Parameters with default values in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a parameter with a default value in Ruby?

A parameter with a default value is a method parameter that has a preset value. If no argument is given when calling the method, Ruby uses this default value.

beginner

How do you define a method with a default parameter in Ruby?

You assign a value to the parameter in the method definition, like def greet(name = "Friend"). If no argument is passed, "Friend" is used.

beginner

What happens if you call a Ruby method with a default parameter and provide an argument?

The provided argument replaces the default value for that call. The method uses the argument you give instead of the default.

intermediate

Can you mix parameters with and without default values in Ruby methods?

Yes, you can. Parameters without default values should come first, followed by those with default values to avoid errors.

beginner

Example: What does this method print?

def say(message = "Hello")
  puts message
end

say
say("Hi")

It prints:

Hello
Hi

First call uses the default "Hello". Second call uses the argument "Hi".

In Ruby, how do you set a default value for a method parameter?

A By setting the value inside the method body

B By assigning a value in the method definition, like <code>def m(param = 5)</code>

C By calling the method with no arguments

D By using a global variable

Default values are set by assigning them in the method definition parameters.

What happens if you call a Ruby method with a default parameter but provide an argument?

A The default value is ignored and the argument is used

B An error occurs

C The method uses the default value anyway

D The method ignores the argument

The argument you provide replaces the default value for that call.

Which order should parameters be in when mixing default and non-default values in Ruby?

A Default parameters first, then non-default

B Only default parameters are allowed

C Order does not matter

D Non-default parameters first, then default parameters

Non-default parameters must come before default parameters to avoid syntax errors.

What will this Ruby code print?

def greet(name = "Guest")
  puts "Hi, #{name}!"
end
greet("Alice")

A Hi, Alice!

B Hi, Guest!

C Hi, !

D Error

The argument "Alice" replaces the default "Guest".

Why are default parameters useful in Ruby methods?

A They force all arguments to be provided

B They make methods slower

C They allow methods to be called with fewer arguments

D They prevent method calls

Default parameters let you skip arguments and still have meaningful values.

Explain how to use parameters with default values in Ruby methods and why they are helpful.

Think about how you can make a method flexible with fewer required inputs.

Describe the correct order of parameters when mixing default and non-default values in Ruby methods and why order matters.

Consider how Ruby matches arguments to parameters.

Practice

1. What does a default parameter value in a Ruby method do?
def greet(name = "Guest")

easy

A. It causes a syntax error if the argument is missing.

B. It makes the parameter mandatory every time the method is called.

C. It changes the method name automatically.

D. It allows the method to be called without that argument, using the default instead.

Solution

1. Step 1: Understand default parameter purpose

   Default parameters provide a fallback value if no argument is given.

2. Step 2: Apply to the example method

   In def greet(name = "Guest"), if no name is passed, "Guest" is used.

3. Final Answer:

   It allows the method to be called without that argument, using the default instead. -> Option D

4. Quick Check:

   Default parameter = fallback value [OK]

Hint: Default means optional argument with fallback value [OK]

Common Mistakes:

• Thinking default parameters are mandatory
• Assuming missing arguments cause errors
• Confusing default values with method overloading

2. Which of the following is the correct syntax to define a method with a default parameter in Ruby?

easy

A. def add(a, b = 5); a + b; end

B. def add(a =, b 5); a + b; end

C. def add(a, b: 5); a + b; end

D. def add(a, b); b = 5; a + b; end

Solution

1. Step 1: Review Ruby default parameter syntax

   Default values are set with = value after the parameter name.

2. Step 2: Check each option

   def add(a, b = 5); a + b; end uses b = 5 correctly. def add(a =, b 5); a + b; end has invalid syntax. def add(a, b: 5); a + b; end uses keyword argument style, not default parameter. def add(a, b); b = 5; a + b; end assigns inside method body, not default parameter.

3. Final Answer:

   def add(a, b = 5); a + b; end -> Option A

4. Quick Check:

   Default param syntax = param = value [OK]

Hint: Use = after parameter name for default values [OK]

Common Mistakes:

• Using colon instead of equals for default values
• Placing = before parameter name
• Assigning default inside method body instead of parameter list

3. What is the output of this Ruby code?

def multiply(x, y = 2)
  x * y
end

puts multiply(4)
puts multiply(4, 3)

medium

A. 8\n12

B. 4\n7

C. Error: missing argument

D. 6\n12

Solution

1. Step 1: Analyze method calls with default parameter

   First call: multiply(4) uses default y=2, so 4*2=8. Second call: multiply(4,3) uses y=3, so 4*3=12.

2. Step 2: Determine output of puts statements

   Outputs are printed on separate lines: 8 then 12.

3. Final Answer:

   8 12 -> Option A

4. Quick Check:

   Default param used when missing = 8, explicit param = 12 [OK]

Hint: Default used if argument missing, else given value [OK]

Common Mistakes:

• Expecting error when argument is missing
• Confusing default parameter with keyword arguments
• Mixing up output values

4. Identify the error in this Ruby method definition:

def greet(name = "Guest", age)
  "Hello #{name}, you are #{age} years old."
end

medium

A. Method name cannot be 'greet' with parameters.

B. Missing return statement causes error.

C. Default parameters must come after non-default parameters.

D. String interpolation syntax is incorrect.

Solution

1. Step 1: Check parameter order rules in Ruby

   Parameters with default values must be placed after parameters without defaults.

2. Step 2: Analyze given method signature

   Here, name has default, but age does not and comes after. This causes syntax error.

3. Final Answer:

   Default parameters must come after non-default parameters. -> Option C

4. Quick Check:

   Default params after required params [OK]

Hint: Put default params after required ones [OK]

Common Mistakes:

• Placing default parameters before required ones
• Assuming return statement is mandatory
• Misreading string interpolation syntax

5. You want to define a method order that takes a product name and an optional quantity with default 1. Which is the best way to define it to allow calling order("apple") and order("apple", 5)?

hard

A. def order(product = "apple", quantity); "Order #{quantity} #{product}(s)"; end

B. def order(product, quantity = 1); "Order #{quantity} #{product}(s)"; end

C. def order(quantity = 1, product); "Order #{quantity} #{product}(s)"; end

D. def order(product, quantity); quantity ||= 1; "Order #{quantity} #{product}(s)"; end

Solution

1. Step 1: Identify correct parameter order and defaults

   Required parameters come first, then optional with default values.

2. Step 2: Evaluate each option

   def order(product, quantity = 1); "Order #{quantity} #{product}(s)"; end correctly places product first and quantity with default 1 second. def order(product = "apple", quantity); "Order #{quantity} #{product}(s)"; end has default before required, invalid. def order(quantity = 1, product); "Order #{quantity} #{product}(s)"; end swaps order incorrectly. def order(product, quantity); quantity ||= 1; "Order #{quantity} #{product}(s)"; end assigns default inside method but requires both arguments.

3. Final Answer:

   def order(product, quantity = 1); "Order #{quantity} #{product}(s)"; end -> Option B

4. Quick Check:

   Required first, default second = def order(product, quantity = 1); "Order #{quantity} #{product}(s)"; end [OK]

Hint: Put required params first, defaults after [OK]

Common Mistakes:

• Placing default parameter before required
• Assigning default inside method body instead of parameter list
• Confusing parameter order