looping throuhg a list 2 items at a time

[David Eppstein]

In article <a92hl1$1v4c at r02n01.cac.psu.edu>,
 Rajarshi Guha <rxg218 at psu.edu> wrote:

> I got this code to work:
> 
> liter = iter(l)
> while(1):
>   try:
>      print liter.next(), liter.next()
>   except StopIteration:
>      break
> 
> Works fine - is it an efficient way to handle this situation?
> (I could arrange the data into tuples if necessary)

Does it work fine?  What if the list has an odd number of items -- the 
last item won't be printed.

In one situation I had a list of items that I wanted to handle three at 
a time, except that, if the number of items in the list was not a 
multiple of three, I wanted to handle the last two as a pair, or the 
first two and last two both as pairs.  I ended up writing the part that 
handled pairs or triples as a separate function, calling it once for an 
initial pair (if any), then having a separate loop for the triples and 
final pair:

      if len(L) % 3 == 1:
         loopbody(L[:2])
         L = L[2:]
      while len(L) > 0:
         loopbody(L[:3])
         L = L[3:]

this worked ok in my situation but is unsatisfactory as a general 
technique because all the slicing can be slow for long lists -- I guess 
you could use an index into your list instead:

      i = 0
      while i < len(L):
          print L[i:i+2]
          i += 2

Unfortunately due to the non-functionalness of print, I don't see a way 
of writing one print statement that will give the same results as
"print L[i],L[i+1]" without sometimes throwing an exception.

-- 
David Eppstein       UC Irvine Dept. of Information & Computer Science
eppstein at ics.uci.edu http://www.ics.uci.edu/~eppstein/