Scala string formatting

Question

val height = 1.9d
val name = "James"
println(f"$name%s is $height%2.2f meters tall")  // James is 1.90 meters tall

I've seen this example both at Better String formatting in Scala and in the Scala docs.

What does the 2 after % stand for? I assume the 2f means two decimal places.

Answer 1

Scala string interpolation formats actually use java's Formatter behind the scenes .

The format string has the following format :

%[argument_index$][flags][width][.precision]conversion

In your case the format string is using a float conversion ( the f at the end)

So your format string is asking for a float conversion with a width of 2 and a precision of 2

In the same way %4.10f would request a float conversion with a width of 4and a precision of 10.

• Passing 1.9d to the format String %4.10f would result in the string 1,9000000000
• Passing 1.9d to the format String %10.4fwould result in the string     1,9000 (it would be padded to a width of 10 with whitespace)

Answer 2

The first 2 indicates the minimum number of characters that you want printed in the output, and second 2 would indicate the number of digits after the decimal point that you need.

Suppose you instead had "%5.2f", and the 'height' was 1.2, it would result in ' 1.20' (with the space in the beginning) [not the quotes; they are here for clarity]

The docs refer to them as Width and Precision respectively (%[width].[precision]).

http://docs.scala-lang.org/overviews/core/string-interpolation.html (this points to the docs for the java Formatter)

https://docs.oracle.com/javase/7/docs/api/java/util/Formatter.html