String to n*n matrix in python

Question

I am an undergraduate student who loves programming. I encountered a problem today and I don't know how to solve this problem. I looked for "Python - string to matrix representation" (Python - string to matrix representation) for help, but I am still confused about this problem.

The problem is in the following:

Given a string of whitespace separated numbers, create an nxn matrix (a 2d list where with the same number of columns as rows)and return it. The string will contain a perfect square number of integers. The int() and split() functions may be useful.

Example:

Input: '1 2 3 4 5 6 7 8 9'

Output: [[1,2,3],[4,5,6],[7,8,9]]

Example 2:

Input: '1'

Output: [[1]]

My answer:

import numpy as np
def string_to_matrix(str_in):
    str_in_split = str_in.split()
    answer = []
    for element in str_in_split:
        newarray = []
    for number in element.split():
        newarray.append(int(number))
    answer.append(newarray)
    print (answer)

The test results are in the following:

Traceback (most recent call last):
  File "/grade/run/test.py", line 20, in test_whitespace
    self.assertEqual(string_to_matrix('1      2 3   4'), [[1,2],[3,4]])
AssertionError: None != [[1, 2], [3, 4]]

Stdout:
[[4]]

as well as

Traceback (most recent call last):
  File "/grade/run/test.py", line 15, in test_small
    self.assertEqual(string_to_matrix('1 2 3 4'), [[1,2],[3,4]])
AssertionError: None != [[1, 2], [3, 4]]

Stdout:
[[4]]

as well as

Traceback (most recent call last):
  File "/grade/run/test.py", line 10, in test_one
    self.assertEqual(string_to_matrix('1'), [[1]])
AssertionError: None != [[1]]

Stdout:
[[1]]

as well as

Traceback (most recent call last):
  File "/grade/run/test.py", line 25, in test_larger
    self.assertEqual(string_to_matrix('4 3 2 1 8 7 6 5 12 11 10 9 16 15 14 13'), [[4,3,2,1], [8,7,6,5], [12,11,10,9], [16,15,14,13]])
AssertionError: None != [[4, 3, 2, 1], [8, 7, 6, 5], [12, 11, 10, 9], [16, 15, 14, 13]]

Stdout:
[[13]]

I am still confused how to solve this problem. Thank you very much for your help!

assertEqual is not going to compare your print result, it uses the value returned by string_to_matrix. string_to_matrix currently returns nothing so the value is None. Change print for return and you will start getting better resutls — Andres
– Andres, Commented Feb 15, 2019 at 2:00
Firstly, the function you created only print out the answer, rather than returning it, thus the assertion will always fail as None is what functions return by default. Replace print (answer) with simply return answer. Secondly, it doesn't seem like you taken into account of "The string will contain a perfect square number of integers". — metatoaster
– metatoaster, Commented Feb 15, 2019 at 2:01

iz_ · Accepted Answer · 2019-02-15 02:04:20Z

2

Assuming you don't want numpy and want to use a list of lists:

def string_to_matrix(str_in):
    nums = str_in.split()
    n = int(len(nums) ** 0.5)
    return list(map(list, zip(*[map(int, nums)] * n)))

nums = str_in.split() splits by any whitespace, n is the side length of the result, map(int, nums) converts the numbers to integers (from strings), zip(*[map(int, nums)] * n) groups the numbers in groups of n, list(map(list, zip(*[map(int, nums)] * n))) converts the tuples produced by zip into lists.

answered Feb 15, 2019 at 2:04

iz_

16.7k4 gold badges29 silver badges43 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Larry Chen Over a year ago

Thanks a lot! Your explanation of "zip(*[map(int, nums)] * n) groups the numbers in groups of n, list(map(list, zip(*[map(int, nums)] * n))) converts the tuples produced by zip into lists." is very helpful for me to understand why it could be written like this.

hpaulj Over a year ago

This use of zip is shown in the grouper recipe at docs.python.org/3/library/itertools.html#itertools-recipes. Unfortunately it doesn't explain how it works. It takes advantage of a couple of Python features that often trip up users.

