For loop only loops 3 times. Why?

Question

I've written a seemingly simple loop. There are 6 items in the list and it should loop six times. However, it only loops 3 times. Why?

list1 = 'one two three four five six'
newlist = list1.split(' ')
print (newlist)
list2 = ['seven', 'eight', 'nine', 'ten', 'eleven', 'twelve']

for number in list2:
    nextnumber = list2.pop()
    print ("Adding number ", nextnumber)
    newlist.append(nextnumber)

print (newlist)

Because `list2.pop()` removes the item of the list - in each loop you reduce list size by one. — game0ver, Feb 09 '18 at 14:56
Well you mutate `list2` by calling `pop()` on each iteration so you're truncating on each iteration, what are you trying to achieve? — EdChum, Feb 09 '18 at 14:56
Do you want list2 to be in reversed order? Because that is what you were trying to do with `pop()` — user3483203, Feb 09 '18 at 15:00
Or if you'd like to append all list2 elements at once: `newlist.extend(reversed(newlist2))`. — erhesto, Feb 09 '18 at 15:01
My [answer](https://stackoverflow.com/a/4082739/355230) to the question [**How to modify list entries during for loop?**](https://stackoverflow.com/questions/4081217/how-to-modify-list-entries-during-for-loop) might be helpful. — martineau, Feb 09 '18 at 15:10

user2390182 · Answer 1 · 2018-02-09T15:10:58.230

1

As mentioned in the comments, you remove items while iterating. You can model this pattern better with a while loop:

while list2:
    newlist.append(list2.pop())

edited Feb 09 '18 at 15:10

answered Feb 09 '18 at 15:03

user2390182

72,016
6
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score 1 · Answer 2 · answered Feb 09 '18 at 15:15

1

Adding list2 to newlist using a for loop could be done like this:

for number in list2:
    print ("Adding number ", number)
    newlist.append(number)

But the short, fast and pythonic way is

newlist.extend(list2)

answered Feb 09 '18 at 15:15

Jesper

1,611
13
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score 1 · Answer 3 · answered Feb 09 '18 at 15:16

1

Use the following code instead:

for number in list2:
    print ("Adding number ", number)
    newlist.append(number)
print (newlist)

Observation: The list2.pop() is reducing your list element and it also reduces the number looping times

answered Feb 09 '18 at 15:16

user3156497

11
2

score 0 · Answer 4 · answered Feb 09 '18 at 15:13

0

It only only loops out three times because you are actively modifying list2. A simple correction would be to make a copy!!

import copy

list2_c = copy.copy(list2)

for num in list2_c:
.
.
.
.

answered Feb 09 '18 at 15:13

Emiliano Isaza Villamizar

357
2
15

You can also use `list2[:]` or `list(list2)`. – bfontaine Feb 09 '18 at 15:14
look at this post https://stackoverflow.com/questions/2612802/how-to-clone-or-copy-a-list – Emiliano Isaza Villamizar Feb 09 '18 at 15:15

score 0 · Answer 5 · answered Feb 09 '18 at 15:17

Quick answer, make a copy of list2:

list1 = 'one two three four five six'
newlist = list1.split(' ')
print (newlist)
list2 = ['seven', 'eight', 'nine', 'ten', 'eleven', 'twelve']

for number in list(list2):
    nextnumber = list2.pop()
    print ("Adding number ", nextnumber)
    newlist.append(nextnumber)

print (newlist)

LeStivi · Answer 6 · 2018-02-09T15:44:21.110

like the previous answers are saying, you are tempering with list2, in other word when you do for number in list2, you are taking 7, 8,9 but never using them, but instead you are removing 12, 11, 10 from the list and printing it. you could do it this way instead:

    for number in list2:
    #Unless you want to append in reverse order(see below for that)
    #comment the next line
        #nextnumber = list2.pop() 
        print ("Adding number ", number) 
        newlist.append(number)#Append the number you just got from list2
    print (newlist)

  out[:]: ['one', 'two', 'three', 'four', 'five', 'six']
  Adding number  seven
  Adding number  eight
  Adding number  nine
  Adding number  ten
  Adding number  eleven
  Adding number  twelve
  ['one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine', 
  'ten', 'eleven', 'twelve']

If you want to append in reverse order, do what @thachnb suggested.

For loop only loops 3 times. Why?

6 Answers6