Summing list of [row, column, value] into 2 dim list in python

Question

I have a list of [row_index, column_index, value]. The row and column can be duplicated. I'd like to summing up this list to a matrix style list.

For instance,

list_of_list = [[0,0,1], [0,0,1], [1,0,3], [1,1,4]]
matrix_style_list = get_matrix_style_list(list_of_list)
print(matrix_style_list)

[[2, 0], [3, 4]]

On above example, value of 0 x 0 is 2 because list_of_list contains [0,0,1], [0,0,1] and 1 + 1 = 2

Answer 1

Build a dictionary that adds each sublist's last value to the value corresponding to a key of a tuple of the sublist's first two values:

>>> list_of_list = [[0,0,1], [0,0,1], [1,0,3], [1,1,4]]
>>> d = {}
>>> for item in list_of_list:
...     *location, datum = item
...     location = tuple(location)
...     d[location] = d.get(location, 0) + datum
...
>>> d
{(1, 0): 3, (0, 0): 2, (1, 1): 4}

Find the size of the matrix:

>>> rows = max(item[0] for item in list_of_list)
>>> columns = max(item[1] for item in list_of_list)

Then you can create a matrix structure with a comprehension:

>>> structure = [[d.get((row, column), 0) for column in range(columns+1)] for row in range(rows+1)]
>>> structure
[[2, 0], [3, 4]]

Answer 2

A simple solution to this would be to first find out the shape of the output array by using max() function , and then create an matrix of that shape with all 0 , and then keep summing the values from your list_of_list to that output_matrix. Example -

list_of_list = [[0,0,1], [0,0,1], [1,0,3], [1,1,4]]
import operator
def get_matrix_style_list(lol):
    x_shape,y_shape = max(lol, key=operator.itemgetter(0,1))[0:2] 
    output_matrix = [[0 for _ in range(y_shape+1)] for _ in range(x_shape +1)]
    for x,y,val in lol:
        output_matrix[x][y] += val 
    return output_matrix

Demo -

>>> list_of_list = [[0,0,1], [0,0,1], [1,0,3], [1,1,4]]
>>> import operator
>>> def get_matrix_style_list(lol):
...     x_shape,y_shape = max(lol, key=operator.itemget
...     output_matrix = [[0 for _ in range(y_shape+1)]
...     for x,y,val in lol:
...         output_matrix[x][y] += val
...     return output_matrix
...
>>> get_matrix_style_list(list_of_list)
[[2, 0], [3, 4]]