嵌套列表及其索引调用

Question

I was wondering how I would go about assigning a value to a specific index in a list that is inside another list. For example:

For a list parentList

for i in range(0,rows, 1):
        parentList[i] = []
        for j in range(0,cols,1):
            parentList[i[j]] = 0

Not sure if that is even the right syntax for assignment like this.

I know I can do .append, but what I need to do for my problem is create a sparse matrix, so everything should be 0 except for the values that the user specifies in the form (row) (col) (value) .

So I thought using the indexes of the lists would help with assignment. If there is a better way, please let me know!

EDIT: This is an example of what is being done

input:

1 2 2

1 3 3

2 1 4

3 3 5

0 0 0


1 3 7

2 2 8

3 2 9

3 3 6

0 0 0

The first matrix in the input is:

0 2 3

4 0 0

0 0 5

The second matrix is:

0 0 7

0 8 0

0 9 6

Their product is:

0 43 18

0 0 28

0 45 30

So the output should be:

1 2 43

1 3 18

2 3 28

3 2 45

3 3 30

0 0 0

I have to use a 1D array of linked lists to represent this in my code and all input and output is done through the Python Shell.

Answer 1

It is easy to work with numpy arrays:

import numpy as np
a=np.empty((3,3))
for i in range(a.shape[0]):
    for j in range(a.shape[1]):
        a[i][j]=0
>>> a
array([[ 0.,  0.,  0.],
       [ 0.,  0.,  0.],
       [ 0.,  0.,  0.]])

Answer 2

Python lists aren't sparse - but there are other packages like numpy and scipy that provide them. If your dataset isn't that big and you are okay just prefilling a list, this will do it:

rows = 100
cols = 13
l = [[0]*cols for _ in rows]

The [0]*cols creates an inner list filled with zeros, and [[0]*cols for _ in rows] repeats that operation for the number of rows you want.

After that, you address individual cells like

l[2][3] = 111