We have spent a lot of time discussing variables, which, as the name indicates, change their values as the program runs. There are, however, pieces of information that do not change value. For example, the mathematical constant π never changes. We call these things constants, and there are two common ways that Objective-C programmers define them: #define and global variables.
In Xcode, create a new Foundation Command Line Tool called Constants.
In the standard C libraries, constants are defined using the #define preprocessor directive. The math part of the standard C library is declared in the file math.h. One of the constants defined there is M_PI. Use it in main.m:
#import <Foundation/Foundation.h> ...