在python中的列表列表上执行行和和列和

Question

I want to calculate the row sum and column sum of a matrix in python; however, because of infosec requirements I cannot use any external libraries. So to create a matrix, I've used a list of lists, as follows:

matrix =  [[0 for x in range(5)] for y in range(5)]
for pos in range(5):
    matrix[pos][pos]=1
matrix[2][2]= 0

Now what I want to do is perform a rowsum and a column sum of the matrix. I know how to do a row sum, that's quite easy:

sum(matrix[0])

but what if I wanted to do a column sum? Is there a more elegant and pythonic way to accomplish that beyond brute-forcing it with a for loop, a la

sumval = 0
for pos in range(len(matrix[0])):
    sumval = matrix[pos][0] + sumval

which would work, but it isn't pythonic at all.

Can anyone help me out?

Answer 1

colsum = sum(row[0] for row in matrix)

As a note for others who look at this question though, this really is a task best left to numpy . OP is not allowed external libraries however.

Answer 2

I'd suggest:

s = 0
for row in matrix:
    s += row[0]

which is the same as you are doing but a bit more readable.

Using something like:

s = sum([row[0] for row in matrix])

is also readable, but slower because you need to do one pass to collect the row[0] elements, and a second to sum.

Answer 3

您可以使用：

sum([matrix[i][0] for i in range(len(matrix[0]))])

Answer 4

I can suggest, define a method to calculate the sum by rows, which returns the list of sums:

def sum_rows(matrix):
  return [sum(row) for row in matrix]

Then define a method that calls sum_rows(matrix) on the transposed matrix:

def sum_cols(matrix):
  return sum_rows(map(list, zip(*matrix)))

For transposing a matrix: Transpose list of lists

Alternative to transpose:

def sum_cols_alt(matrix):
  return [ sum(row[i] for row in matrix) for i, _ in enumerate(matrix) ]

Answer 5

One way to do this is to use the map function:

for sum_row, sum_col in zip(map(sum, matrix), map(sum, zip(*matrix))):
    print(sum_row, sum_col)