Variable in loop on Python don't change [duplicate]

Question

I can't understand why in loop variable don't change, but I explicitly try it. So here is my code:

a=[1,2,3]
b=["a","b","c"]
d=[a,b]
for i in d:
    for a in i:
         a*2
         print(a)

And when I run I see :

1
2
3
a
b
c

Instead expected:

2
4
6
aa
bb
cc

You are not assigning the new value to any variable. Replace a*2 by a=a*2. — Sven Rusch
– Sven Rusch, Commented Feb 4, 2017 at 15:00
The line a*2 gets "lost", it doesn't change the value of a. To do that you'd need to either reassign a its new value by doing a = a * 2 or just print the desired value by doing print a * 2. Also, you are using the variable a for both the array and the inner variable in the second loop. Avoid this at all costs. — dabadaba
– dabadaba, Commented Feb 4, 2017 at 15:02
by the way I don't understand why people are downvoting this. Sure, it's an absolutely basic question, but for a person who is clearly completely new to programming, I'd say it's a totally legit question. — dabadaba
– dabadaba, Commented Feb 4, 2017 at 15:03
Rather than being a duplicate (the other question is about an entirely different confusion), this should have been closed as a typo. The expected behaviour makes no sense at all. — Karl Knechtel
– Karl Knechtel, Commented Oct 2, 2022 at 0:07

mkrieger1 · Accepted Answer · 2022-01-23 19:49:14Z

1

In order to change the a when iterating i, you must assign the value to the variable.

so instead of

for a in i:
    a*2
    print(a)

try

for a in i:
    a = a*2
    print(a)

mkrieger1

24.2k7 gold badges68 silver badges84 bronze badges

answered Feb 4, 2017 at 15:04

K-man

3641 silver badge14 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

its me · Accepted Answer · 2017-02-04 15:01:46Z

0

Because you are not assigning value to the variable. It will change if you do a = a*2, instead of just a*2. Try this in python shell:

>>> a=[1,2,3]
>>> b=["a","b","c"]
>>> d=[a,b]
>>> for i in d:
...     for a in i:
...         a=a*2
...         print(a)
... 
2
4
6
aa
bb
cc
>>>

answered Feb 4, 2017 at 15:01

its me

1272 silver badges8 bronze badges

Hasan Alper Ocalan · Accepted Answer · 2017-02-04 15:02:52Z

0

a=[1,2,3]
b=["a","b","c"]
d=[a,b]
for i in d:
    for a in i:
         a=a*2 # change this line like that
         print(a)

answered Feb 4, 2017 at 15:02

3181 silver badge9 bronze badges