Lambda creation and behavior in Ruby - Mini Project: Build & Apply

Lambda creation and behavior

📖 Scenario: You are working on a small Ruby program that uses lambdas to perform simple math operations. Lambdas are like little machines that take input and give output. You will create a lambda, set up a value to use with it, run the lambda, and then show the result.

🎯 Goal: Build a Ruby program that creates a lambda to double a number, applies it to a given number, and prints the result.

📋 What You'll Learn

Create a lambda called double that takes one argument and returns it multiplied by 2

Create a variable called number and set it to 7

Call the double lambda with number as input and store the result in doubled_number

Print the value of doubled_number

💡 Why This Matters

🌍 Real World

Lambdas are useful for small tasks you want to reuse, like math operations or filtering lists.

💼 Career

Understanding lambdas helps you write cleaner, more flexible Ruby code, which is valuable in many programming jobs.

1

Create a lambda called double

Create a lambda called double that takes one argument named x and returns x * 2.

Ruby

# Create a lambda called double that doubles its input
# Your code here

Hint

Use ->(x) { } to create a lambda in Ruby.

2

Create a variable number with value 7

Create a variable called number and set it to 7.

Ruby

double = ->(x) { x * 2 }
# Create a variable number and set it to 7
# Your code here

Hint

Just write number = 7 to store the number.

3

Call the lambda and store the result

Call the lambda double with number as input and store the result in a variable called doubled_number.

Ruby

double = ->(x) { x * 2 }
number = 7
# Call double with number and store in doubled_number
# Your code here

Hint

Use double.call(number) to run the lambda.

4

Print the doubled number

Print the value of doubled_number using puts.

Ruby

double = ->(x) { x * 2 }
number = 7
doubled_number = double.call(number)
# Print doubled_number
# Your code here

Hint

Use puts doubled_number to show the result.

Practice

1. What is a lambda in Ruby?

easy

A. A type of loop that repeats code multiple times

B. A special variable that holds numbers only

C. A method that automatically runs when a program starts

D. A small block of code stored in a variable that can be called later

Solution

1. Step 1: Understand what a lambda represents

   A lambda is a reusable piece of code stored in a variable, like a mini-function.

2. Step 2: Compare with other options

   Options A, C, and D describe loops, methods, or variables, not lambdas.

3. Final Answer:

   A small block of code stored in a variable that can be called later -> Option D

4. Quick Check:

   Lambda = reusable code block [OK]

Hint: Lambdas are like mini-functions stored in variables [OK]

Common Mistakes:

• Confusing lambdas with loops
• Thinking lambdas run automatically
• Believing lambdas hold only numbers

2. Which of the following is the correct way to create a lambda in Ruby?

easy

A. ->(x) { x * 2 }

B. def lambda(x) x * 2 end

C. lambda(x) => x * 2

D. lambda(x) { x * 2 }

Solution

1. Step 1: Identify valid lambda syntax

   In Ruby, lambdas can be created using the arrow syntax ->(params) { code } or lambda { |params| code }.

2. Step 2: Check each option

   lambda(x) { x * 2 }: invalid syntax (lambda keyword expects block with |params|, not parenthesized args). def lambda(x) x * 2 end: method definition, not a lambda. lambda(x) => x * 2: invalid syntax. ->(x) { x * 2 }: correct arrow syntax.

3. Final Answer:

   ->(x) { x * 2 } -> Option A

4. Quick Check:

   Arrow syntax ->(x) { ... } creates lambda [OK]

Hint: Use ->(params) { code } or lambda { |params| code } [OK]

Common Mistakes:

• Using def instead of lambda
• Incorrect arrow syntax
• Confusing method and lambda syntax

3. What will be the output of this Ruby code?

my_lambda = ->(x) { x + 3 }
puts my_lambda.call(4)

medium

A. 3

B. 4

C. 7

D. Error

Solution

1. Step 1: Understand the lambda behavior

   The lambda adds 3 to the input number x.

2. Step 2: Calculate the output

   Calling my_lambda.call(4) computes 4 + 3 = 7.

3. Final Answer:

   7 -> Option C

4. Quick Check:

   4 + 3 = 7 [OK]

Hint: Call lambda with .call and input to get output [OK]

Common Mistakes:

• Forgetting to use .call to invoke lambda
• Confusing input and output values
• Expecting syntax error

4. What is wrong with this Ruby code?

my_lambda = lambda { |x, y| x + y }
puts my_lambda.call(5)

medium

A. It will raise an ArgumentError

B. It will print nil

C. It will print 5

D. It will raise a SyntaxError

Solution

1. Step 1: Check lambda argument requirements

   Lambdas check arguments strictly. This lambda expects 2 arguments (x and y).

2. Step 2: Analyze the call with one argument

   Calling my_lambda.call(5) provides only 1 argument, causing ArgumentError.

3. Final Answer:

   It will raise an ArgumentError -> Option A

4. Quick Check:

   Lambda argument count mismatch = ArgumentError [OK]

Hint: Lambdas need exact argument count or error [OK]

Common Mistakes:

• Assuming missing arguments default to nil
• Expecting no error on fewer arguments
• Confusing lambdas with procs (which are lenient)

5. Given this code, what will be the output?

def test_lambda
  l = -> { return "Inside lambda" }
  result = l.call
  return "Outside lambda"
end
puts test_lambda

hard

A. Inside lambda

B. Outside lambda

C. nil

D. Error

Solution

1. Step 1: Understand lambda return behavior

   Return inside a lambda exits only the lambda, not the enclosing method.

2. Step 2: Trace method execution

   The lambda returns "Inside lambda" to variable result, but method continues and returns "Outside lambda".

3. Final Answer:

   Outside lambda -> Option B

4. Quick Check:

   Lambda return exits lambda only, method returns after [OK]

Hint: Return inside lambda exits lambda, not method [OK]

Common Mistakes:

• Thinking return inside lambda exits method
• Expecting "Inside lambda" as output
• Confusing lambda with proc return behavior