How to draw bottom half of a circle in canvas

Question

I am trying to draw the the bottom half of the circle using the appropriate x=cos(theta), y=sin(theta) functions. If I iterate theta from Math.PI to 2*Math.PI I seem to be getting the upper half of the circle instead:

What am I doing wrong in this code snippet:

    window.onload = function() 
    {
        var canvas = document.getElementById('circle-canvas');

        if (canvas && canvas.getContext) {
            var context = canvas.getContext('2d');
            if (context) {
                context.strokeStyle = "#369";
                context.lineWidth = 4;


                j = canvas.width / 2;
                k = canvas.height / 2;
                r = canvas.width / 4; 

                function computeX(theta, r, j, k){ return r * Math.cos(theta) + j; }
                function computeY(theta, r, j, k){ return r * Math.sin(theta) + k; }

                start = Math.PI;
                context.lineTo(computeX(start, r, j, k), computeY(start, r, j, k));
                for(var theta = start; theta <= (2*Math.PI); theta += .1) 
                {
                    x = computeX(theta, r, j, k);
                    y = computeY(theta, r, j, k),

                    context.lineTo(x, y);
                }
                context.stroke();
                context.closePath();
            }
        }
    }

EDIT: I am aware of the arc function. I need to implement the arc this way because this will be used as part of a bigger problem where I need to compute each individual point of the arc.

Answer 1

There is an arcfunction.

var canvas = document.getElementById("circle-canvas");
var context = canvas.getContext("2d");

var centerX = canvas.width / 2; 
var centerY = canvas.height / 4; 

var radius = canvas.width / 4; 

// I think these values are the angles for the bottom half - otherwise use other values
var startingAngle = Math.PI;
var endingAngle = 0;
var counterclockwise = true;

context.arc(centerX, centerY, radius, startingAngle,
    endingAngle, counterclockwise);

context.lineWidth = 4;
context.strokeStyle = "#369";
context.stroke();

Answer 2

Just put a - before r

y = computeY(theta, -r, j, k),

Tested and it works