Ruby conditional judgment

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

Ruby if...else Statement

Syntax

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

if Expressions are used for conditional execution. The values ​​false and nil are false, and all other values ​​are true. Note that Ruby uses elsif, not else if and elif.

If conditional is true, code is executed. If conditional is not true, the code specified in the else clause is executed.

Usually we omit the reserved word then. If you want to write the complete if formula in one line, you must separate the conditional formula and the program block with then. As shown below:

if a == 4 then a = 7 end

Example

#!/usr/bin/ruby
# -*- coding: UTF-8 -*-

x=1
if x > 2
   puts "x 大于 2"
elsif x <= 2 and x!=0
   puts "x 是 1"
else
   puts "无法得知 x 的值"
end

The above example output result:

x 是 1

Ruby if Modifier

Grammar

code if condition

The if modifier phrase means that the formula on the left side of if will only be executed when the condition on the right side of if is true. That is, if conditional is true, execute code.

Example

#!/usr/bin/ruby

$debug=1
print "debug\n" if $debug

The above example output result:

debug

Ruby unless Statement

Grammar

unless conditional [then]
   code
[else
   code ]
end

unless expression and if expression have the opposite effect, that is, if conditional is false, code will be executed. If conditional is true, execute the code specified in the else clause.

Example

#!/usr/bin/ruby
# -*- coding: UTF-8 -*-

x=1
unless x>2
   puts "x 小于 2"
 else
  puts "x 大于 2"
end

The output result of the above example is:

x 小于 2

Ruby unless Modifier

Syntax

code unless conditional

If conditional is false, code is executed.

Example

#!/usr/bin/ruby
# -*- coding: UTF-8 -*-

$var =  1
print "1 -- 这一行输出\n" if $var
print "2 -- 这一行不输出\n" unless $var

$var = false
print "3 -- 这一行输出\n" unless $var

The above example output result:

1 -- 这一行输出
3 -- 这一行输出

Ruby case Statement

Syntax

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

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

It uses the === operator to compare the expression specified by when. If they are consistent, the when part is executed. Content.

Usually we omit the reserved word then. If you want to write the complete when expression in one line, you must separate the conditional expression and the program block with then. It looks like this:

when a == 4 then a = 7 end

So:

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

is basically similar to:

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

Instance

#!/usr/bin/ruby
# -*- coding: UTF-8 -*-

$age =  5
case $age
when 0 .. 2
    puts "婴儿"
when 3 .. 6
    puts "小孩"
when 7 .. 12
    puts "child"
when 13 .. 18
    puts "少年"
else
    puts "其他年龄段的"
end

Above The example output result is:

小孩

When the "expression" part of the case is omitted, the first when condition part is true expression will be 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
# 显示 "bar is true"