CODEWITHSUNDEEP
javascala
Chris Stewart
Chris Stewart

Reputation: 1689

How do I pad a negative integer with leading zeroes?

Here is how my method is currently defined in Scala, i've been following this Stackoverflow answer

  def digitFormatter(long: Long, numDigits: Int): String = {
    String.format(s"%0${numDigits}d", long)
  }

This seems to work ok with postive integers, i get this expected behavior

    assert(digitFormatter(0, 1) == "0")
    assert(digitFormatter(0, 2) == "00")
    assert(digitFormatter(1, 2) == "01")
    assert(digitFormatter(10, 2) == "10")

However this doesn't seem to work with negative numbers, this is what I would expect my output to be

    assert(
      digitFormatter(-1, 2) == "-01")

However the result I get is just -1. How do I pad leading zeroes on negative numbers using the java std libraries?

Upvotes: 1

Views: 1149

Answers (4)

Martijn
Martijn

Reputation: 12102

Unfortunately, there is no Java format available that does this: They format on total string width, not on number of zeroes. The easiest way is to vary the length of the padding depending on whether the argument is negative:

def digitFormatter(long: Long, numDigits: Int): String = {
  val padlength = if (long >= 0) numDigits else numDigits + 1
  String.format(s"%0${padlength}d", long)
}

Upvotes: 1

Tomer Shetah
Tomer Shetah

Reputation: 8529

String.format should not be used at Scala. You can read the post The Scala String format approach (and Java String.format) by Alvin Alexander.

To make it work in Scala you need to do:

def digitFormatter(long: Long, numDigits: Int): String = {
  val padlength = if (long < 0) numDigits + 1 else numDigits
  s"%0${padlength}d".format(long)
}

You can find the format method in Scala docs. Code run can be found at Scastie.

Upvotes: 0

rzwitserloot
rzwitserloot

Reputation: 103018

You've mischaracterised what String.format does. In your code, you have ${numDigits} which suggests you think that number in front of the d is the number of digits.

No, it's the number of characters.

In other words:

String.format("%05d", -12)

produces the string -0012, because you asked for a string which is at minimum 5 characters long, and which applies 0-padding to get to 5 characters.

If you want a method which turns e.g. -12 into -00012 (6 characters, 5 digits) and +12 into 00012 (5 characters, 5 digits), then you'd have to do something like:

def digitFormatter(long: Long, numDigits: Int): String = {
    var numChars = numDigits + (if (long < 0) 1 else 0)
    String.format(s"%0${numChars}d", long)
}

NB: I'm not a scala programmer, I'm taking a bit of a stab with that ternary operator, but I think you can do that in scala.

Upvotes: 2

Mustafa Poya
Mustafa Poya

Reputation: 3027

You can do it just as below:

String result = ((value < 0) ? "-" : "") + String.format("%05d", Math.abs(value));

Upvotes: -1

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