How to get mathemical PI constant in Swift

With Swift 3 & 4, pi is now defined as a static variable on the floating point number types Double, Float and CGFloat, so no specific imports are required any more:

Double.pi
Float.pi
CGFloat.pi

Also note that the actual type of .pi can be inferred by the compiler. So, in situations where it's clear from the context that you are using e.g. CGFloat, you can just use .pi (thanks to @Qbyte and @rickster for pointing that out in the comments).

For older versions of Swift:

M_PI is originally defined in Darwin but is also contained in Foundation and UIKit, so importing any of these will give you the right access.

import Darwin // or Foundation or UIKit

let pi = M_PI

Note: As noted in the comments, pi can also be used as unicode character in Swift, so you might as well do

let π = M_PI

alt + p is the shortcut (on US-keyboards) that will create the π unicode character.

import Darwin is not needed all M_x are visible with the import Foundation

( Xcode Version 6.4 (6E35b) )