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Python Python Collections (Retired) Lists Redux Manipulating Lists

Posted by David Hsu
David Hsu
2,574 Points

I have some confusion around using a for loop and remove function

I was trying to solve this using a for loop and remove, and it seems when it did this, the for loop skipped the next item in the list after the previous item was removed.

my code:

the_list = ["a", 2, 3, 1, False, [1, 2, 3]]

for item in the_list:
  print("evaluating {}, {}".format(item,type(item)))
  if type(item) is str:
    the_list.remove(item)
  elif type(item) is bool:
    the_list.remove(item)
  elif type(item) is list:
    the_list.remove(item)
  else:
    print "continuing"
  print the_list

the output while debugging:

evaluating a, <type 'str'>
[2, 3, 1, False, [1, 2, 3]]
evaluating 3, <type 'int'>
continuing
[2, 3, 1, False, [1, 2, 3]]
evaluating 1, <type 'int'>
continuing
[2, 3, 1, False, [1, 2, 3]]
evaluating False, <type 'bool'>
[2, 3, 1, [1, 2, 3]]

so after "a" is removed, the next time in the for loop, it seemed to skip over evaluating 2. The same happened when it removed False; it never evaluates [1,2,3] as a list.

is the generated list of things in the for loop static, so remove() is making the next item in the list skipped?

2 Answers

David Hsu
David Hsu
2,574 Points
David Hsu
David Hsu
2,574 Points

after poking around, I learned about the reversed() function for the for loop. If I did:

the_list = ["a", 2, 3, 1, False, [1, 2, 3]]

for item in reversed(the_list):
  print("evaluating {}, {}".format(item,type(item)))
  if type(item) is str:
    the_list.remove(item)
  elif type(item) is bool:
    the_list.remove(item)
  elif type(item) is list:
    the_list.remove(item)
  else:
    print "continuing"
  print the_list

then it functioned as I wanted. I could have made a copy of the_list to manipulate, and that would have worked as well.

Jason Anello
Jason Anello
Courses Plus Student 94,610 Points

Hi David,

Do you still need an explanation of why your original code didn't work or do you think you have it figured out?

David Hsu
David Hsu
2,574 Points
David Hsu
David Hsu
2,574 Points

I have it figured out, thanks.