Is there a "do ... while" loop in Ruby?
I'm using this code to let the user enter in names while the program stores them in an array until they enter an empty string (they must press enter after each name):
people = []
info = 'a' # must fill variable with something, otherwise loop won't execute
while not info.empty?
info = gets.chomp
people += [Person.new(info)] if not info.empty?
end
This code would look much nicer in a do ... while loop:
people = []
do
info = gets.chomp
people += [Person.new(info)] if not info.empty?
while not info.empty?
In this code I don't have to assign info to some random string.
Unfortunately this type of loop doesn't seem to exist in Ruby. Can anybody suggest a better way of doing this?
CAUTION :
The begin <code> end while <condition> is rejected by Ruby's author Matz. Instead he suggests using Kernel#loop , eg
loop do
# some code here
break if <condition>
end
For more details please refer to: http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/6745 (or via Wayback), and this wiki: http://rosettacode.org/wiki/Loops/Do-while#Ruby
Originally written by Jeremy Voorhis. The content has been copied here because it seems to have been taken down from the originating site. Copies can also be found in the Web Archive and at Ruby Buzz Forum. -Bill the Lizard
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 walln"
# ...
n -= 1
end while n > 0
buf << "no more bottles of beer"
end
end
Like this:
people = []
begin
info = gets.chomp
people += [Person.new(info)] if not info.empty?
end while not info.empty?
Reference: Ruby's Hidden do {} while () Loop
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