Send for calling methods dynamically in Ruby

Introduction

Sometimes, you want to call a method when you only know its name during the program. send helps you do that easily.

You want to call different methods based on user input.

You need to run a method whose name is stored in a variable.

You want to avoid writing many if-else statements for method calls.

You want to make your code flexible and reusable.

You want to call private methods from outside the class (careful!).

Syntax

Ruby

object.send(:method_name, arg1, arg2, ...)

object is where the method lives.

:method_name is a symbol or string of the method's name.

Examples

Calls the upcase method on the string name dynamically.

Ruby

name = "hello"
puts name.send(:upcase)

Calls the greet method with argument "Alice" dynamically.

Ruby

class Greeter
  def greet(name)
    "Hello, #{name}!"
  end
end

g = Greeter.new
puts g.send(:greet, "Alice")

Calls the method add dynamically using a variable holding the method name.

Ruby

def add(a, b)
  a + b
end

method_name = :add
puts send(method_name, 5, 3)

Sample Program

This program creates a calculator object and calls the multiply method dynamically using send. It prints the result.

Ruby

class Calculator
  def add(a, b)
    a + b
  end

  def multiply(a, b)
    a * b
  end
end

calc = Calculator.new
method_to_call = :multiply
result = calc.send(method_to_call, 4, 5)
puts "Result: #{result}"

Important Notes

Be careful when using send with user input to avoid security risks.

send can call private methods, so use it wisely.

Summary

send lets you call methods when you only know their names during the program.

It makes your code flexible and helps avoid many if-else statements.

Use symbols or strings for method names, and pass arguments after the method name.

Practice

1. What does the Ruby method send do when used like obj.send(:method_name)?

easy

A. Calls the method named method_name on obj dynamically.

B. Creates a new method called method_name on obj.

C. Deletes the method named method_name from obj.

D. Returns the name of the method as a string.

Solution

1. Step 1: Understand send usage

   The send method calls a method on an object using the method's name given as a symbol or string.

2. Step 2: Analyze the example obj.send(:method_name)

   This calls the method named method_name on obj dynamically at runtime.

3. Final Answer:

   Calls the method named method_name on obj dynamically. -> Option A

4. Quick Check:

   send calls methods dynamically [OK]

Hint: Remember: send calls methods by name on objects [OK]

Common Mistakes:

• Thinking send creates or deletes methods
• Confusing send with method name retrieval
• Assuming send returns method names as strings

2. Which of the following is the correct syntax to call a method named greet on object user using send?

easy

A. user.send(greet)

B. user.send('greet')()

C. user.send['greet']

D. user.send(:greet)

Solution

1. Step 1: Recall send syntax

   The method name should be passed as a symbol or string without parentheses after send.

2. Step 2: Check each option

   user.send(greet) misses quotes or symbol, causing error. user.send(:greet) correctly uses a symbol :greet. user.send('greet')() wrongly adds parentheses after send call. user.send['greet'] uses incorrect bracket syntax.

3. Final Answer:

   user.send(:greet) -> Option D

4. Quick Check:

   Use symbol for method name in send [OK]

Hint: Use colon before method name symbol in send [OK]

Common Mistakes:

• Passing method name without quotes or colon
• Adding parentheses after send call
• Using brackets [] instead of parentheses ()

3. What is the output of this Ruby code?

class Person
  def greet(name)
    "Hello, #{name}!"
  end
end

p = Person.new
puts p.send(:greet, "Alice")

medium

A. Hello, Alice!

B. greet

C. NoMethodError

D. Hello, name!

Solution

1. Step 1: Understand the method call with send

   The send method calls greet on p with argument "Alice".

2. Step 2: Evaluate the method output

   The greet method returns "Hello, Alice!" which is printed by puts.

3. Final Answer:

   Hello, Alice! -> Option A

4. Quick Check:

   send calls greet with argument "Alice" [OK]

Hint: send passes arguments after method name [OK]

Common Mistakes:

• Expecting method name string as output
• Confusing argument passing with no arguments
• Thinking send returns method object

4. Identify the error in this Ruby code snippet:

class Calculator
  def add(a, b)
    a + b
  end
end

calc = Calculator.new
calc.send(:add, 5)

medium

A. send cannot be used with arguments

B. Method add does not exist

C. Missing second argument for method add

D. Syntax error in method definition

Solution

1. Step 1: Check method definition

   The method add requires two arguments: a and b.

2. Step 2: Analyze the send call

   The send call provides only one argument (5), missing the second argument.

3. Final Answer:

   Missing second argument for method add -> Option C

4. Quick Check:

   Method arguments must match when using send [OK]

Hint: Match method arguments count when using send [OK]

Common Mistakes:

• Passing fewer arguments than method requires
• Assuming send works without correct arguments
• Thinking send changes method signature

5. You want to call different arithmetic methods (add, subtract, multiply) on a Calculator object dynamically based on user input stored in operation. Which Ruby code correctly uses send to achieve this?

class Calculator
  def add(a, b); a + b; end
  def subtract(a, b); a - b; end
  def multiply(a, b); a * b; end
end

calc = Calculator.new
operation = :multiply
# Fill in the blank below to call the method dynamically
result = ???
puts result

hard

A. calc.send(:operation, 4, 5)

B. calc.send(operation, 4, 5)

C. calc.send('operation', 4, 5)

D. calc.send(operation())

Solution

1. Step 1: Understand dynamic method calling

   The variable operation holds the symbol of the method to call.

2. Step 2: Use send with variable method name and arguments

   Passing operation as the first argument to send calls the correct method with arguments 4 and 5.

3. Final Answer:

   calc.send(operation, 4, 5) -> Option B

4. Quick Check:

   send uses variable method names with arguments [OK]

Hint: Pass method name variable and args directly to send [OK]

Common Mistakes:

• Passing method name as string literal instead of variable
• Adding parentheses after variable name incorrectly
• Using symbol literal instead of variable for method name