Scala Slice: Substring, List Slice

Use the slice function to create substrings and get slices of lists.

Slice, substring. A slice is part of a list. It may only contain the elements at positions 4 through 8. A substring is a string slice. It might only contain some characters.

With StringOps in Scala, we apply slice to strings. We take substrings with a start index, and a last index (not a length). List uses the same syntax.

Substring example. In Scala we can call the Substring method as in Java. But a string can also be sliced with the slice function. Some implicit conversions make this possible.

First argument: The first character index where we want to start a substring. To get the first character, we use 0.

Second argument: The last index of our substring. This is not a character count or length—it should be larger than the first argument.

Scala program that uses string slice val phrase = "a soft orange cat" // Use slice on a string from scala.collection.immutable.StringOps. // ... Use first index, second index. val result = phrase.slice(2, 2 + 4) println("[" + result + "]") Output [soft]

List example. A list is like a string, but has other types of elements like Ints or Doubles. We can use slice on a list. We use a first and a last index.List

Here: We begin a slice at the second element (index 1, value 3.5) and continue until the third index.

Next: We begin at element 3 (index 2, value 10.3) and continue until the end of the list (with the list's length).

Scala program that uses list slice val points = List(1.5, 3.5, 10.3, 11.3) println(points) // Get slice of list. val slice1 = points.slice(1, 3) println(slice1) // Get slice until end of list. val slice2 = points.slice(2, points.length) println(slice2) Output List(1.5, 3.5, 10.3, 11.3) List(3.5, 10.3) List(10.3, 11.3)

String slice, char. We can access single chars from a string with an index. But with slice, we can get one-char strings (strings of length 1). Strings are sometimes more useful than chars.

Tip: The Scala compiler helpfully warns us when we compare a string slice (a substring) to a char.

Scala program that accesses chars, slices from string val letters = "scala" // This is a char. val result1 = letters(0) // This is a string. val result2 = letters.slice(0, 1) println(result1) println(result2) // Cannot compare a char and a string. // ... This always returns false. println(result1 == result2) Output C:\programs\program.scala:13: warning: comparing values of types Char and String using '==' will always yield false println(result1 == result2) ^ one warning found s s false

Slice versus substring. The slice() and substring methods receive the same arguments. And they return the same results. The returned string parts are equal in this program.

Scala program that uses slice, substring val example = "California" // Slice and substring have the same results on a string. val firstPart = example.slice(1, 4) val firstPart2 = example.substring(1, 4) // Print results. println(firstPart, firstPart2) if (firstPart == firstPart2) { println(true) } Output (ali,ali) true

Performance. I tested the slice method and the substring method on a string. I found the performance was about the same. So we can use either one for top performance.

Version 1: This version of the code uses the slice() method on a string and tests its result.

Version 2: Here we use the substring() method on a string. This code does the same thing as version 1.

Opinion: The word "substring" makes it clearer what we are trying to do. For this reason is might be a better choice.

Scala program that benchmarks slice, substring val data = "abcdef" // Warm up the JIT. val test = data.slice(3, 5) val test2 = data.substring(3, 5) val t1 = System.currentTimeMillis() // Version 1: use slice. for (i <- 0 to 100000000) { val part = data.slice(3, 5) if (part != "de") println(false) } val t2 = System.currentTimeMillis() // Version 2: use substring. for (i <- 0 to 100000000) { val part = data.substring(3, 5) if (part != "de") println(false) } val t3 = System.currentTimeMillis() // Print times. println(t2 - t1) println(t3 - t2) Output 969 ms, slice 967 ms, substring

A summary. With slice we remove elements from a collection. We specify the elements we want to keep—the rest vanish. An immutable, new collection is returned.

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Scala Slice: Substring, List Slice

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