# Take 2

This time, we'll use math to get the precise angle that the magnetic field forms with the X and Y axes of the magnetometer.

We'll use the `atan2` function. This function returns an angle in the `-PI` to `PI` range. The graphic below shows how this angle is measured:

Although not explicitly shown in this graph the X axis points to the right and the Y axis points up.

Here's the starter code. `theta`, in radians, has already been computed. You need to pick which LED to turn on based on the value of `theta`.

``````#![deny(unsafe_code)]
#![no_std]

extern crate aux15;
extern crate m;

use aux15::prelude::*;
use aux15::{Direction, I16x3};
use m::Float;

fn main() {
let (mut leds, mut lsm303dlhc, mut delay, _itm) = aux15::init();

loop {
let I16x3 { x, y, .. } = lsm303dlhc.mag().unwrap();

let _theta = (y as f32).atan2(x as f32);

// FIXME pick a direction to point to based on `theta`
let dir = Direction::Southeast;

leds.iter_mut().for_each(|led| led.off());
leds[dir].on();

delay.delay_ms(100_u8);
}
}

``````

Suggestions/tips:

• A whole circle rotation equals 360 degrees.
• `PI` radians is equivalent to 180 degrees.
• If `theta` was zero, what LED would you turn on?
• If `theta` was, instead, very close to zero, what LED would you turn on?
• If `theta` kept increasing, at what value would you turn on a different LED?