if and else with match and recursion

Protein Translation
Protein Translation in Scala
import scala.annotation.tailrec

object ProteinTranslation {

  def proteins(input: String): Seq[String] = {
    codonsToProteins(input, Seq())

  private def codonsToProteins(
      input: String,
      proteins: Seq[String]
  ): Seq[String] = {
    if (input.length < 3)
      codonToProtein(input.take(3)) match {
        case "STOP" => proteins
        case protein =>
          codonsToProteins(input.drop(3), proteins :+ protein)

  private def codonToProtein(codon: String): String = {
    codon match {
      case "AUG"                         => "Methionine"
      case "UUU" | "UUC"                 => "Phenylalanine"
      case "UUA" | "UUG"                 => "Leucine"
      case "UCU" | "UCC" | "UCA" | "UCG" => "Serine"
      case "UAU" | "UAC"                 => "Tyrosine"
      case "UGU" | "UGC"                 => "Cysteine"
      case "UGG"                         => "Tryptophan"
      case "UAA" | "UAG" | "UGA"         => "STOP"

This approach starts by importing from packages for what is needed.

The proteins() method starts by having calling the codonsToProteins() method, passing in the input String and en empty Sequence.

The codonsToProteins() method is annotated with the @tailrec annotation to verify that the method can be compiled with tail call optimization.

A tail call is a particular form of recursion where the last call in the method is a call to the same method and nothing else.

In other words, if the last call in recurMe() is recurMe(arg1, arg2) + 1, the + 1 makes the recursion non-tail recursive.

If the last call in recurMe() is recurMe(arg1, arg2, acc + 1), then the recursion is a tail call, because only the method is being called with no other operation being peformed on it.

If the length of the input String is less than 3, the method returns the Sequence of proteins. Otherwise, a match expression is used to perform pattern matching on the result of passing the codon to the codonToProtein() method. The codon is set by using the take() method to get the first three Chars of the input String.

The codonToProtein() method uses a match to look up the protein for the codon and returns the protein from the method.

If the protein is for a STOP codon, the match returns the Sequence of proteins from the method. Otherwise, the method calls itself, using the drop() method to pass in all but the first 3 Chars of the input String, and passing in the existing Sequence of proteins with the protein added to the end of it with the :+ operator.


The take() and drop() methods treat a string as a plain sequence of Char code units and makes no attempt to keep surrogate pairs or codepoint sequences together. The user is responsible for making sure such cases are handled correctly. Failing to do so may result in an invalid Unicode string. The take() and drop() methods work here because all of the characters are ASCII.

17th Apr 2024 · Found it useful?