Map/collect for transformation in Ruby - Step-by-Step Execution

Concept Flow - Map/collect for transformation

Start with an array

↓

Apply map/collect block

↓

Transform each element

↓

Collect transformed elements

↓

Return new array with transformed values

This flow shows how map/collect takes each element from an array, transforms it, and returns a new array with the transformed elements.

Execution Sample

Ruby

numbers = [1, 2, 3]
squares = numbers.map { |n| n * n }
puts squares.inspect

This code squares each number in the array and prints the new array.

Execution Table

Step	Current Element (n)	Block Operation	Result for Element	New Array State
1	1	1 * 1	1	[1]
2	2	2 * 2	4	[1, 4]
3	3	3 * 3	9	[1, 4, 9]
4	-	End of array	-	[1, 4, 9]

💡 All elements processed, map returns new array with squared values.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
numbers	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]
n	-	1	2	3	-
squares	[]	[1]	[1, 4]	[1, 4, 9]	[1, 4, 9]

Key Moments - 2 Insights

Why does the original array 'numbers' not change after using map?

Because map returns a new array with transformed values and does not modify the original array, as shown in the variable_tracker where 'numbers' stays the same.

What happens if the block inside map does not return a value?

If the block returns nil or nothing, the new array will have nil for that element. Each block's return value becomes the new element, as seen in the execution_table's 'Result for Element' column.

Visual Quiz - 3 Questions

Look at the execution_table, what is the new array state after processing the second element?

A [1, 2]

B [4]

C [1, 4]

D [1, 4, 9]

💡 Hint

Check the 'New Array State' column at step 2 in the execution_table.

At which step does the variable 'n' have the value 3?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Current Element (n)' column in the execution_table.

If the block was changed to 'n + 1', what would be the final 'squares' array in variable_tracker?

A [1, 2, 3]

B [2, 3, 4]

C [1, 4, 9]

D [0, 1, 2]

💡 Hint

Think about adding 1 to each element in the original array [1, 2, 3].

Concept Snapshot

map/collect syntax: array.map { |element| transformation }
Returns a new array with each element transformed.
Original array stays unchanged.
Block return value becomes new element.
Useful for changing data without side effects.

Full Transcript

This visual execution shows how Ruby's map or collect method works. Starting with an array of numbers, each element is taken one by one. The block multiplies each element by itself, creating a square. Each result is added to a new array. The original array remains unchanged. The final output is a new array of squared numbers. This helps transform data cleanly and simply.

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