How to specify floating-point precision in a string

Swift version: 5.4

Paul Hudson @twostraws June 1st 2019

Swift's string interpolation makes it easy to put floating-point numbers into a string, but it lacks the ability to specify precision. For example, if a number is 45.6789, you might only want to show two digits after the decimal place.

Here's an example using basic string interpolation:

let angle = 45.6789
let raw = "Angle: \(angle)"

That will make the raw value equal to "Angle: 45.6789". But if you wanted to round the angle to two decimal places, you would use this code instead:

let formatted = String(format: "Angle: %.2f", angle)

The "%f" format string means "a floating point number," but "%.2f" means "a floating-point number with two digits after the decimal point. When you use this initializer, Swift will automatically round the final digit as needed based on the following number.