What does means (-nan) in C programming languange? And why this happens?

Question

Im using a loop to calculate the distance between particles, in the first loop, everything is fine, but in the second step, the programming os returning (-nan).
https://code.sololearn.com/cwytl4cfpzpg/?ref=app
https://code.sololearn.com/cwytl4cfpzpg/?ref=app

Answer 1

Your variables loops and natom haven’t been declared. They need to be set to some value before running.

Answer 2

The nan means not a number so that must mean your natom variable isn’t a number. You will have to check the code where it is generated.

Answer 3

But they are

Answer 4

I dont writed the all code, but in the top they are

Answer 5

In bottom of the code natom is
# define natom = 100