How to convert an Int to a String of a given length with leading zeros to align?

The Java library has pretty good (as in excellent) number formatting support which is accessible from StringOps enriched String class:

scala> "%07d".format(123)
res5: String = 0000123

scala> "%07d".formatLocal(java.util.Locale.US, 123)
res6: String = 0000123

Edit post Scala 2.10: as suggested by fommil, from 2.10 on, there is also a formatting string interpolator (does not support localisation):

val expr = 123
f"$expr%07d"
f"${expr}%07d"

Edit Apr 2019:

• If you want leading spaces, and not zero, just leave out the 0 from the format specifier. In the above case, it'd be f"$expr%7d".Tested in 2.12.8 REPL. No need to do the string replacement as suggested in a comment, or even put an explicit space in front of 7 as suggested in another comment.
• If the length is variable, s"%${len}d".format("123")

Short answer:

"1234".reverse.padTo(7, '0').reverse

Long answer:

Scala StringOps (which contains a nice set of methods that Scala string objects have because of implicit conversions) has a padTo method, which appends a certain amount of characters to your string. For example:

"aloha".padTo(10,'a')

Will return "alohaaaaaa". Note the element type of a String is a Char, hence the single quotes around the 'a'.

Your problem is a bit different since you need to prepend characters instead of appending them. That's why you need to reverse the string, append the fill-up characters (you would be prepending them now since the string is reversed), and then reverse the whole thing again to get the final result.

Hope this helps!

The padding is denoted by %02d for 0 to be prefixed to make the length 2:

scala> val i = 9 
i: Int = 9

scala> val paddedVal = f"${num}%02d"
paddedVal: String = 09

scala> println(paddedVal)             
09

huynhjl beat me to the right answer, so here's an alternative:

"0000000" + 123 takeRight 7

def leftPad(s: String, len: Int, elem: Char): String = {
 elem.toString * (len - s.length()) + s
}