How to modify list entries during for loop

Question

Now I know that it is not safe to modify the list during an iterative looping  However  suppose I have a list of strings  and I want to strip the strings themselves  Does replacement of mutable values count as modification

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You can do something like this   a    1 2 3 4 5  b    i  2 for i in a    It s called a list comprehension  to make it easier for you to loop inside a list

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The answer given by Jemshit Iskenderov  and Ignacio Vazquez-Abrams is really good  It can be further illustrated with this example  imagine that  a  A list with two vectors is given to you   b  you would like to traverse the list and reverse the order of each one of the arrays  Let s say you have  v   np array  1  2 3 4   b   np array  3 4 6    for i in  v  b       i   i   -1      this command does not reverse the string  print  v b     You will get   array  1  2  3  4    array  3  4  6      On the other hand  if you do   v   np array  1  2 3 4   b   np array  3 4 6    for i in  v  b      i      i   -1      this command reverses the string  print  v b     The result is   array  4  3  2  1    array  6  4  3

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No you wouldn t alter the  content  of the list  if you could mutate strings that way  But in Python they are not mutable  Any string operation returns a new string   If you had a list of objects you knew were mutable  you could do this as long as you don t change the actual contents of the list   Thus you will need to do a map of some sort  If you use a generator expression it  the operation  will be done as you iterate and you will save memory

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Since the loop below only modifies elements already seen  it would be considered acceptable   a     a    b    c      d     for i  s in enumerate a       a i    s strip    print a    - gt    a    b    c    d     Which is different from   a       s strip   for s in a    in that it doesn t require the creation of a temporary list and an assignment of it to replace the original  although it does require more indexing operations   Caution  Although you can modify entries this way  you can t change the number of items in the list without risking the chance of encountering problems    Here s an example of what I mean   deleting an entry messes-up the indexing from that point on   b     a     b    c      d     for i  s in enumerate b       if s strip      b i      leading or trailing whitespace          del b i   print b     - gt    a    c       WRONG     The result is wrong because it didn t delete all the items it should have     Update  Since this is a fairly popular answer  here s how to effectively delete entries  in-place   even though that s not exactly the question    b     a    b    c      d     b       entry for entry in b if entry strip      entry   print b     - gt    a      CORRECT

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One more for loop variant  looks cleaner to me than one with enumerate     for idx in range len list        list idx        set a new value       some other code which doesn t let you use a list comprehension

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It is not clear from your question what the criteria for deciding what strings to remove is  but if you have or can make a list of the strings that you want to remove   you could do the following   my strings     a   b   c   d   e   undesirable strings     b   d   for undesirable string in undesirable strings      for i in range my strings count undesirable string            my strings remove undesirable string    which changes my strings to   a    c    e

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In short  to do modification on the list while iterating the same list   list        Modify the list  for each element in list  Condition Check     example   list       list remove each element  for each element in list if each element in   data1    data2

User · Answer

Modifying each element while iterating a list is fine  as long as you do not change add remove elements to list    You can use list comprehension   l     a     list    of      string    l    item strip   for item in l    or just do the C-style for loop   for index  item in enumerate l       l index    item strip

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It s considered poor form  Use a list comprehension instead  with slice assignment if you need to retain existing references to the list   a    1  3  5  b   a a       x   2 for x in a  print b

[python] How to modify list entries during for loop?

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