Map/collect for transformation in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the map method do in Ruby?

The map method takes each element of a collection, applies a block of code to it, and returns a new array with the transformed elements.

beginner

What is the difference between map and each in Ruby?

map returns a new array with transformed elements, while each simply iterates over elements without changing or returning a new array.

beginner

How do you use map to square each number in an array [1, 2, 3]?

You write:
[1, 2, 3].map { |n| n * n } which returns [1, 4, 9].

intermediate

Can map be used with hashes in Ruby?

Yes, map can be used with hashes. It transforms each key-value pair and returns an array of results. To get a hash back, you can use map.to_h.

intermediate

What happens if you use map without a block?

If you call map without a block, it returns an Enumerator, which you can later use to chain or pass a block.

What does [1, 2, 3].map { |x| x + 1 } return?

A [2, 3, 4]

B [1, 2, 3]

C [1, 4, 9]

D An error

The map method adds 1 to each element, returning [2, 3, 4].

Which method returns a new array with transformed elements?

A each

B map

C select

D delete

map returns a new array with each element transformed by the block.

What type of object does map return when called without a block?

A Array

B Nil

C Hash

D Enumerator

Without a block, map returns an Enumerator.

How can you convert the result of map on a hash back to a hash?

A Use <code>.to_a</code>

B Use <code>.join</code>

C Use <code>.to_h</code>

D Use <code>.flatten</code>

You use .to_h to convert an array of key-value pairs back to a hash.

What will [1, 2, 3].map { |n| n * 2 } output?

A [2, 4, 6]

B [1, 2, 3]

C [3, 4, 5]

D An error

Each element is multiplied by 2, so the result is [2, 4, 6].

Explain how the map method works in Ruby and give a simple example.

Think about changing each item in a list and getting a new list back.

Describe the difference between map and each in Ruby.

One changes the collection, the other just looks at each item.

Practice

1. What does the Ruby method map (or collect) do to an array?

easy

A. It sorts the array in ascending order.

B. It deletes items from the original array.

C. It creates a new array by changing each item based on the block given.

D. It returns the first item of the array.

Solution

1. Step 1: Understand the purpose of map

   The map method takes each item in the array and applies the block's code to it.

2. Step 2: Result of map

   This creates a new array with the changed items, leaving the original array unchanged.

3. Final Answer:

   It creates a new array by changing each item based on the block given. -> Option C

4. Quick Check:

   map changes items and returns new array [OK]

Hint: Remember: map returns a new changed array, original stays same [OK]

Common Mistakes:

• Thinking map changes the original array
• Confusing map with delete or sort methods
• Believing map returns a single value

2. Which of the following is the correct syntax to double each number in the array [1, 2, 3] using map?

easy

A. [1, 2, 3].map { |x| x * 2 }

B. [1, 2, 3].map(x => x * 2)

C. [1, 2, 3].map do x x * 2 end

D. [1, 2, 3].map { x -> x * 2 }

Solution

1. Step 1: Identify correct block syntax

   In Ruby, map uses a block with { |variable| code } or do |variable| code end.

2. Step 2: Check each option

   [1, 2, 3].map { |x| x * 2 } uses correct block syntax with { |x| x * 2 }. Others have syntax errors or wrong block format.

3. Final Answer:

   [1, 2, 3].map { |x| x * 2 } -> Option A

4. Quick Check:

   Use curly braces and pipe for block variables [OK]

Hint: Use { |var| ... } for map blocks in one line [OK]

Common Mistakes:

• Missing pipes around block variable
• Using arrow syntax like JavaScript
• Incorrect do-end block without pipes

3. What is the output of this Ruby code?

[4, 5, 6].map { |n| n + 1 }

medium

A. [4, 5, 6]

B. Error: undefined method

C. [3, 4, 5]

D. [5, 6, 7]

Solution

1. Step 1: Understand the block operation

   The block adds 1 to each number in the array: 4+1=5, 5+1=6, 6+1=7.

2. Step 2: Result of map

   The map method returns a new array with these values: [5, 6, 7].

3. Final Answer:

   [5, 6, 7] -> Option D

4. Quick Check:

   Each item +1 = [5, 6, 7] [OK]

Hint: Add inside block to transform each item [OK]

Common Mistakes:

• Expecting original array to change
• Confusing map with each (which returns original array)
• Thinking map returns a single value

4. Find the error in this code snippet:

numbers = [1, 2, 3]
numbers.map { n n * 2 }

medium

A. The block should use parentheses

B. Missing pipe symbols around block variable

C. map cannot be used on arrays

D. The array should be a hash

Solution

1. Step 1: Check block syntax

   The block is missing pipes around the variable: it should be { |n| n * 2 }.

2. Step 2: Understand Ruby block rules

   Without pipes, Ruby does not know the block variable, causing a syntax error.

3. Final Answer:

   Missing pipe symbols around block variable -> Option B

4. Quick Check:

   Always use |var| in map blocks [OK]

Hint: Always put pipes | | around block variables [OK]

Common Mistakes:

• Omitting pipes in block parameters
• Thinking map is invalid on arrays
• Trying to use parentheses instead of pipes

5. Given an array of strings ["apple", "banana", "cherry"], how do you use map to create a new array with the length of each string?

hard

A. ["apple", "banana", "cherry"].map { |fruit| fruit.length }

B. ["apple", "banana", "cherry"].map { |fruit| fruit.upcase }

C. ["apple", "banana", "cherry"].map { |fruit| fruit[0] }

D. ["apple", "banana", "cherry"].map { |fruit| fruit.reverse }

Solution

1. Step 1: Identify the desired transformation

   We want the length of each string, so we use fruit.length inside the block.

2. Step 2: Apply map with correct block

   Using map { |fruit| fruit.length } returns a new array of lengths.

3. Final Answer:

   ["apple", "banana", "cherry"].map { |fruit| fruit.length } -> Option A

4. Quick Check:

   Map each string to its length [OK]

Hint: Use .length inside map block to get string sizes [OK]

Common Mistakes:

• Using unrelated methods like upcase or reverse
• Trying to get length without block variable
• Confusing map with each for transformation