Question

what is the meaning of: i=-n

in here: for(i=-NUM; i<=NUM; i++)

Answer 1

I guess this is not Python.
In a number of languages (let's say C and its children C++, Java, ..., but probably a number of others too), the first statement in the for loop is an initialization. So here it's just stating that given a NUM constant (I guess it's a constant because it's uppercase, and in many languages there's an agreement to write the constants uppercase), you will enumerate from -NUM to +NUM.
As a test, you could print i in the for loop.