Ruby 简明教程

Ruby - if…​else, case, unless

Ruby 提供条件结构，这是现代语言中非常普遍的。在此，我们将解释 Ruby 中提供的所有条件语句和修饰符。

Ruby offers conditional structures that are pretty common to modern languages. 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 表达式用于条件执行。值 false 和 nil 为 false，其他所有值为 true。请注意，Ruby 使用 elsif，而不是 else if 或 elif。

if expressions are used for conditional execution. The values false and nil are false, and everything else are true. Notice Ruby uses elsif, not else if nor elif.

在条件为真时执行代码。如果条件不为真，则执行 else 子句中指定代码。

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

if 表达式的条件由保留字 then、换行符或分号与代码分隔。

An if expression’s conditional is separated from code by the reserved word then, a newline, or a semicolon.

Example

#!/usr/bin/ruby

x = 1
if x > 2
   puts "x is greater than 2"
elsif x <= 2 and x!=0
   puts "x is 1"
else
   puts "I can't guess the number"
end
x is 1

Ruby if modifier

Syntax

code if condition

在条件为真时执行代码。

Executes code if the conditional is true.

Example

#!/usr/bin/ruby

$debug = 1
print "debug\n" if $debug

这会产生以下结果 −

This will produce the following result −

debug

Ruby unless Statement

Syntax

unless conditional [then]
   code
[else
   code ]
end

在条件为假时执行代码。如果条件为真，则执行 else 子句中指定代码。

Executes code if conditional is false. If the conditional is true, code specified in the else clause is executed.

Example

#!/usr/bin/ruby

x = 1
unless x>=2
   puts "x is less than 2"
 else
   puts "x is greater than 2"
end

这会产生以下结果 −

This will produce the following result −

x is less than 2

Ruby unless modifier

Syntax

code unless conditional

在条件为假时执行代码。

Executes code if conditional is false.

Example

#!/usr/bin/ruby

$var =  1
print "1 -- Value is set\n" if $var
print "2 -- Value is set\n" unless $var

$var = false
print "3 -- Value is set\n" unless $var

这会产生以下结果 −

This will produce the following result −

1 -- Value is set
3 -- Value is set

Ruby case Statement

Syntax

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

比较用例指定的表达式和使用 === 运算符指定的表达式，并执行与之匹配的 when 子句的代码。

Compares the expression specified by case and that specified by when using the === operator and executes the code of the when clause that matches.

用例指定的表达式作为左操作数进行评估。如果没有与之匹配的 when 子句，case 将执行 else 子句的代码。

The expression specified by the when clause is evaluated as the left operand. If no when clauses match, case executes the code of the else clause.

when 语句的表达式由保留字 then、换行符或分号与代码分隔。因此：

A when statement’s expression is separated from code by the reserved word then, a newline, or a semicolon. Thus −

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

基本上类似于以下内容：

is basically similar to the following −

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

Example

#!/usr/bin/ruby

$age =  5
case $age
when 0 .. 2
   puts "baby"
when 3 .. 6
   puts "little child"
when 7 .. 12
   puts "child"
when 13 .. 18
   puts "youth"
else
   puts "adult"
end

这会产生以下结果 −

This will produce the following result −

little child