Advanced Exercism • crystal

Reverse String

Challenge Overview

Reverse a given string.

Introduction

Reversing strings (reading them from right to left, rather than from left to right) is a surprisingly common task in programming.

For example, in bioinformatics, reversing the sequence of DNA or RNA strings is often important for various analyses, such as finding complementary strands or identifying palindromic sequences that have biological significance.

Instructions

Your task is to reverse a given string.

Some examples:

Turn "stressed" into "desserts".
Turn "strops" into "sports".
Turn "racecar" into "racecar".

Dig Deeper

Use an array

Using an array

module ReverseString
  def self.reverse(input : String) : String
    result = Array(Char).new(input.size)
    input.each_char do |char|
      result.unshift(char)
    end
    result.join
  end
end

The method takes a String as an argument and returns a String. An empty array stores the reversed string which stores Char. The reason for using Char instead of String is because Char is lighter than String, and we only store one character at a time. The Array is given the size of the string to be reversed, this makes the array doesn’t have to be resized later which is more efficient.

The each_char method iterates over each character in the string. For each character so are they unshifted into the array. The unshift method adds the character to the beginning of the array. This means the first character in the string will be the last character in the array.

The join method converts the array back to a string.

Uses String::Builder

Using String::Builder

module ReverseString
  def self.reverse(input : String) : String
    String.build(input.size) do |builder|
      (0...input.size).reverse_each do |i|
        builder << input[i]
      end
    end
  end
end

The method takes a String as an argument and returns a String. The constructor, String.build, takes the size of the string as an argument. The String.build method is from the String::Builder class. The String::Builder allows writing to a buffer in memory.

The constructor is given the size of the string to be reversed. This is due to the fact that we already know the size of the string. This is more efficient than using the default size of 16 and then resizing the string later.

A Range is created from 0 to the size of the string (exclusive). The reverse_each method iterates over the range in reverse order. So the last index of the string is the first value given to the block. For each index, the character at that index is appended to the string builder.

The String.build method is what is implicitly returned from the method.

Uses Reverse

Reverse

module ReverseString
  def self.reverse(input : String) : String
    input.reverse
  end
end

This is the simplest and most exemplary solution, using the built-in String#reverse method. Internally, it uses a very optimized implementation, using a buffer and working directly on the string’s bytes. If curious, check the source code.

Source: Exercism crystal/reverse-string