The first of these is usually written as r, the second as
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A 2-vector (x, y) can be described by two numbers that are not coefficients
in a sum: its length, and the angle its vector makes with the x axis.
The first of these is usually written as r, the second as .
These parameters obey 
the inverse relations are 
r and
are called polar coordinates.
Calculating the angle 
in polar coordinates is a bit tricky; the obvious thing to try is atan(y, x)
but that is defined only between 
and
,
while
has a domain of size 2
.
Here is something that works: 
This gives theta in the range -
to .
If you want it to have range 0 to 2
you can add if(y < 0,8 * atan(1), 0) to it.
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