Python two-dimensional list adds value to element

Question

I want to add a value to a two-dimensional list item. Instead, a value is added to all elements in the column. Can anyone help?

ROWS=3 COLUMNS=5 a=[[0] * (ROWS)] * (COLUMNS) for i in range(ROWS): print (a[i]) print("") x=5 a[2][1]=a[2][1]+x for i in range(ROWS): print (a[i]) 

Answer 1

Create your list as follows:

 In [6]: a = [[0 for _ in range(rows)] for _ in range(cols)] In [7]: a[2][1] = a[2][1] + 5 In [8]: a Out[8]: [[0, 0, 0], [0, 0, 0], [0, 5, 0], [0, 0, 0], [0, 0, 0]] 

Since you are doing this now, it actually just repeats the same list object, so changing one actually changes each of them.

 In [11]: a = [[0] * (rows)] * (cols) In [12]: [id(x) for x in a] Out[12]: [172862444, 172862444, 172862444, 172862444, 172862444] #same id(), ie same object 

Answer 2

Line

 a=[[0] * (ROWS)] * (COLUMNS) 

Creates copies of lists, not new lists for each column or row.

Use list comprehension instead:

 a = [[0 for _ in range(ROWS)] for _ in range(COLUMNS)] 

A quick demonstration of the difference:

 >>> demo = [[0]] * 2 >>> demo [[0], [0]] >>> demo[0][0] = 1 >>> demo [[1], [1]] >>> demo = [[0] for _ in range(2)] >>> demo [[0], [0]] >>> demo[0][0] = 1 >>> demo [[1], [0]]