List containing lists (matrix) as input() with python 3

Question

I'm trying to convert a given input to a list containing lists as shown in the command-line code below:

matrix = input('asking')
asking[[2, 0, 4],[1, 2, 4],[4, 4, 2]]
matrix
'[[2, 0, 4],[1, 2, 4],[4, 4, 2]]'

desired output:

[[2,0,4],[1,2,4],[4,4,2]]

Attempts

list(matrix)
['[', '[', '2', ',', ' ', '0', ',', ' ', '4', ']', ',', ' ', '[', '1', ',', ' ', '2', ',', ' ', '4', ']', ',', ' ', '[', '4', ',', ' ', '4', ',', ' ', '2', ']', ']']

x = [int(a) for a in matrix] 
builtins.TypeError: int() argument must be a string, a bytes-like object or a number, not 'list'

matrix.split(',')
['[[2', '0', '4]', '[1', '2', '4]', '[4', '4', '2]]']

Code:

fin_result = 0

def perform_check():
    print("Enter form \n [[a,b,c] \n [d,e,f] \n [g,h,i]]")
    #ask_for_input = [[4,-1,1],[4,5,3],[-2,0,0]]
    ask_for_input = [[2,0,4],[1,2,4],[4,4,2]]
    print(type(ask_for_input))

    #call function to cont
    calculate_determinate_matrix(ask_for_input)


def calculate_determinate_matrix(matrix3x3):
    matrix_list2x2 = []

    matrix_list2x2.append([[matrix3x3[1][1], matrix3x3[1][2]], [matrix3x3[2][1], matrix3x3[2][2]]])

    matrix_list2x2.append([[matrix3x3[1][0], matrix3x3[1][2]],[matrix3x3[2][0], matrix3x3[2][2]]])

    matrix_list2x2.append([[matrix3x3[1][0], matrix3x3[1][1]],[matrix3x3[2][0], matrix3x3[2][1]]])

    count = 0
    for count, matrix_2x2 in enumerate(matrix_list2x2):
        if count % 2 == 1:
            calculate_2x2_matrix(matrix_2x2, matrix3x3[0][count] * -1)
        else:
            calculate_2x2_matrix(matrix_2x2, matrix3x3[0][count])

def calculate_2x2_matrix(matrix, mult):
    global fin_result
    result = matrix[0][0] * matrix[1][1]
    result_2 = matrix[0][1] * matrix[1][0]
    fin_result += mult * (result - result_2)    

def main():
    perform_check()
    print(fin_result)

if __name__ == "__main__":
    main()

Clearly this code needs work, but I can't figure this in terms of list comprehension.

zipa · Accepted Answer · 2017-11-14 10:25:11Z

2

Use json loads() method:

matrix = '[[2, 0, 4],[1, 2, 4],[4, 4, 2]]'
matrix = json.loads(matrix)
type(matrix)
<type 'list'>

And in case you want to check for errors in input you can wrap it in try-except:

try:
    matrix = json.loads(matrix)
except ValueError:
    #code goes here

answered Nov 14, 2017 at 10:25

zipa

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2 Comments

Snuffles Over a year ago

Beautiful :) Thank you very much. Didn't even know that library existed.

zipa Over a year ago

You are welcome :) FYI accepting an answer will increase your reputation.

L3viathan · Accepted Answer · 2017-11-14 10:27:49Z

0

An alternative to json.loads is ast.literal_eval. It has the advantage that it recognizes Python syntax even if it was invalid JSON, like single-quoted strings. In your exact case, it doesn't make a difference:

import ast
matrix = '[[2, 0, 4],[1, 2, 4],[4, 4, 2]]'
matrix = ast.literal_eval(matrix)

answered Nov 14, 2017 at 10:27

L3viathan

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2 Comments

Snuffles Over a year ago

Thanks Zipa was quika though :P I don't think i understand the reason for using ast.literal_eval yet, i will have a fiddle.

L3viathan Over a year ago

In your case, use json.loads. But let's say you want a list of strings, e.g. ['foo', 'bar', 'bat']. This will not work with json.loads (although ["foo", "bar", "bat"] will), only with ast.literal_eval. I know zipa's answer is the right solution for you, but StackOverflow is as much for the person asking the questions as it is for those coming here by googling.

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