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How to specify floating-point precision in a string

Written by Paul Hudson    @twostraws

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.

Available from iOS 7.0

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