A programming kata is an exercise which helps a programmer hone his skills through practice and repetition.
This article is part of the series “Scala Tutorial Through Katas”. Articles are divided into easy, medium and hard. Beginners should start with easy ones and move towards more complicated once they feel more comfortable programming in Scala.
For the complete list of Scala katas and solutions please visit the index page
The article assumes that the reader is familiar with the basic usage of ScalaTest asserts and knows how to run them from his favorite Scala IDE (ours is IntelliJ IDEA with the Scala plugin).
Tests that prove that the solution is correct are displayed below. Recommended way to solve this kata is to write the implementation for the first test, confirm that it passes and move to the next. Once all of the tests pass, the kata can be considered solved.
One possible solution is provided below the tests. Try to solve the kata by yourself first.
Word Wrap
Write Word Wrapper, that has a single static function named wrap that takes two arguments, a string, and a column number.
The function returns the string, but with line breaks inserted at just the right places to make sure that no line is longer than the column number.
Like a word processor, break the line by replacing the last space in a line with a newline.
Solution can assume that no word is longer than the maximum number characters in a line.
Following is a set of unit tests that can be used to solve this kata in the TDD fashion.
[UNIT TESTS]
import org.scalatest.{Matchers, FlatSpec}
import com.wordpress.technologyconversations.learning.kata.solutions.WordWrap._
class WordWrapTest extends FlatSpec with Matchers {
"WordWrap" should "return empty string is no text is specified" in {
"".wordWrap(10) should be ("")
}
it should "work with a single word" in {
"kata".wordWrap(4) should be ("kata")
}
it should "return wrapped text with small sample" in {
val input = "This kata"
val expected = "Thisnkata"
input.wordWrap(4) should be (expected)
}
it should "return wrapped text" in {
val input = "This kata should be easy unless there are hidden, or not so hidden, obstacles. Let's start!"
val expected = "This katanshould beneasy unlessnthere arenhidden, ornnot sonhidden,nobstacles.nLet's start!"
input.wordWrap(12) should be (expected)
}
it should "return the same text if max length is the same as the length of the text" in {
val input = "Lorem ipsum dolor sit amet."
val expected = "Lorem ipsum dolor sit amet."
input.wordWrap(input.length) should be (expected)
}
}
Test code can be found in the GitHub WordWrap.scala.
[ONE POSSIBLE SOLUTION]
object WordWrap {
implicit class StringImprovements(s: String) {
def wordWrap(maxLength: Int): String = {
s.split(" ").foldLeft(Array(""))( (out, in) => {
if ((out.last + " " + in).trim.length > maxLength) out :+ in
else out.updated(out.size - 1, out.last + " " + in)
}).mkString("n").trim
}
}
}
The solution code can be found in WordWrap.scala solution.
What was your solution? Post it as a comment so that we can compare different ways to solve this kata.
Technology Conversations 
Indeed, Katas are great way to learn a new language. I would probably try some concurrent algorithms and design as well e.g. producer/consumer.
Is it just me or does the example fail if you set the length to be the same as actual length of the string (it replaces the last space with a newline) ?
Chris, can you post a test that fails?
Following was successful:
it should “work with a single word” in {
“kata”.wordWrap(4) should be (“kata”)
}
Mixed java/scala project with tests in java right now – but I ran:
Where TEXT is “Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.”
And I get
java.lang.AssertionError:
expected:
but was:
That may not show up well here – but in the actual I get a newline before the final laborum
Aargh – the test results didn’t show – this time without any angle brackets:
java.lang.AssertionError:
expected:Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
but was:Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est
laborum.
That may not show up well here – but in the actual I get a newline before the final laborum
You’re right. Trim was missing in the if statement. I added a new test that reproduces the bug and the solution.
Before:
if ((out.last + ” ” + in).length > maxLength) out :+ in
After:
if ((out.last + ” ” + in).trim.length > maxLength) out :+ in
Yes – that worked. Thanks 🙂
Now – the only thing this doesn’t do that the different java versions I have do is splitting words that are longer than the maxLength at maxLength-1 and then append a hyphen before the newline – but my scala skills are sadly not good enough to try adding that 🙂
Hi,
I’ve done a tail recursive version:
def wrap(someString: String, columnSize: Int) : String = {
if (columnSize >= someString.length) {
someString
}
else {
someString.take(columnSize) + “\n” + wrap(someString.takeRight(someString.length – columnSize), columnSize)
}
}
Efficient alternative, avoiding lot of array copies (remember arr :+ creates a new copy of the array
def wrap(input: String, column: Int): String = {
if(input == “”) {
input
} else if(input.length <= column) {
input
} else {
val inputArray = input.toCharArray
var start = 0
while(start < inputArray.length) {
var end = start + column
if(end < inputArray.length) {
while (inputArray(end) != ' ') {
end -= 1
}
inputArray(end) = '\n'
}
start = end + 1
}
inputArray.mkString
}
}