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

User · Answer

I guess this is what you want   l    range 100    index   0                        for i in l                               print i                    try          print l pop index 1                             print l pop index 1      except          pass     index    1   It is quite handy to code when the number of item to be popped is a run time decision  But it runs with very a bad efficiency and the code is hard to maintain

User · Answer

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  lt  len some list       print i                               print some list pop 0                         print some list pop 0

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

Use a while loop that checks for the truthfulness of the array   while array      value   array pop 0        do some calculation here   And it should do it without any errors or funny behaviour

User · Answer

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

[python] Modifying list while iterating

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