This post originated from an RSS feed registered with Ruby Buzz
by Jeremy Voorhis.
Original Post: Ruby's Hidden do {} while () Loop
Feed Title: JVoorhis
Feed URL: http://feeds.feedburner.com/jvoorhis
Feed Description: JVoorhis is a Rubyist in northeast Ohio. He rambles about Ruby on Rails, development practices, other frameworks such as Django, and on other days he is just full of snark.
I found the following snippet while reading the source for Tempfile#initialize in the Ruby core library:
begin
tmpname = File.join(tmpdir, make_tmpname(basename, n))
lock = tmpname + '.lock'
n += 1
end while @@cleanlist.include?(tmpname) or
File.exist?(lock) or File.exist?(tmpname)
At first glance, I assumed the while modifier would be evaluated before the contents of begin...end, but that is not the case. Observe:
>> begin
?> puts "do {} while ()"
>> end while false
do {} while ()
=> nil
As you would expect, the loop will continue to execute while the modifier is true.
>> n = 3
=> 3
>> begin
?> puts n
>> n -= 1
>> end while n > 0
3
2
1
=> nil
While I would be happy to never see this idiom again, begin...end is quite powerful. The following is a common idiom to memoize a one-liner method with no params:
def expensive
@expensive ||= 2 + 2
end
Here is an ugly, but quick way to memoize something more complex:
def expensive
@expensive ||=
begin
n = 99
buf = ""
begin
buf << "#{n} bottles of beer on the wall\n"
# ...
n -= 1
end while n > 0
buf << "no more bottles of beer"
end
end
Previous Topic
