Array sorting and reversing in Ruby

Introduction

Sorting and reversing arrays help organize data so it's easier to find or use. Sorting arranges items in order, and reversing flips their order.

When you want to list names alphabetically.

When you need to show numbers from smallest to largest or largest to smallest.

When you want to reverse the order of items, like showing the latest messages first.

When preparing data for reports that require sorted or reversed lists.

Syntax

Ruby

class Array
  # Sorts the array in ascending order and returns a new array
  def sorted_array
    self.sort
  end

  # Reverses the array and returns a new array
  def reversed_array
    self.reverse
  end
end

The sort method returns a new array sorted in ascending order.

The reverse method returns a new array with elements in reverse order.

Examples

This sorts numbers from smallest to largest.

Ruby

numbers = [3, 1, 4, 2]
sorted_numbers = numbers.sort
puts sorted_numbers.inspect

This sorts words alphabetically.

Ruby

words = ["banana", "apple", "cherry"]
sorted_words = words.sort
puts sorted_words.inspect

Reversing an empty array returns an empty array.

Ruby

empty_array = []
reversed_empty = empty_array.reverse
puts reversed_empty.inspect

Reversing an array with one element returns the same array.

Ruby

single_element = [42]
reversed_single = single_element.reverse
puts reversed_single.inspect

Sample Program

This program shows the original array, then the sorted version, and finally the reversed version.

Ruby

numbers = [5, 2, 9, 1, 7]
puts "Original array: #{numbers.inspect}"

sorted_numbers = numbers.sort
puts "Sorted array: #{sorted_numbers.inspect}"

reversed_numbers = numbers.reverse
puts "Reversed array: #{reversed_numbers.inspect}"

Important Notes

Sorting an array usually takes time proportional to the number of elements times the logarithm of that number (O(n log n)).

Reversing an array takes time proportional to the number of elements (O(n)).

A common mistake is to expect sort or reverse to change the original array. They return new arrays instead.

Use sort! or reverse! if you want to change the original array.

Summary

Sorting arranges array elements in order, usually ascending.

Reversing flips the order of elements in the array.

Both methods return new arrays unless you use the versions with ! to modify in place.

Practice

1. What does the Ruby method sort do when called on an array?

easy

A. It arranges the elements in ascending order.

B. It reverses the order of elements.

C. It removes duplicate elements.

D. It shuffles the elements randomly.

Solution

1. Step 1: Understand the purpose of sort

   The sort method arranges elements in ascending order by default.

2. Step 2: Compare with other methods

   Unlike reverse which flips order, sort orders elements from smallest to largest.

3. Final Answer:

   It arranges the elements in ascending order. -> Option A

4. Quick Check:

   sort = ascending order [OK]

Hint: Remember: sort arranges ascending, reverse flips order [OK]

Common Mistakes:

• Confusing sort with reverse
• Thinking sort removes duplicates
• Assuming sort shuffles elements

2. Which of the following is the correct syntax to reverse an array arr in Ruby?

easy

A. arr.reverse!

B. arr.reverse[]

C. reverse.arr()

D. arr.reverse()!

Solution

1. Step 1: Identify the correct method call

   The method to reverse an array in place is reverse! called on the array object.

2. Step 2: Check syntax correctness

   Only arr.reverse! is valid Ruby syntax; others are incorrect method calls or misplaced symbols.

3. Final Answer:

   arr.reverse! -> Option A

4. Quick Check:

   reverse! modifies array in place [OK]

Hint: Use reverse! to reverse array in place [OK]

Common Mistakes:

• Using parentheses incorrectly
• Placing exclamation mark after parentheses
• Calling reverse as a standalone function

3. What is the output of the following Ruby code?

arr = [3, 1, 4, 2]
sorted_arr = arr.sort
reversed_arr = sorted_arr.reverse
puts reversed_arr.inspect

medium

A. [1, 2, 3, 4]

B. [3, 1, 4, 2]

C. [4, 3, 2, 1]

D. [2, 4, 1, 3]

Solution

1. Step 1: Sort the original array

   Calling arr.sort returns a new array sorted ascending: [1, 2, 3, 4].

2. Step 2: Reverse the sorted array

   Calling sorted_arr.reverse flips the order to [4, 3, 2, 1].

3. Final Answer:

   [4, 3, 2, 1] -> Option C

4. Quick Check:

   sort then reverse = descending order [OK]

Hint: Sort then reverse = descending order [OK]

Common Mistakes:

• Assuming reverse modifies original array
• Confusing sorted array with original
• Expecting reverse to sort ascending

4. Identify the error in this Ruby code snippet:

arr = [5, 2, 9]
arr.sort!
arr.reverse
puts arr.inspect

medium

A. Missing parentheses in method calls cause errors.

B. The sort! method is invalid syntax.

C. The array arr cannot be reversed after sorting.

D. Using reverse does not change arr in place.

Solution

1. Step 1: Understand sort! effect

   sort! sorts the array arr in place, so arr becomes [2, 5, 9].

2. Step 2: Check reverse method usage

   reverse returns a new reversed array but does not modify arr itself.

3. Final Answer:

   Using reverse does not change arr in place. -> Option D

4. Quick Check:

   reverse without ! returns new array [OK]

Hint: Use reverse! to modify array in place [OK]

Common Mistakes:

• Assuming reverse changes original array
• Thinking sort! is invalid
• Believing method calls need parentheses always

5. Given an array arr = [7, 3, 5, 3, 9, 1], how can you create a new array that is sorted in descending order without changing the original array?

hard

A. Use arr.sort! then arr.reverse!

B. Use arr.sort.reverse

C. Use arr.reverse.sort

D. Use arr.reverse! then arr.sort!

Solution

1. Step 1: Sort the array without modifying original

   arr.sort returns a new sorted array ascending, original arr stays unchanged.

2. Step 2: Reverse the sorted array to get descending order

   Calling reverse on the sorted array returns a new array reversed, still not changing original.

3. Final Answer:

   Use arr.sort.reverse -> Option B

4. Quick Check:

   sort then reverse = descending new array [OK]

Hint: Chain sort then reverse to get descending without changing original [OK]

Common Mistakes:

• Using sort! or reverse! which modify original array
• Reversing before sorting
• Expecting reverse.sort to work the same