Python 3 Adding Diagonals in an Array

Question

I can't seem to find anything about adding both diagonals in a 4x4 array.

row = 4
column = 4
lis1 = [23.5, 30.1, 56.2, 11.9]
lis2 = [45.1, 8.9, 77.3, 54.1]
lis3 = [6.6, 7.7, 8.8, 2.2]
lis4 = [9.9, 8.9, 7.8, 23.6]
array = [lis1, lis2, lis3, lis4]

def diagonalSum(array):
    count = 0
    for i in range (len(array)):
        count += array[i][i]
    print ('The total of the elements in both diagonals equals  %.2f' %(count))
    return count

When I call on the function, it prints the total for lis1[0]+lis2[1]+lis3[2]+lis4[3] but I need it to also calculate the total for lis4[0]+lis3[1]+lis2[2]+lis1[3] and display the total of both diagonals. Any suggestions?

whwright · Accepted Answer · 2016-03-30 16:01:16Z

1

You are only counting the diagonal from the top left to the bottom right, in this case 0,0 1,1 2,2 3,3. You also need to count the other diagonal from the top right to the bottom left, or 0,3 1,2 2,1 3,0

To use one loop you could add another index that you increment or decrement yourself, something like this

j = len(array) - 1
for i in range(len(array)):
    # diagonal from top left to bottom right
    count += array[i][i]
    # diagonal from bottom left to top right
    count += array[j][i]
    j -= 1

answered Mar 30, 2016 at 16:01

whwright

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1 Comment

MikeD Over a year ago

I knew it had to be something obvious... brain is just fried. Thank you for the help.

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