any way to loop iteration with same item in python?

Question

Hello.

It's a common programming task to loop iteration while not receiving next item. For example:

for sLine in oFile :
  if ... some logic ... :
    sLine = oFile.next()
    ... some more logic ...
    # at this point i want to continue iteration but without
    # getting next item from oFile. How can this be done in python?

Answer 1

I first thought you wanted the continue keyword, but that would of course get you the next line of input.

I think I'm stumped. When looping over the lines of a file, what exactly should happen if you continued the loop without getting a new line?

Do you want to inspect the line again? If so, I suggest adding an inner loop that runs until you're "done" with the input line, which you can then break out of, or use maybe the while-condition and a flag variable to terminate.

Answer 2

Instead of looping through the file line by line, you can do a file.readlines() into a list. Then loop through the list (which allows you to look at the next item if you want). Example:

list = oFile.readlines()
for i in range(len(list))
    #your stuff
    list[i] #current line
    list[i-1] #previous line
    list[i+1] #next line

you can go to the previous or next line by simply using [i-1] or [i+1]. You just need to make sure that no matter what, i does not go out of range.

Answer 3

What you need is a simple, deterministic finite state machine. Something like this...

state = 1
for sLine in oFile:
   if state == 1:
      if ... some logic ... :
         state = 2
   elif state == 2:
      if ... some logic ... :
         state = 1

Answer 4

jle's approach should work, though you might as well use enumerate():

for linenr, line in enumerate(oFile):
    # your stuff

Answer 5

Ugly. Wrap the body in another loop.

for sLine in oFile :
  while 1:
    if ... some logic ... :
      sLine = oFile.next()
      ... some more logic ...
      continue
    ... some more logic ...
    break

A little better:

class pushback_iter(object):
    def __init__(self,iterable):
        self.iterator = iter(iterable)
        self.buffer = []
    def next(self):
        if self.buffer:
            return self.buffer.pop()
        else:
            return self.iterator.next()
    def __iter__(self):
        return self
    def push(self,item):
        self.buffer.append(item)

it_file = pushback_iter(file)
for sLine in it_file:
    if ... some logic ... :
        sLine = it_file.next()
        ... some more logic ...
        it_file.push(sLine)
        continue
    ... some more logic ...

Unfortunately no simple way of referencing the current iterator.

Answer 6

You can assign your iterator to an variable then use the .next get te next one.

iter = oFile.xreadlines() # is this the correct iterator you want?
try:
  sLine = iter.next()
  while True:
    if ... some logic ... :
      sLine = iter.next()
      ... some more logic ...
      continue
    sLine = iter.next()
except StopIterator:
  pass

Answer 7

Simply create an iterator of your own that lets you push data back on the front of the stream so that you can give the loop a line that you want to see over again:

next_lines = []
def prependIterator(i):
    while True:
        if next_lines:
            yield(next_lines.pop())
        else:
            yield(i.next())

for sLine in prependIterator(oFile):
    if ... some logic ... :
        sLine = oFile.next()
        ... some more logic ...
        # put the line back so that it gets read
        # again as we head back up to the "for
        # statement
        next_lines.append(sLine)

If the prepend_list is never touched, then the prependIterator behaves exactly like whatever iterator it is passed: the if statement inside will always get False and it will just yield up everything in the iterator it has been passed. But if items are placed on the prepend_list at any point during the iteration, then those will be yielded first instead before it returns back to reading from the main iterator.

Answer 8

You just need to create a variable out of your iterator, and then manipulate that:

% cat /tmp/alt-lines.py 
#! /usr/bin/python -tt

import sys

lines = iter(open(sys.argv[1]))
for line in lines:
    print line,
    lines.next()

% cat /tmp/test
1
2
3
4
% /tmp/alt-lines.py /tmp/test
1
3

...note that in this case we unconditionally do lines.next() so the above fails for files with odd lines, but I assume that isn't going to be the case for you (and adding the error checking is fairly trivial -- just catch and throw away StopIteration on the manual .next()).