Undeclared identifier M_PI_2 issues


At first I was confused as to where M_PI_2 came from but Google says it’s a mathematical constant… cool.

However, when I build and run the program to convert cartesian into polar, Xcode tells me that M_PI_2 is an undeclared identifier.

I did some more Googling and found that this constant doesn’t exist in C99 which apparently I’m running. I would have to define M_PI_2 explicitly as the reciprocal of pi squared?