Block syntax (do..end and curly braces) in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a block in Ruby?

A block is a chunk of code enclosed between do..end or curly braces { } that can be passed to methods and executed.

beginner

When should you use do..end versus curly braces { } for blocks?

do..end is usually used for multi-line blocks, while curly braces { } are preferred for single-line blocks.

beginner

How do you pass a block to a method in Ruby?

You write the method call followed by a block using either do..end or { }. The method can then execute the block using yield.

beginner

Example: What does this code print?

3.times do
  puts "Hello"
end

It prints "Hello" three times, each on a new line.

intermediate

Can you use both do..end and curly braces { } interchangeably?

Yes, both define blocks, but style and readability guide when to use each. Curly braces for short blocks, do..end for longer ones.

Which block syntax is preferred for single-line blocks in Ruby?

A do..end

B square brackets []

C parentheses ()

D {} (curly braces)

Curly braces {} are preferred for single-line blocks in Ruby.

What keyword inside a method executes the passed block?

A yield

B call

C execute

D run

The keyword yield runs the block passed to the method.

Which of these is a valid way to write a block in Ruby?

A method do |x| x * 2 end

B method (|x| x * 2)

C method { |x| x * 2 }

D method [|x| x * 2]

Blocks are written with do..end (A) or curly braces {} (C).

What will this code output?
5.times { print "*" }

A *****

B * * * * *

C 5

D Error

It prints five stars in a row without spaces.

Which block syntax is usually used for multi-line blocks?

A {} (curly braces)

B do..end

C () parentheses

D <> angle brackets

do..end is preferred for multi-line blocks in Ruby.

Explain the difference between do..end and curly braces { } when writing blocks in Ruby.

Think about block length and style.

Describe how a Ruby method can use a block passed to it.

Focus on method and block interaction.

Practice

1. Which block syntax is generally preferred for multi-line blocks in Ruby?

easy

A. Both are equally preferred for multi-line blocks

B. { }

C. Neither; Ruby does not support blocks

D. do..end

Solution

1. Step 1: Understand block syntax options

   Ruby supports two block syntaxes: do..end and curly braces { }.

2. Step 2: Identify usage for multi-line blocks

   do..end is preferred for multi-line blocks because it improves readability.

3. Final Answer:

   do..end -> Option D

4. Quick Check:

   Multi-line blocks use do..end [OK]

Hint: Use do..end for multi-line blocks, braces for single-line [OK]

Common Mistakes:

• Using curly braces for multi-line blocks reduces readability
• Thinking Ruby doesn't support blocks
• Believing both syntaxes are identical in all cases

2. Which of the following is the correct syntax for a single-line block that prints each number in an array?

easy

A. [1, 2, 3].each { |n| puts n }

B. [1, 2, 3].each { n puts | }

C. [1, 2, 3].each (|n| puts n )

D. [1, 2, 3].each do |n| puts n

Solution

1. Step 1: Identify correct block syntax for single-line

   Single-line blocks use curly braces { } with parameters inside pipes | |.

2. Step 2: Check syntax correctness

   [1, 2, 3].each { |n| puts n } correctly uses { |n| puts n }. Others have syntax errors.

3. Final Answer:

   [1, 2, 3].each { |n| puts n } -> Option A

4. Quick Check:

   Single-line block with braces and pipes [OK]

Hint: Use { |var| ... } for single-line blocks [OK]

Common Mistakes:

• Missing pipes around block parameters
• Using do..end without proper line breaks
• Incorrect placement of pipes or braces

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

(1..3).map { |x| x * 2 }

medium

A. [1, 2, 3]

B. [2, 4, 6]

C. [3, 6, 9]

D. SyntaxError

Solution

1. Step 1: Understand the range and map method

   The range (1..3) produces numbers 1, 2, 3. The map method applies the block to each number.

2. Step 2: Calculate each mapped value

   Each number is multiplied by 2, resulting in 2, 4, and 6.

3. Final Answer:

   [2, 4, 6] -> Option B

4. Quick Check:

   Map doubles each element [OK]

Hint: map applies block to each element and returns new array [OK]

Common Mistakes:

• Confusing map with each (which returns original array)
• Expecting a syntax error due to block syntax
• Misunderstanding range behavior

4. Identify the error in this Ruby code:

5.times do |i| puts i }

medium

A. Missing closing end for the block

B. Missing block parameter pipes | |

C. Incorrect use of curly brace to close a do..end block

D. No error, code runs fine

Solution

1. Step 1: Check block opening and closing syntax

   The block starts with do but incorrectly ends with a curly brace }.

2. Step 2: Understand correct block closure

   Blocks opened with do must close with end, not }.

3. Final Answer:

   Incorrect use of curly brace to close a do..end block -> Option C

4. Quick Check:

   do..end blocks cannot close with braces [OK]

Hint: Match do with end, braces with braces [OK]

Common Mistakes:

• Mixing do..end with curly braces
• Forgetting to close the block
• Misplacing block parameter pipes

5. Given the array [1, 2, 3, 4], which block syntax correctly filters even numbers and returns them as a new array?

hard

A. [1, 2, 3, 4].select { |n| n.even? }

B. [1, 2, 3, 4].select do |n| n.even?

C. [1, 2, 3, 4].select { n.even? }

D. [1, 2, 3, 4].select do n.even? end

Solution

1. Step 1: Understand the select method and block parameters

   select filters elements for which the block returns true. The block must receive each element as a parameter.

2. Step 2: Check syntax correctness

   [1, 2, 3, 4].select { |n| n.even? } correctly uses curly braces with block parameter |n| and calls n.even?. [1, 2, 3, 4].select do |n| n.even? has a syntax error (missing end). [1, 2, 3, 4].select { n.even? } misses block parameters. [1, 2, 3, 4].select do n.even? end misses pipes around parameter.

3. Final Answer:

   [1, 2, 3, 4].select { |n| n.even? } -> Option A

4. Quick Check:

   Curly braces with pipes for single-line filter [OK]

Hint: Use { |var| condition } for concise filtering blocks [OK]

Common Mistakes:

• Omitting block parameters
• Mixing do..end without pipes for parameters
• Using incorrect block syntax causing errors