Parameters with default values in Ruby - Step-by-Step Execution

Concept Flow - Parameters with default values

Define method with default parameters

↓

Call method with arguments?

No→Use default values

Yes↓

Assign passed arguments to parameters

↓

Execute method body

↓

Return result

↓

End

The method is defined with default values for parameters. When called, if arguments are missing, defaults are used; otherwise, passed values are used.

Execution Sample

Ruby

def greet(name = "Friend")
  "Hello, #{name}!"
end

puts greet("Alice")
puts greet

Defines a method with a default parameter and calls it with and without an argument.

Execution Table

Step	Method Call	Parameter 'name'	Action	Output
1	greet("Alice")	"Alice"	Use passed argument	"Hello, Alice!"
2	puts output	-	Print output	Hello, Alice!
3	greet	"Friend"	Use default value	"Hello, Friend!"
4	puts output	-	Print output	Hello, Friend!
5	-	-	End of calls	-

💡 No more method calls; execution ends.

Variable Tracker

Variable	Start	After Call 1	After Call 2	Final
name	-	"Alice"	"Friend"	-
output	-	"Hello, Alice!"	"Hello, Friend!"	-

Key Moments - 2 Insights

Why does the method use "Friend" when no argument is given?

Because the parameter 'name' has a default value "Friend" defined, so when no argument is passed (see execution_table row 3), it uses that default.

What happens if we pass an argument to the method?

The passed argument overrides the default value (see execution_table row 1), so 'name' becomes the passed value.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'name' during the second method call?

A nil

B "Alice"

C "Friend"

D "Hello"

💡 Hint

Check execution_table row 3 where the method is called without arguments.

At which step is the output "Hello, Alice!" printed?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table row 2 where puts prints the output.

If we change the default value to "Guest", what will be printed at step 4?

A "Hello, Guest!"

B "Hello, Friend!"

C "Hello, nil!"

D "Hello, Alice!"

💡 Hint

Refer to variable_tracker for default value changes affecting output.

Concept Snapshot

def method_name(param = default)
  # method body
end

- If argument is passed, param = argument
- If no argument, param = default
- Allows flexible calls with optional arguments

Full Transcript

This example shows how Ruby methods can have parameters with default values. When the method greet is called with an argument like "Alice", it uses that value. When called without an argument, it uses the default "Friend". The execution table traces each call and output. The variable tracker shows how 'name' changes. This helps beginners understand how default parameters work in Ruby methods.

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