Nested Lists and their index calls

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:

The first matrix in the input is:

The second matrix is:

Their product is:

So the output should be:

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.

Use a `dict` with a `tuple` (row, col) is probably the simplest way to emulate a sparse matrix using only Python builtins — Jon Clements, Oct 24 '14 at 17:53
I don't follow. Could you show an example input and the corresponding desired output? — Karl Knechtel, Oct 24 '14 at 17:54
`range(0,rows,1)` is equivalent to `range(rows)`. Starting at 0 and incrementing by 1 are the defaults, so you don't need them. — User, Oct 24 '14 at 17:54
Not sure why, but the instructor specified this: "Your program should represent the sparse matrix as a one-dimensional array of linked lists. " — Udent, Oct 24 '14 at 17:55
@Udent It'll be a good idea to [edit](http://stackoverflow.com/questions/26553392/interrelated-resources) your post with that and any other requirements/guidance you've been given so others can help you better... (and not suggest ways that you're not allowed/expected to do it etc...) — Jon Clements, Oct 24 '14 at 17:56
One good way to manage a sparse matrix in Python is by using Scipy library: http://docs.scipy.org/doc/scipy-0.14.0/reference/sparse.html — mvillaress, Oct 24 '14 at 17:57
Please specify in your post what the output should be. Is it supposed to be a m*m matrix (list of lists)? — User, Oct 24 '14 at 17:57
Ok, put most of the stuff in that he was asking for, sorry just learning how to use this website, will be more thorough from the getgo in the future. — Udent, Oct 24 '14 at 18:01

score 0 · Accepted Answer · answered Oct 24 '14 at 17:58

0

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.]])

answered Oct 24 '14 at 17:58

Irshad Bhat

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for this, are the values automatically converted to floating points? – Udent Oct 24 '14 at 18:09
You can set datatype of your choice as: **a=np.empty((3,3),dtype=int)** – Irshad Bhat Oct 24 '14 at 18:11
This might seem like a dumb question, but when you specify what position of shape, that is just like which one of the 3's specified earlier correct? So for: a = np.empty((x,y)) x = a.shape[0] y = a.shape[1]? – Udent Oct 24 '14 at 18:20

score 0 · Answer 2 · answered Oct 24 '14 at 18:00

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

Nested Lists and their index calls

2 Answers2