Ruby Conditional Statements

Ruby provides several very common conditional structures. Here, we will explain all the conditional statements and the modifiers available in Ruby.

Ruby if...else statement

syntax

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

if expression is used for conditional execution. Value false and nil False, all other values are true. Please note that Ruby uses elsif, not else if and elif.

if conditional If it is true, then execute code. If conditional is not true, then execute the else clause specified code.

Generally, we omit the keyword 'then'. If you want to write the complete 'if' statement on a single line, you must separate the condition from the code block with 'then'. As shown below:

if a ==　4　then a =　7　end

Online Example

!#/usr/bin/ruby
#　-*-　coding: UTF-8　-*-
　
x=1
if x >　2
　　　puts "x is greater than　2"
elsif x <=　2　and x!=0
　　　puts "x is　1"
else
　　　puts "Unable to determine the value of x"
end

The output result of the above example is:

x is　1

Ruby if modifier

syntax

code if condition

The 'if' modifier group indicates that the expression on the left of 'if' is executed only when the condition on the right of 'if' is true. That is, if conditional If it is true, then execute code.

Online Example

!#/usr/bin/ruby
　
$debug=1
print "debug\n" if $debug

The output result of the above example is:

debug

Ruby unless statement

syntax

unless conditional [then]
　　　code
[else
　　　code ]
end

The 'unless' statement is the opposite of the 'if' statement, that is, if conditional If it is false, then execute code. If conditional If it is true, then execute the else clause specified code.

Online Example

!#/usr/bin/ruby
#　-*-　coding: UTF-8　-*-
　
x=1
unless x>2
　　　puts "x is less than　2"
　else
　　puts "x is greater than　2"
end

The output result of the above example is:

x is less than　2

Ruby unless modifier

syntax

code unless conditional

if conditional If it is false, then execute code.

Online Example

!#/usr/bin/ruby
#　-*-　coding: UTF-8　-*-
　
$var =　　1
print "1　--　This line outputs
"　if　$var
print "2　--　This line does not output
"　unless　$var
　
$var = false
print "3　--　This line outputs
"　unless　$var

The output result of the above example is:

1　--　This line outputs
3　--　This line outputs

Ruby case statement

syntax

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

case first checks a expression for matching judgment, and then performs branch selection based on the matching result.

It uses ===operator comparison when specified expression, if consistent, execute when Part of the content.

Generally, we omit the keyword 'then'. If you want to write the complete 'when' statement on a single line, you must separate the condition from the code block with 'then'. As shown below:

when a ==　4　then a =　7　end

Therefore:

case expr0
when expr1, expr2
　　　stmt1
when expr3, expr4
　　　stmt2
else
　　　stmt3
end

Basically similar to:

_tmp = expr0
if expr1　=== _tmp || expr2　=== _tmp
　　　stmt1
elsif expr3　=== _tmp || expr4　=== _tmp
　　　stmt2
else
　　　stmt3
end

Online Example

!#/usr/bin/ruby
#　-*-　coding: UTF-8　-*-
　
$age =　　5
case $age
when 0 ..　2
　　　　puts "Infant"
when　3　..　6
　　　　puts "Child"
when　7　..　12
　　　　puts "child"
when　13　..　18
　　　　puts "Youth"
else
　　　　puts "Other age groups"
end

The output result of the above example is:

Child

When the 'expression' part of case is omitted, the expression of the first when condition that is true is calculated.

foo = false
bar = true
quu = false
　
case
when foo then puts 'foo is true'
when bar then puts 'bar is true'
when quu then puts 'quu is true'
end
# Display "bar is true"