`joel_kleier.electric_froth = \`

I got a ball rolling!

2014-11-05

In my last post I went through my process of discovering how Nim works, at least a little bit. This time around I want to talk about what I did to get a circle bouncing back and forth on the screen! Not much of an acheivment, all things considered, but I spent most of my time just reading a little about SDL2_gfx.

First, I defined a type `TBall`

to describe the position, size, and velocity of my little circle:

```
type
TBall = object
x: float
y: float
rad: float
vx: float
vy: float
```

Then, I instantiated what will become the bouncing circle:

```
var
ball = TBall(
x: 320,
y: 240,
rad: 10,
vx: 100,
vy: 0)
```

I do some checks to make sure the ball position is in bounds, and moving in the correct direction:

```
ball.x += ball.vx * dt
if ball.x < ball.rad:
ball.x = ball.rad
ball.vx *= -1
elif ball.x > 640-ball.rad:
ball.x = 640-ball.rad
ball.vx *= -1
```

And finally, I render the ball with a handy method in the `SDL2_gfx`

library:

`renderer.filledCircleRGBA(int16(ball.x), int16(ball.y), int16(ball.rad), 0,255,0,255)`

The only tricky thing here is the type conversion – in nim there’s a distinct difference between type *conversion* and type *casting*.

Type *conversion* is used in a manner like `int16(123.3)`

, and the program will do it’s best to translate the value into the specified type.

Type *casting* is used in a manner like `cast[int16](123.3)`

, and the program will keep the same bit patter, but change the values type. Which will lead, inevitably to different values than *conversion*.

Until next time!