coping without do-while

[gusl]

You'd like to write code like:

do {
  A
} while(test)

but your language does not offer do-while (a.k.a do-loop). What do you do?

(a)

A
while(test){
  A
}

(b)

while(TRUE){
  A
  if (!test) break
}

Other?

Comments

[demarko (from livejournal.com)]

(a) mostly because I find it easier to parse

[gregh1983]

(a), for this reason as well, but I see the point the (b) crowd is making about catching errors.

[ernunnos.livejournal.com]

B, because it's more resistant to coding errors. If you make a change in the repeated code, you only have to change it in one place, and not two. And although the construct isn't as neat as "do while", "while true break" is understandable to anyone who regularly works in a language that needs it.

[gfish.livejournal.com]

This. And I'd rather have readability over neatness anyway.

[gustavolacerda.livejournal.com]

IMHO you're getting both. I find repetitive code to be one of the worst things... (though in some other cases repetition can help readability)

[gfish.livejournal.com]

Oh, absolutely. I was more referring to using a do-while construction at all. I'd rather just have while, so there are as few types of structures as possible.

[gustavolacerda.livejournal.com]

Trouble is, "break" can break assumptions people make about post-conditions.

A naive onlooker might say that code running after the loop is evidence that you just refuted TRUE.

[en-ki.livejournal.com]

I like Ruby's "loop" construct

loop {
  # do stuff
  break if ...;
  # do other stuff
  break if ...;
}

It's an infinite loop if you don't break out of the loop somehow, so in practice you use it whenever you want to do multiple tests, multiple exits, or exits in odd places. It doesn't do anything different from what while(TRUE) does, but it's a clear syntactic marker that you're doing things in this way.

I do wish you could do

do
  stuff()
until CONDITION

as well as

until CONDITION
   stuff()
end

but loop does ok.

[gustavolacerda.livejournal.com]

until(test) is equivalent to while(!test).

[en-ki.livejournal.com]

Yes, what I was illustrating there was the idea you were talking about in your post, running the loop contents once before doing the test. Generally when I want to do that I also want "until", if only to be more English-like. Ruby has

until ...
end

(a while loop with negated test) but not

do ...
until

(a while loop with negated test that unconditionally runs once before the test).

[shaktool.livejournal.com]

I basically do (b) even in languages that support "do". I kind of forget that "do" even exists.

[wjl.livejournal.com]

(c)

let fun loop () =
  (A;
   if test then loop () else ())
in
  loop ()
end

[demarko (from livejournal.com)]

I c wat u did thar

[wjl.livejournal.com]

what? it's true! :P

[bhudson.livejournal.com]

B unless you're in the not infrequent case where the first iteration is slightly different.

[gwillen.livejournal.com]

If I find myself in that case, my instinct is to try to refactor until they're the same. I feel like usually it works.

[bhudson.livejournal.com]

I'm thinking of situations like printing a comma-separated list (i.e. implementing perl's "join"). Or summing up a list over a set that has no zero element (or where the extra addition matters to performance).

[gwillen.livejournal.com]

Oh yeah, I've always wondered if there might not be a looping construct we could add that would better handle the delimited-list case, since it's not an uncommon thing to have to write.

while (x = next_element()) {
printf(x);
} delimited-by {
printf(", ");
};

or something.

[wjl.livejournal.com]

i think i tried to come up with a solid higher-order function interface for this once, but i don't remember it being very satisfying.. i think i had it taking three pieces of code: one for the beginning, one for the end, and one for the in-betweens. but i guess you really don't need the beginning code or the end code, since you could just put it outside the loop..

[gustavolacerda.livejournal.com]

For stuff like that, my instinct is to use a Reduce in which the two-argument function simply inserts a comma between the two arguments.

[raflacerda.livejournal.com]

for(i=0;i<1;){
A
if(test)i++;
}

[gustavolacerda.livejournal.com]

This is the same as (b), except for introducing a variable that is never used.

[gustavolacerda.livejournal.com]

I guess there is one advantage: you're not using break...