Multiple variable control in for loops. Doable in Python?

Derek Martin code at pizzashack.org
Fri Jul 18 23:28:32 CEST 2008


On Fri, Jul 18, 2008 at 12:21:49PM -0700, mark floyd wrote:
> I'm new to Python and have been doing work converting a few apps
> from Perl to Python.  I can not figure out the comparable Python
> structures for multi-variable for loop control.
[...]
> I spent a good part of yesterday looking for a way to handle this
> style for loop in Python and haven't been able to find an
> appropriate way to handle this control style.  

One wonders why... :)

> We have this style for loop all over the place and not being able to
> find a similar structure in Python could be a problem.  Any pointers
> to a Python equivalent structure would be much appreciated

Even if Python didn't offer a way to write a for loop in a similar
fashion (someone else replied about that already), why should it be a
problem?  In general control structures can be rewritten as some other
kind of control structure.  For example, this does exactly what your
for loop examples do:

  i = 0
  j = 0
  while i < 5 and j < 10:
      print i, j
      i += 1 
      j += 1

Though, this example is silly, as it will always terminate after the
5th iteration of the loop, and there is no need to have j being used
as a control variable... it's termination condition will never be met.
Though the example illustrates the techique, even if the example is
bogus.

Another way is to use functions to modify the values of i and j.
Writing your loops this way, you can have as many control variables as
you need, and your formula for incrementing those control variables
can be as varied as complicated as you can imagine.

  def some_increment_function(i):
      # Do some complicated processing of i
      i = ...
      return i

  def other_incrjmental_function(j):
      # Do some complicated processing of j
      j = ...
      return j

  i = 0
  j = 0
  while i < 5 and j < 10:
      print i, j
      i = some_increment_function(i)
      j = other_increment_function(j)

And of course, you could also replace the loop terminating conditions
with functions that return a value which can be interpreted as a truth
value.  If you wanted to get really crazy, you could even code the
control structure as a recursive function:

  def control(i, j):
      print i,j
      if not (i < 5 or j < 10):
          return
      else:
          control(some_increment_function(i), other_increment_function(j))


Should you really write control structures this way, generally?
Absolutely not (unless you're writing LISP or Scheme :)).  But the
point is, even if a given language doesn't have a particular syntactic
element that you're looking for, it's pretty much guaranteed to
provide a way to do what you're trying to do.  You just need to stop
thinking about your problem in terms of a particular syntactic
element, and start thinking about it in more general terms of what you
are actually trying to accomplish, and apply whatever syntax (usually
one of several) your language provides to do that.

-- 
Derek D. Martin
http://www.pizzashack.org/
GPG Key ID: 0x81CFE75D

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