How to randomly generate -1 or +1, in one line.

A hobby of mine is avoiding if statements, with mathematical formula that return the right number based on other conditions.

It is easy to generate a 0 or a 1 randomly, simply use Math.round(Math.random());

That will work in a number of languages, or something similar.

For a random -1 or +1, try this:

var randNum = -1 + (2*Math.round(Math.random));

If the random number generated by Math.random() is 0.5 or above, then the answer will be 1. If the random number generated is less than 0.5, the answer will be 1.

Enjoy!

How to make a RevoluteJoint in Box2D/CreateJS

There are plenty of posts on how to get box2d and createjs working, Lee Brimelow has a great youtbe video that covers how to get started very nicely.

But when I tried to make a revolute joint, I made a silly mistake that took me hours to find the source of. I'm a bit of a newbie, but I didn't find the problem to my solution online, so I thought I'd post it in case it happens to anyone else. And then maybe they can go to sleep before 5 in the morning...

Does this sound familiar?

"I can make the two objects, but when I try to create the joint, I get an 'Uncaught TypeError: undefined is not a function' message.'

Well, first, since I was working from the Lee Brimelow example as a starting point, I hadn't made sure I had setup the shortcut to the actual RevoluteJoint parts of box2d.

eg.
var box2d = {
b2Vec2 : Box2D.Common.Math.b2Vec2,
b2BodyDef : Box2D.Dynamics.b2BodyDef,
b2Body: Box2D.Dynamics.b2Body,
b2FixtureDef: Box2D.Dynamics.b2FixtureDef,
b2Fixture: Box2D.Dynamics.b2Fixture,
b2RevoluteJointDef: Box2D.Dynamics.Joints.b2RevoluteJointDef,
b2RevoluteJoint: Box2D.Dynamics.Joints.b2RevoluteJoint,
b2World: Box2D.Dynamics.b2World,
b2MassData: Box2D.Collision.Shapes.b2MassData,
b2PolygonShape: Box2D.Collision.Shapes.b2PolygonShape,
b2CircleShape: Box2D.Collision.Shapes.b2CircleShape,
b2DebugDraw: Box2D.Dynamics.b2DebugDraw
};

Still wouldn't work though. So here was the problem, my makeJoint function looked like this:
function makeJoint() {
var jDef = new box2d.b2RevoluteJointDef();
jDef.bodyB = c;
jDef.bodyA = a;
jDef.collideConnected = false;
jDef.localAnchorB = new box2d.b2Vec2(0/SCALE,80/SCALE);
jDef.localAnchorA = new box2d.b2Vec2(0,0);
j = world.CreateJoint(jDef);
}

where a (I thought) was the anchor body, and c was the other body I wanted to attach to it.

Unfortunately, a and c were actually the fixtures of those bodies, instead of the bodies themselves.

So in my function that made the anchor body, it said:
a = world.CreateBody(anchorBodyDef).CreateFixture(anchorFixtureDef) ;

So I changed it to:
a = world.CreateBody(anchorBodyDef);
a.CreateFixture(anchorFixtureDef);

Hope that helps.