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?

10 Answers

CAUTION:

The begin <code> end while <condition> is rejected by Ruby's author Matz. Instead he suggests using Kernel#loop, e.g.

loop do # some code here break if <condition> end 

Here's an email exchange in 23 Nov 2005 where Matz states:

|> Don't use it please. I'm regretting this feature, and I'd like to |> remove it in the future if it's possible. | |I'm surprised. What do you regret about it? Because it's hard for users to tell begin <code> end while <cond> works differently from <code> while <cond> 

RosettaCode wiki has a similar story:

During November 2005, Yukihiro Matsumoto, the creator of Ruby, regretted this loop feature and suggested using Kernel#loop.

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 

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

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

How about this?

people = [] until (info = gets.chomp).empty? people += [Person.new(info)] end 

Here's the full text article from hubbardr's dead link to my blog.

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 

This works correctly now:

begin # statment end until <condition> 

But, it may be remove in the future, because the begin statement is counterintuitive. See:

Matz (Ruby’s Creator) recommended doing it this way:

loop do # ... break if <condition> end 

From what I gather, Matz does not like the construct

begin <multiple_lines_of_code> end while <cond> 

because, it's semantics is different than

<single_line_of_code> while <cond> 

in that the first construct executes the code first before checking the condition, and the second construct tests the condition first before it executes the code (if ever). I take it Matz prefers to keep the second construct because it matches one line construct of if statements.

I never liked the second construct even for if statements. In all other cases, the computer executes code left-to-right (eg. || and &&) top-to-bottom. Humans read code left-to-right top-to-bottom.

I suggest the following constructs instead:

if <cond> then <one_line_code> # matches case-when-then statement while <cond> then <one_line_code> <one_line_code> while <cond> begin <multiple_line_code> end while <cond> # or something similar but left-to-right 

I don't know if those suggestions will parse with the rest of the language. But in any case I prefere keeping left-to-right execution as well as language consistency.

a = 1 while true puts a a += 1 break if a > 10 end 

Here's another one:

people = [] 1.times do info = gets.chomp unless info.empty? people += [Person.new(info)] redo end end 

ppl = [] while (input=gets.chomp) if !input.empty? ppl << input else p ppl; puts "Goodbye"; break end end