Map/collect for transformation in Ruby

Introduction

Map (also called collect) helps you change each item in a list to something new, making a new list with these changed items.

When you want to add 1 to every number in a list.

When you want to turn a list of names into a list of greetings.

When you want to get the length of each word in a list.

When you want to change all temperatures from Celsius to Fahrenheit.

When you want to make all words uppercase in a list.

Syntax

Ruby

new_array = old_array.map { |item| transformation }

You can also use 'collect' instead of 'map'; they do the same thing.

The block inside curly braces { } tells Ruby how to change each item.

Examples

Multiply each number by 2.

Ruby

numbers = [1, 2, 3]
doubled = numbers.map { |n| n * 2 }
# doubled is [2, 4, 6]

Get the length of each word.

Ruby

words = ["cat", "dog"]
lengths = words.map { |w| w.length }
# lengths is [3, 3]

Make a greeting for each name.

Ruby

names = ["Alice", "Bob"]
greetings = names.map { |name| "Hello, #{name}!" }
# greetings is ["Hello, Alice!", "Hello, Bob!"]

Sample Program

This program changes all fruit names to uppercase and prints the new list.

Ruby

fruits = ["apple", "banana", "cherry"]
uppercase_fruits = fruits.map { |fruit| fruit.upcase }
puts uppercase_fruits

Important Notes

Map does not change the original list; it makes a new one.

You can use map with any kind of list, like numbers, words, or objects.

If you want to change the original list, use map! with an exclamation mark.

Summary

Map/collect changes each item in a list and returns a new list.

Use a block to tell Ruby how to change each item.

Original list stays the same unless you use map!.

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