = r * sin(a);

    var y = r * cos(a);

    vertex(x, y);

  }

  endShape(CLOSE);

}