the list for forloop index is changing?

Question

I have some easy code:

a = [1,2,3,4,3,3,5]
for letter in a:
  print (letter)
  if letter == 3:
     a.remove(letter)

print ('a=',a)

The output shows:

1
2
3
3
5
a= [1, 2, 4, 3, 5]

I expect to see:

1
2
3
4
3
3
5
a=[1,2,4,5]

What is happening? It seems in the loop that the "a" list is also being updated simultaneously. How can I fix the for loop to get the expected output?

Most pythonic way: `a = [x for x in a if x != 3]` – Hayley Guillou Oct 28 '15 at 23:39 — Hayley Guillou, Oct 28 '15 at 23:39

philngo · Accepted Answer · 2015-10-28T23:49:44.657

-1

It's removing the item at index 3, not the item with the value of 3. The following list comprehension does what you want.

a = [letter for letter in a if letter != 3]

[edit] Nope, I was wrong about "remove" - I confused it with "pop"

edited Oct 28 '15 at 23:49

answered Oct 28 '15 at 23:39

philngo

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from `help(list.remove)`: `L.remove(value) -- remove first occurrence of value.` – kindall Oct 28 '15 at 23:45

the list for forloop index is changing?

1 Answers1