What are the reserved words BEGIN or END used for

问题:

This is a very hard to find word because in most cases they are not sensitive during a search. The best I could find outside of documentation is a test in IRB.

 BEGIN{puts x = 10}
 10

回答1:

As all keywords BEGIN and END are documented as public instance methods of Object (even though you won't see them returned from Object.public_instance_methods)

BEGIN Designates, via code block, code to be executed unconditionally before sequential execution of the program begins. Sometimes used to simulate forward references to methods.

puts times_3(gets.to_i)

BEGIN {
  def times_3(n)
    n * 3
  end
}

END Designates, via code block, code to be executed just prior to program termination.

END { 
  puts "Bye!" 
}

Some more detailed explanation from Programming Ruby The Pragmatic Programmer's Guide

BEGIN and END Blocks

Every Ruby source file can declare blocks of code to be run as the file is being loaded (the BEGIN blocks) and after the program has finished executing (the END blocks).

BEGIN {   
  begin code 
}

END {
  end code 
}

A program may include multiple BEGIN and END blocks. BEGIN blocks are executed in the order they are encountered. END blocks are executed in reverse order.

回答2:

One thing that hasn't been mentioned is that in earlier versions of Ruby, BEGIN was unconditional:

if false
  BEGIN { puts "Up is down, hot is cold, good is evil!" }
end

If you try that with Ruby 1.8.7, the sentence is printed, even though it's in the branch of if that isn't taken.

Under Ruby 2.0.0, it's a syntax error to use BEGIN outside of the top-level (a much smarter way to handle that):

unconditional.rb:2: BEGIN is permitted only at toplevel
  BEGIN { puts "Up is down, hot is cold, good is evil!" }
       ^

Edit: In a way, nobody has answered the question you raise in your comment: Why does Ruby have BEGIN at all? I'll try. BEGIN comes to Ruby (like many things) from Perl. Perl has it because it existed in awk. It made a ton of sense in awk because by default, an awk file consists of a series of patterns and actions:

/foo/ { print $1 }
/bar/ { print $2 }

Every pattern is checked for every line. If the pattern matches, then the action is performed. Otherwise, awk moves on to the next pattern. So in the mini script above, if the line matches 'foo', then the first field is printed. If the line matches 'bar', then the second field is printed.

But by now you can see the gap that BEGIN (and END) blocks fill: What if you want to do something unconditionally before any intput has been tested or after all the input has been seen (like print a header at the top of your report or print a row of totals at the end of the report)? Normal awk lines of pattern + action can't help you there.

That's why BEGIN and END exist. But I'm not sure how useful they are for modern, idiomatic Ruby scripts. But as dbenhur points out in the comments, you can still use Ruby very well for awk-like one-liners. (I also have a recollection that MiniTest, the standard Ruby testing library, used to use an at_exit function for testing, but I'm not sure it does any longer.)

Two good links about Ruby, awk and Ruby one-liners:

• AWK-ward Ruby by Ryan Tomayko
• Ruby one-liners

回答3:

From The Ruby Programming Language:

---

BEGIN and END Blocks

Every Ruby source file can declare blocks of code to be run as the file is being loaded (the BEGIN blocks) and after the program has finished executing (the END blocks).

BEGIN {
  # begin code
} 
END {
  # end code
}

A program may include multiple BEGIN and END blocks. BEGIN blocks are executed in the order they are encountered. END blocks are executed in reverse order.

---

So:

$ cat beginend.rb
END { puts :end }
BEGIN { puts :begin }
END { puts :end2 }
BEGIN { puts :begin2 }

puts :run
$ ruby beginend.rb 
begin
begin2
run
end2
end

回答4:

The BEGIN block is exactly what you may assume, and that is that the block given will run before the rest of the code in your program.

This being an example.

puts "Goodbye cruel world!"

BEGIN {
puts "Hello World!"
}

I hope that helps.

There is a working example of this in a minitest where a collection of values is put out of the way at the end of the file, but evaluated first.

回答5:

the BEGIN/END is really handy when using the -e option to process a stream. For example, to total a file of numbers:

cat <<EOF > numbers
1
5 
10
20
EOF

cat numbers | ruby -ane 'BEGIN { $t=0}; END {puts $t}; $t += $_.to_i'

note how the BEGIN zeros out the global, and the END prints the result.

回答6:

BEGIN and END is also used to comment such as:

=begin
This is a comment line
It can explain what the rest of the program is about
This was inspired from the perl style of programming
=end

You can check the same here: https://en.wikipedia.org/wiki/Comparison_of_programming_languages_(syntax)#Comments