Flow Control statements in Ruby

Flow control uses a conditional statement to perform decision making operations based on that conditional statement. Decision making process in programming is similar to decision making in real life. For example, a certain block of code needs to be executed when some condition is fulfilled.

A programming language uses conditional statements to control the flow of execution of the program based on certain conditions.

There are various types of flow control statements in Ruby.

• if, elsif, else Statement
• ?: ternary operator
• unless statement
• case statement
• and and or

Ruby if, elsif, else statement

This is the basic type of conditional statement. It executes a code block if condition is true. If condition is false, code block of else clause is executed.

Syntax

if condition [then]
   code...
elsif condition [then]
   code...
else
   code...
end

The elsif, then, and else part are optionals and required on usage basis.

Example

number = 10

answer = if number.even?
           "Number is even"
         elsif number.odd?
           "Number is odd"
         else
           "Invalid number"
         end

puts answer

if modifier

Executes code if the condition evaluates as true. This can be used when the else part is unnecessary. This is also called a “guard clause”, or an “if statement as a guard clause”.

number = 10
puts "Number is even" if number.even?

Ruby Ternary Operator ( ?: )

The ternary operator results to only one branch being evaluated based on a conditional statements evaluation. It is used in place of the if-else statement.

Syntax

# Pseudo-code
condition ? evaluate this statement if condition is true : evaluate this statement if condition false

Example

5 < 10 ? puts("5 is less than 10") : puts("5 is equal to or greater than 10")

The output would be:

5 is less than 10

unless statement

It is similar to if-else statement however it executes the code block when the condition evaluates as false. If the condition evaluates as true, the code block in the else clause is executed.

Syntax

unless condition [then]
  code
else
  code
end

The else part is optional and required on usage basis.

The else is discouraged. If there is an else statement with unless, then we might consider that the if statement is “upside down”.

Example

state = "hungry"

unless state == 'hungry'
  puts "I can wait to eat."
else
  puts "I need to eat."
end

The output would be:

I need to eat.

unless modifier

Executes code if condition is false. This can be used when else part is unnecessary.

stop = false
puts "Continue coding ..." unless stop

Ruby case statement

Ruby case statement is similar to switch statement in other languages.

Syntax

case expression
when expression [, expression ...] [then]
  code
else
  code
end

It compares the expression specified by case with the condition of when clause(s) and execute the code block of matching when clause.

If there is no when clauses match, case executes the code of the else clause.

Example

number = 5
case
when number < 5 then
  puts "Number is less than 5"
when number == 5 then
  puts "Number equals to 5"
when number > 5 then
  puts "Number is greater than 5"
else
  puts "Invalid number"
end

# output
Number equals to 5

Ruby case is basically similar to if-elsif-else statement.

The above example can be translated as:

number = 5
if number < 5
  puts "Number is less than 5"
elsif number == 5
  puts "Number equals to 5"
elsif number > 5
  puts "Number is greater than 5"
else
  puts "Invalid number"
end