Modifying list while iterating

Question

l  = range(100)                         
for i in l:                         
    print i,                         
    print l.pop(0),                  
    print l.pop(0)

The above python code gives the output quite different from expected. I want to loop over items so that I can skip an item while looping.

Please explain.

Answer 1

The general rule of thumb is that you don't modify a collection/array/list while iterating over it.

Use a secondary list to store the items you want to act upon and execute that logic in a loop after your initial loop.

Answer 2

Try this. It avoids mutating a thing you're iterating across, which is generally a code smell.

for i in xrange(0, 100, 3):
    print i

See xrange.

Answer 3

I've been bitten before by (someone else's) "clever" code that tries to modify a list while iterating over it. I resolved that I would never do it under any circumstance.

You can use the slice operator mylist[::3] to skip across to every third item in your list.

mylist = [i for i in range(100)]
for i in mylist[::3]:
  print(i),

Other points about my example relate to new syntax in python 3.0.

• I use a list comprehension to define mylist because it works in Python 3.0 (see below)
• print is a function in python 3.0

Python 3.0 range() now behaves like xrange() used to behave, except it works with values of arbitrary size. The latter no longer exists.

Answer 4

Never alter the container you're looping on, because iterators on that container are not going to be informed of your alterations and, as you've noticed, that's quite likely to produce a very different loop and/or an incorrect one. In normal cases, looping on a copy of the container helps, but in your case it's clear that you don't want that, as the container will be empty after 50 legs of the loop and if you then try popping again you'll get an exception.

What's anything BUT clear is, what behavior are you trying to achieve, if any?! Maybe you can express your desires with a while...?

i = 0
while i < len(L):
    print i,                         
    print L.pop(0),                  
    print L.pop(0)

BTW, I'm using L, not l, because lowercase L and lowercase I look far too similar in most fonts, so using both is just looking for trouble and confusion.

Answer 5

This slice syntax makes a copy of the list and does what you want:

l  = range(100)  
for i in l[:]:  
    print i,  
    print l.pop(0),  
    print l.pop(0)