user2263572 · Accepted Answer · 2019-02-15 02:02:08Z

1

Assuming you want to make this dynamic.

str_in = '1 2 3 4 5 6 7 8 9'
a = str_in.split(" ")
r_shape = int(math.sqrt(len(a)))
np.array([int(x) for x in a]).reshape(r_shape, r_shape)

answered Feb 15, 2019 at 2:02

user2263572

5,6165 gold badges40 silver badges61 bronze badges

Comments

Nikolas Stevenson-Molnar · Accepted Answer · 2019-02-15 17:18:47Z

1

Use split, create the 1D numpy array, then use reshape:

>>> s = '1 2 3 4 5 6 7 8 9'
>>> np.array([s.split(), dtype=int).reshape(3,3)
array([[1, 2, 3],
       [4, 5, 6],
       [7, 8, 9]])

If you don't know the size of the array, but you know it's square (same width / height), then you can use math.sqrt to get the inputs for reshape:

>>> import math
>>> s = '1 2 3 4 5 6 7 8 9'
>>> arr = np.array(s.split(), dtype=int)
>>> size = int(math.sqrt(len(arr)))
>>> arr.reshape(size, size)
array([[1, 2, 3],
       [4, 5, 6],
       [7, 8, 9]])

edited Feb 15, 2019 at 17:18

answered Feb 15, 2019 at 1:58

Nikolas Stevenson-Molnar

4,7452 gold badges25 silver badges31 bronze badges

2 Comments

hpaulj Over a year ago

You don't need the int map or comprehension. Just ask np.array to produce an integer array: np.array(s.split(), dtype=int)

Nikolas Stevenson-Molnar Over a year ago

Ah neat! Thanks for the tip!

Chris · Accepted Answer · 2019-02-15 02:01:47Z

0

Given that you will always get perfect square number of ints:

import numpy as np

input_strings = '1 2 3 4 5 6 7 8 9'
arr = np.array(input_strings.split(), dtype=int)
n = int(len(arr) ** 0.5)
arr.reshape(n, n)
array([[1, 2, 3],
       [4, 5, 6],
       [7, 8, 9]])

Note: in your case, str.split is better off without explicit sep in order to work fine with multiple whitespaces in between the digits.

answered Feb 15, 2019 at 2:01

Chris

29.8k3 gold badges34 silver badges56 bronze badges

Comments

Andres · Accepted Answer · 2019-02-15 02:04:41Z

0

import numpy as np
def string_to_matrix(str_in):
   str_in_split = str_in.split()
   numbers = list(map(int, str_in_split))
   size = r_shape = int(np.sqrt(len(numbers)))
   return np.array(numbers).reshape(r_shape, r_shape)

This is why you always got: AssertionError: None != ...

assertEqual(A, string_to_matrix("...")) verifies if A is equals to the value returned by string_to_matrix. In your code you don't return anything so it is None

The other issue is how you splitted the string, the easier options is to split everything and convert to number, and then reshape to sqrt(number of elements). This assumes that input length can be splited to form a nxn matrix

answered Feb 15, 2019 at 2:04

Andres

4,5318 gold badges42 silver badges53 bronze badges

Comments

user11065131 · Accepted Answer · 2019-02-15 02:52:24Z

-1

import math
string = "1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16"
stringItems = string.split(" ")
numberOfItems = len(stringItems)
if math.sqrt(numberOfItems) - int(math.sqrt(numberOfItems)) == 0:
    width = int(math.sqrt(numberOfItems))
    array = []
    finalArray = []
    for i in range (0, width):
        for j in range (0, width):
            array.insert(j, stringItems[0])
            stringItems.pop(0)
        finalArray.insert(i, array)
        array = []
    print finalArray
else:
    print "I require a string with length equal to a square number please"

answered Feb 15, 2019 at 2:52

user11065131

1

1 Comment

RaminS Over a year ago

Answers should consist of more than a mere code-dump. If you think the question is poorly asked, you can flag it or post a comment.

Collectives™ on Stack Overflow

String to n*n matrix in python

6 Answers 6

2 Comments

Comments

2 Comments

Comments

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

2 Comments

Comments

2 Comments

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related