在python中打印矩阵的右对角线值

Question

I have a 5 x 5 matrix of integers, and need to use certain code in Python. I need to construct a list containing the values in the cells on the diagonal from top-right to bottom-left of matrix.

matrix = [[ 1,  2,  3,  4,  5],
          [ 6,  7,  8,  9, 10],
          [11, 12, 13, 14, 15],
          [16, 17, 18, 19, 20],
          [21, 22, 23, 24, 25]]

Desired output:

[5, 9, 13, 17, 21]

Partial code attempt:

diagonal = []
for posi in matrix???:
    diagonal.append(??? ???)

Answer 1

Given that you want the diagonal from the upper right to the bottom left, you need matrix positions [(0, n), (1, n-1), ..., (n-1, 1), (n, 0)]. The first value of each pair is easily obtained using range(len(matrix)) . The second value can be obtained by subtracting the first value (ie the row number) from the length or width of the square matrix (and then subtracting one more due to zero based indexing). Now you just look up each row/column tuple pair to get the index.

diagonal = []
for row in range(len(matrix)):
    col = len(matrix) - row - 1  # zero based indexing
    diagonal.append(matrix[row][col])

>>> diagonal
[5, 9, 13, 17, 21]

Answer 2

I would suggest using the corresponding numpy classes, in this case numpy.matrix . Numpy then allows us to flip values from left to right and extract the diagonal.

import numpy as np

mymatrix = np.matrix([[ 1,  2,  3,  4,  5],
          [ 6,  7,  8,  9, 10],
          [11, 12, 13, 14, 15],
          [16, 17, 18, 19, 20],
          [21, 22, 23, 24, 25]])

mydiagonal = np.matrix.diagonal(np.fliplr(mymatrix))

print(mydiagonal)
[[ 5  9 13 17 21]]

If the output has to be a list, it's easy to convert using tolist() :

print(mydiagonal.tolist()[0])
[5, 9, 13, 17, 21]

Answer 3

many code for easy thing

matrix = [[ 1,  2,  3,  4,  5],
          [ 6,  7,  8,  9, 10],
          [11, 12, 13, 14, 15],
          [16, 17, 18, 19, 20],
          [21, 22, 23, 24, 25]]

#this mode is faster
matrix=[matrix[c][d] for c in xrange(5) for d in xrange(5)]
print matrix

matrix=[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25]

#then we work better
print'Result',matrix[4:-1:4]

Result [5, 9, 13, 17, 21]

#and the asc diagonal
print'Result',matrix[::6]

Result [1, 7, 13, 19, 25]