Python array -- elements replaced with last value

Question

Given the code

  def sumset(a, b):
    bands=[[0, 0]]*len(a)*len(b)
    current=-1
    for ba in a:
      for bb in b:
        current+=1
        bands[current][0]=ba[0]+bb[0]
        bands[current][1]=ba[1]+bb[1]
        print(bands[current])

    print(bands)

The output of sumset([[1,2], [2,4]], [[0,1], [8, 9]]) gives

[1, 3]
[9, 11]
[2, 5]
[10, 13]
[[10, 13], [10, 13], [10, 13], [10, 13]]

I cannot understand why bands is filled with bands[3].

EDIT: I am using Python 3.4.2 on Ubuntu 14.10.

use bands = [[0,0] for i in range(len(a)*len(b))] to avoid it. Check the answer below — Piotr Dabkowski
– Piotr Dabkowski, Commented Nov 10, 2014 at 23:39

yedpodtrzitko · Accepted Answer · 2014-11-10 23:38:12Z

1

By multiplying a list you're creating a copies of the initial list. So when you modify one of them, the changes are transfered into the other copies as well.

answered Nov 10, 2014 at 23:38

yedpodtrzitko

9,4292 gold badges48 silver badges47 bronze badges

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

Collectives™ on Stack Overflow

Python array -- elements replaced with last value

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related