Array sorting and reversing in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you sort an array in Ruby in ascending order?

Use the sort method. For example, [3, 1, 2].sort returns [1, 2, 3].

beginner

How do you reverse the order of elements in an array in Ruby?

Use the reverse method. For example, [1, 2, 3].reverse returns [3, 2, 1].

intermediate

What does the sort! method do in Ruby?

It sorts the array in place, changing the original array instead of creating a new one.

intermediate

How can you sort an array in descending order in Ruby?

You can use sort.reverse or sort { |a, b| b <=> a } to sort in descending order.

intermediate

What is the difference between reverse and reverse! methods?

reverse returns a new array with elements reversed, while reverse! reverses the elements in the original array itself.

Which Ruby method sorts an array without changing the original array?

A reverse

B sort

C sort!

D reverse!

The sort method returns a new sorted array and does not modify the original array.

What does [5, 3, 8].reverse return?

A [8, 5, 3]

B [8, 3, 5]

C [3, 5, 8]

D [5, 3, 8]

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

How do you sort an array in descending order in Ruby?

A array.reverse!

B array.reverse.sort

C array.sort!

D array.sort.reverse

Sorting ascending then reversing gives descending order.

Which method changes the original array when reversing it?

A reverse!

B reverse

C sort

D sort!

reverse! modifies the original array in place.

What does sort! do?

A Sorts and returns a new array

B Does nothing

C Sorts the original array in place

D Reverses the array

sort! sorts the original array itself, changing it.

Explain how to sort and reverse an array in Ruby, including how to do it without changing the original array.

Think about methods that return new arrays versus those that change the original.

Describe the difference between sort and sort!, and when you might want to use each.

Consider if you want to keep the original array unchanged or not.

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