从矩阵python 2的右上角到左下角打印对角线

Question

have a question regarding my code, and I'm really lost I need to create empty diagonal list outputting number from maxtrix from top-right ot bottom-left so it is [5, 9, 13, 17, 21].

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]]

diagonal = []
for posi in matrix: 
    diagonal.append(posi[len(matrix)-1])

print diagonal

This is how far I got but it only outputs the last set of the numbers.

I can only change the value of

for posi in ##here##: 
    diagonal.append(##here## ##here##)

Options I have are:

matrix[posi]
matrix
range(len(matrix)-1)
posi
[len(matrix)-1]
[len(matrix)-1-posi)
posi
And like [4][0], [3][1], [1],[2]
[0,1,2,3,4]

It been trying to figure it out and lost now, any help is greatly appreciated Thank you

Answer 1

If you have to use one of those options, I'd go with:

for posi in [0,1,2,3,4]: 
    diagonal.append(matrix[posi][len(matrix)-1-posi])

Answer 2

Perhaps, here is the simple solution with Enumerate

diagonal = []
for i, j in enumerate(matrix):
    diagonal.append(j[-(i+1)])

# or using list comprehension along with enumerate
diagonal = [j[-(i+1)] for i, j in enumerate(matrix)]

print diagonal

Answer 3

While the option being looked for is probably:

for posi in range(len(matrix)):
    diagonal.append(matrix[posi][len(matrix)-1-posi])

This isn't very pythonic and generally you would iterate over the matrix vs. indices and just use a negative index to count from the back, so a simple list comprehension:

diagonal = [row[-posi] for posi, row in enumerate(matrix, 1)]
# [5, 9, 13, 17, 21]