On 09/07/2008, at 23.16, David-Sarah Hopwood wrote:
>Jonathan S. Shapiro wrote:
>> I have just realized that the specification of DO is not what we
wanted.
>> I followed the Scheme specification of DO here. You will find on the
web
>> statements in various places that Scheme DO implements DO-UNTIL. Those
>> statements are incorrect. What Scheme DO implements is in fact
>> WHILE-NOT. That is: a Scheme DO loop may not perform any executions of
>> the body at all.
>>
>> I tend to believe that there are good uses for all of the following
>> constructs:
>>
>> WHILE perform body while test remains tr
>> WHILE-NOT perform body while test remains false
>> DO-UNTIL perform body until test becomes true, running at least
>> once.
>>
>> Of the three, I believe that WHILE-NOT is the least commonly useful,
and
>> that it is trivially subsumed by WHILE.
>
>C-family languages have DO..WHILE rather than DO..UNTIL, of course.
>The (small) advantages of this are:
> - familiarity to C, C++, Java and C# programmers,
> - changing a loop between the WHILE and DO..WHILE forms does not
> require inverting the condition.
Whatever it is named, we need a construct that guarantees at least one
execution of the body, because the value of the loop condition is set
inside the body.
>
>I briefly looked at uses of 'do {...} while (condition)' in my C code,
>and it did not seem like "negative" conditions were generally any more
>common than "positive" ones (i.e. that the code would have been made
>clearer by being able to invert the condition).
>
>A minor advantage of DO..UNTIL is that the condition acts as an
>assertion that is true after the loop (rather than false after the
>loop, which might require a reader of the code to mentally invert it
>in more cases). I would consider this to be less important than the
>familiarity issue.
>
>--
>David-Sarah Hopwood
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