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 executewhile not info.empty?info = gets.chomppeople += [Person.new(info)] if not info.empty?endThis code would look much nicer in a do ... while loop:
people = []doinfo = gets.chomppeople += [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?
Best Answer
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 herebreak 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 tellbegin <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#initializein the Ruby core library:begintmpname = File.join(tmpdir, make_tmpname(basename, n))lock = tmpname + '.lock'n += 1end while @@cleanlist.include?(tmpname) orFile.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 falsedo {} while ()=> nilAs 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 > 0321=> nilWhile 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 + 2endHere is an ugly, but quick way to memoize something more complex:
def expensive@expensive ||=beginn = 99buf = "" beginbuf << "#{n} bottles of beer on the wall\n" # ...n -= 1end while n > 0buf << "no more bottles of beer" endend
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 = []begininfo = gets.chomppeople += [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)]endThis works correctly now:
begin# statmentend until <condition>But, it may be remove in the future, because the begin statement is counterintuitive. See: http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/6745
Matz (Ruby’s Creator) recommended doing it this way:
loop do# ...break if <condition>endHere'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:
begintmpname = File.join(tmpdir, make_tmpname(basename, n))lock = tmpname + '.lock'n += 1end while @@cleanlist.include?(tmpname) orFile.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 falsedo {} while ()=> nilAs 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 > 0321=> nilWhile 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 + 2endHere is an ugly, but quick way to memoize something more complex:
def expensive@expensive ||=beginn = 99buf = "" beginbuf << "#{n} bottles of beer on the wall\n" # ...n -= 1end while n > 0buf << "no more bottles of beer" endendFrom 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 computerexecutes code left-to-right (eg. || and &&) top-to-bottom. Humans read code left-to-righttop-to-bottom.
I suggest the following constructs instead:
if <cond> then <one_line_code> # matches case-when-then statementwhile <cond> then <one_line_code><one_line_code> while <cond>begin <multiple_line_code> end while <cond> # or something similar but left-to-rightI don't know if those suggestions will parse with the rest of the language. But in any caseI prefere keeping left-to-right execution as well as language consistency.
a = 1while trueputs aa += 1break if a > 10endHere's another one:
people = []1.times doinfo = gets.chompunless info.empty? people += [Person.new(info)]redoendendppl = []while (input=gets.chomp)if !input.empty?ppl << inputelsep ppl; puts "Goodbye"; breakendend