How to Find a Point on a Circle in Kotlin: Tutorial

Understanding the Math Equation of a Circle

Before we jump to the code, let's get a small lesson on how are circles represented in a polar coordinate system.

Imagine a circle with a radius r at the Origin. The circle's center can be depicted as (0,0) and (0,r) is a point on the circle.

Another way to represent this point on the circle would be
𝑥=𝑟 sin 𝜃, 𝑦= 𝑟 cos 𝜃

Here 𝜃 is the angle in radians.

Now that we know how to represent a point on the circle, let's translate this into some Kotlin code.

Writing the Code

fun printPointOnTheCircle(thetaInDegrees: Float, radius: Float, cX: Float, cY: Float) {
	
    val x = cx + (radius * kotlin.math.sin(Math.toRadians(thetaInDegrees)).toFloat())
    val y = cY + (radius * kotlin.math.cost(Math.toRadians(thetaInDegrees)).toFloat())
    
    println("(x,y) = ($x,$y)")    

}

The idea for this blog came up when I was writing a blog about coding Custom Progress Bar with Jetpack Compose Canvas API. Here is what I was trying to achieve.

While the mathematical equation for a point of a circle was straightforward to remember. I still had to tailor it a bit, to position the white dot accurately in the arcs. This is because the coordinate system of android has its origin based on the top-left corner and not the actual center of the screen.

I drafted the whole process by hand, here are the notes with visual illustrations to show you how I approached the problem.