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Loop-and-a-half (Re: Curious assignment behaviour)

Donn Cave donn at u.washington.edu
Thu Oct 11 12:55:05 EDT 2001 
Quoth Dale Strickland-Clark <dale at riverhall.NOSPAMco.uk>:
...
| The general loop construct as found in C and Java where you have three
| expressions:  initialisation, increment/iteration, test is a very
| flexible and powerful approach which I really like. 

I sort of regret wasting bandwidth on this purely academic question,
but it's a matter of truly venerable tradition that C's loop is a
bit shy of perfection.  The idea as I understand it, is that there
are essentially two control points in a loop, the entry, and the
conditional repeat.  C conflates those two in one, because both
entry and the conditional repeat have to be at the "top".

There are plenty of applications for a separate entry, and that's
where we came in.  The most frequently cited application for an
assignment expression is

   while line = file.readline():
      ...

A very flexible and powerful approach to looping would allow you
to enter the loop in time to set up for the condition, however many
statements that might take.  C and Python give you one expression,
and in C that expression can be an assignment.  If you can't do what
you need there, then you have to choose between A) repeat the same
code before the loop:

   line = file.readline()
   while line:
      ...
      line = file.readline()

or B) take the condition out of the loop control (standard Python solution)

   while 1:
      line = file.readline()
      if not line:
         break

People who claim that C fixes this just expose the limitations of
their thinking.  C does not fix this, by allowing an assignment
there, except in the restricted case where the pre-conditional
part of the loop can fit into one expression.  (Note that I'm not
talking about the initialization section of for(), which is a
pure non-feature - those statements can just as well be written
before the loop.)

Anyone writing a new language should obviously think about that (if
the language has any looping constructs at all - there are so many
"functional" languages crowding the landscape that I'm not sure anyone
invents procedural languages these days.)

	Donn Cave, donn at u.washington.edu
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