Python : how to append new elements in a list of list?

by

You must do

a = [[] for i in xrange(3)]

not

a = [[]]*3

Now it works:

$ cat /tmp/3.py
a = [[] for i in xrange(3)]

print str(a)

a[0].append(1)
a[1].append(2)
a[2].append(3)

print str(a[0])
print str(a[1])
print str(a[2])

$ python /tmp/3.py
[[], [], []]
[1]
[2]
[3]

When you do something like a = [[]]*3 you get the same list [] three times in a list. The same means that when you change one of them you change all of them (because there is only one list that is referenced three times).

You need to create three independent lists to circumvent this problem. And you do that with a list comprehension. You construct here a list that consists of independent empty lists []. New empty list will be created for each iteration over xrange(3) (the difference between range and xrange is not so important in this case; but xrange is a little bit better because it does not produce the full list of numbers and just returns an iterator object instead).

More Related Contents:

  1. I have a list of file names and I need to be able to count how many of the same file gets repeated for each file name [closed]
  2. How do I check if a list is empty?
  3. What’s the difference between lists and tuples?
  4. How to remove list elements in a for loop in Python? [duplicate]
  5. Iterate over all pairs of consecutive items in a list [duplicate]
  6. How Big can a Python List Get?
  7. Why is a list comprehension so much faster than appending to a list?
  8. How to remove \n from a list element?
  9. Nested List and count()
  10. Modify a list while iterating
  11. Sorting Python list based on the length of the string
  12. rect collision with list of rects
  13. How do I create a list with numbers between two values?
  14. Sort multidimensional array based on 2nd element of the subarray
  15. Sort list by frequency
  16. List assignment with [:]
  17. Why is tuple faster than list in Python?
  18. Identify if list has consecutive elements that are equal
  19. Initializing a list to a known number of elements in Python [duplicate]
  20. Python data structure sort list alphabetically
  21. Finding the indices of matching elements in list in Python
  22. Difference between using [] and list() in Python
  23. How to implement list subtraction?
  24. How do I subtract one list from another?
  25. Python: Elegant way to check if at least one regex in list matches a string
  26. Python – How to convert only numbers in a mixed list into float?
  27. What is the time complexity of Python List Reverse?
  28. Check if list contains only item x
  29. How do I send list of dictionary as Body parameter together with Files in FastAPI?
  30. deque.popleft() and list.pop(0). Is there performance difference?

Leave a Comment