http://qs1969.pair.com?node_id=949

Until loops are just the opposite of whiles. If the control statement evaluates to false code within the loop is executed once and repeated until the control statement tests to true.
Heres a quick example: 
$year=1900;
until($year==2000){
   print "Mission-critical electronics working in the year $year\n";
   #now it works;
   $year++;
}
print "Not working anymore\n";
As you can see the mission-critical electronics work until the year 2000.
I suggest if you've just survived that harrowing ordeal you learn all about do while loops

Replies are listed 'Best First'.
RE: until loops
by turnstep (Parson) on May 24, 2000 at 05:01 UTC
    The "$year++" part was added later, *after* the first couple replies.
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Re: until loops
by Frey (Novice) on May 13, 2001 at 20:34 UTC
    Actually, this is not an infinite loop. This loop is always false until it evaluates 2000,
    in which case it is true, and it stops. I got hung up on this too, I think the backwardness of until can be confusing.
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[d/l]
      I dont think that is correct, the expression "$year==2000" will be false if $year is not 2000 but the actual statement of unless($year==2000) will be true unless year equals 2000. Also an infinite loop is one where the condition will never be met and so will never terminate, in this case if year will never become 2000. For example, if year was initialised to 2001 or if year was 1999 and never incremented then it is an infinite loop.
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RE: until loops
by Anonymous Monk on May 05, 2000 at 20:27 UTC
    If I'm not mistaking that until loop is an infinite loop due to not having an increment.
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RE: until loops
by Simplicus (Monk) on May 22, 2000 at 21:57 UTC
    Yes, and your loop will still be executing in the year 3000.
    Simplicus
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      The increment takes place with "$year++"
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        so does $year++ mean that year gets incremented by 1 ?? or 2 ?
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Re: until loops
by lvanhout (Curate) on Jun 16, 2004 at 05:11 UTC
    I usually find it a little cleaner to say 
    until($year >= 2000)

    [download]
    That is to say continue until year is greater than or equal to 2000. In case you go back and change your code within the loop and do something more complicated than increment.

    Unless you really mean that only 2000 is an escape loop case and 2001 and up will be continue loop cases.
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[d/l]
      On the other hand, with only the == instead of >=, it's a perfectly appropriate demonstration of the concept. I'm not sure, but if compiled, the == might actually execute faster than the >=, though by such a minute margin that it's effectively useless to differentiate.

      - apotheon
      CopyWrite Chad Perrin
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Unless vs. Until?
by inblosam (Monk) on May 22, 2002 at 06:53 UTC
    Can't you just use an "unless" just the same? Is there an advantage/disadvantage to one or the other? 

    $year=1900;
unless($year==2000){     #notice unless instead of until
   print "blah blah $year\n";
   #now it works;
   $year++;
}
print "Done\n";

    [download]


    Michael Jensen
    InShift Technologies
    http://www.inshift.com
    michael@inshift.com
[reply]
[d/l]

      Unfortunately this won't work. The until loop will continue redoing the block until the condition is met. Unless will only execute the block one time, leaving the counter at 1901 instead of 2000.

      Update: crazyinsomniac has pointed out a better way to say this: The unless(condition) statement is saying the same thing as to if( not condition). The until(condition) statement is equivelant to while( not condition).

      That is to say, until() and while() are loops, and will repeat until the condition is met, while if() and unless() are not, and will only execute the block 0 or 1 times. Therefore, you can't trade unless() for until().

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