红宝石 |关键词

关键字或保留字是一种语言中用于某些内部流程或表示某些预定义操作的字词。因此，这些词不允许用作变量名或对象或常量。这样做可能会导致编译时错误。

例子：

# Ruby program to illustrate Keywords 
  
#!/usr/bin/ruby
  
# here 'if' is a keyword
# it can't be used as variable
if = 20
  
# Here 'if' and 'end' are keywords.  
# if condition to check whether  
# your age is enough for voting 
if if >= 18
  puts "You are eligible to vote."
end

编译时错误：

Error(s), warning(s):

source_file.rb:7: syntax error, unexpected ‘=’
if = 20
^
source_file.rb:12: syntax error, unexpected >=
if if >= 18
^
source_file.rb:14: syntax error, unexpected keyword_end, expecting end-of-input

编程需要懂一点英语

Ruby 中共有 41 个关键字，如下所示：

Keyword	Description
__ENCODING__	The script encoding of the current file.
__LINE__	The line number of this keyword in the current file.
__FILE__	The path to the current file.
BEGIN	Runs before any other code in the current file.
END	Runs after any other code in the current file.
alias	Creates an alias between two methods (and other things).
and	Short-circuit Boolean and with lower precedence than &&
begin	Starts an exception handling block.
break	Leaves a block early.
case	Starts a case expression.
class	Creates or opens a class.
def	Defines a method.
defined?	Returns a string describing its argument.
do	Starts a block.
else	The unhandled condition in case, if and unless expressions.
elsif	An alternate condition for an if expression.
end	The end of a syntax block. Used by classes, modules, methods, exception handling and control expressions.
ensure	Starts a section of code that is always run when an exception is raised.
false	Boolean false.
for	A loop that is similar to using the each method.
if	Used for if and modifier if expressions.
in	Used to separate the iterable object and iterator variable in a for loop.
module	Creates or opens a module.
next	Skips the rest of the block.
nil	A false value usually indicating “no value” or “unknown”.
not	Inverts the following boolean expression. Has a lower precedence than !
or	Boolean or with lower precedence than \|\|
redo	Restarts execution in the current block.
rescue	Starts an exception section of code in a begin block.
retry	Retries an exception block.
return	Exits a method.
self	The object the current method is attached to.
super	Calls the current method in a superclass.
then	Indicates the end of conditional blocks in control structures.
true	Boolean true.
undef	Prevents a class or module from responding to a method call.
unless	Used for unless and modifier unless expressions.
until	Creates a loop that executes until the condition is true.
when	A condition in a case expression.
while	Creates a loop that executes while the condition is true.
yield	Starts execution of the block sent to the current method.

例子：

# Ruby program to illustrate the use of Keywords 
  
#!/usr/bin/ruby 
     
# defining class Vehicle 
# using the 'class' keyword
class GFG
     
# defining method 
# using 'def' keyword
def geeks 
     
    # printing result 
    puts "Hello Geeks!!"
     
# end of the method 
# using 'end' keyword
end
     
# end of class GFG 
# using 'end' keyword
end
     
# creating object 
obj = GFG.new
     
# calling method using object 
obj.geeks

输出：

Hello Geeks!!

参考： http://ruby-doc.org/docs/keywords/1.9/Object.html