Avatar of davearonson

davearonson's solution

to Matrix in the Python Track

Published at May 16 2019 · 0 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

Given a string representing a matrix of numbers, return the rows and columns of that matrix.

So given a string with embedded newlines like:

9 8 7
5 3 2
6 6 7

representing this matrix:

    1  2  3
  |---------
1 | 9  8  7
2 | 5  3  2
3 | 6  6  7

your code should be able to spit out:

  • A list of the rows, reading each row left-to-right while moving top-to-bottom across the rows,
  • A list of the columns, reading each column top-to-bottom while moving from left-to-right.

The rows for our example matrix:

  • 9, 8, 7
  • 5, 3, 2
  • 6, 6, 7

And its columns:

  • 9, 5, 6
  • 8, 3, 6
  • 7, 2, 7

Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of raise Exception, you should write:

raise Exception("Meaningful message indicating the source of the error")

Running the tests

To run the tests, run the appropriate command below (why they are different):

  • Python 2.7: py.test matrix_test.py
  • Python 3.4+: pytest matrix_test.py

Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version): python -m pytest matrix_test.py

Common pytest options

  • -v : enable verbose output
  • -x : stop running tests on first failure
  • --ff : run failures from previous test before running other test cases

For other options, see python -m pytest -h

Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the $EXERCISM_WORKSPACE/python/matrix directory.

You can find your Exercism workspace by running exercism debug and looking for the line that starts with Workspace.

For more detailed information about running tests, code style and linting, please see Running the Tests.

Source

Warmup to the saddle-points warmup. http://jumpstartlab.com

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

matrix_test.py

import unittest

from matrix import Matrix


# Tests adapted from `problem-specifications//canonical-data.json` @ v1.1.0

class MatrixTest(unittest.TestCase):
    def test_extract_row_from_one_number_matrix(self):
        matrix = Matrix("1")
        self.assertEqual(matrix.row(1), [1])

    def test_can_extract_row(self):
        matrix = Matrix("1 2\n3 4")
        self.assertEqual(matrix.row(2), [3, 4])

    def test_extract_row_where_numbers_have_different_widths(self):
        matrix = Matrix("1 2\n10 20")
        self.assertEqual(matrix.row(2), [10, 20])

    def test_can_extract_row_from_non_square_matrix(self):
        matrix = Matrix("1 2 3\n4 5 6\n7 8 9\n8 7 6")
        self.assertEqual(matrix.row(3), [7, 8, 9])

    def test_extract_column_from_one_number_matrix(self):
        matrix = Matrix("1")
        self.assertEqual(matrix.column(1), [1])

    def test_can_extract_column(self):
        matrix = Matrix("1 2 3\n4 5 6\n7 8 9")
        self.assertEqual(matrix.column(3), [3, 6, 9])

    def test_can_extract_column_from_non_square_matrix(self):
        matrix = Matrix("1 2 3\n4 5 6\n7 8 9\n8 7 6")
        self.assertEqual(matrix.column(3), [3, 6, 9, 6])

    def test_extract_column_where_numbers_have_different_widths(self):
        matrix = Matrix("89 1903 3\n18 3 1\n9 4 800")
        self.assertEqual(matrix.column(2), [1903, 3, 4])


if __name__ == '__main__':
    unittest.main()

daves_matrix_test.py

import pytest

from matrix import Matrix


# First some fairly normal tests:

class TestWith3x3Matrix:

    matrix = Matrix("1 2 3\n4 5 6\n7 8 9")

    def test_extract_first_col(self):
        assert self.matrix.column(1) == [1, 4, 7]

    def test_extract_first_row(self):
        assert self.matrix.row(1) == [1, 2, 3]

    def test_extract_middle_col(self):
        assert self.matrix.column(2) == [2, 5, 8]

    def test_extract_middle_row(self):
        assert self.matrix.row(2) == [4, 5, 6]

    def test_extract_last_col(self):
        assert self.matrix.column(3) == [3, 6, 9]

    def test_extract_last_row(self):
        assert self.matrix.row(3) == [7, 8, 9]


# make sure it works with non-rectangular matrix

class TestWith2x3Matrix:

    matrix = Matrix("1 2\n3 4\n5 6")

    def test_extract_first_row(self):
        assert self.matrix.row(1) == [1, 2]

    def test_extract_middle_row(self):
        assert self.matrix.row(2) == [3, 4]

    def test_extract_last_row(self):
        assert self.matrix.row(3) == [5, 6]

    def test_extract_first_col(self):
        assert self.matrix.column(1) == [1, 3, 5]

    def test_extract_last_col(self):
        assert self.matrix.column(2) == [2, 4, 6]


# make sure it works with matrix as skinny as possible

class TestWith1x3Matrix:

    matrix = Matrix("1\n2\n3")

    def test_extract_first_row(self):
        assert self.matrix.row(1) == [1]

    def test_extract_middle_row(self):
        assert self.matrix.row(2) == [2]

    def test_extract_last_row(self):
        assert self.matrix.row(3) == [3]

    def test_extract_col(self):
        assert self.matrix.column(1) == [1, 2, 3]


# make sure it works with matrix as short as possible

class TestWith3x1Matrix:

    matrix = Matrix("1 2 3")

    def test_extract_row(self):
        assert self.matrix.row(1) == [1, 2, 3]

    def test_extract_first_col(self):
        assert self.matrix.column(1) == [1]

    def test_extract_middle_col(self):
        assert self.matrix.column(2) == [2]

    def test_extract_last_col(self):
        assert self.matrix.column(3) == [3]


# make sure it works with matrix as small as possible

class TestWith1x1Matrix:

    matrix = Matrix("1")

    def test_extract_row(self):
        assert self.matrix.row(1) == [1]

    def test_extract_col(self):
        assert self.matrix.column(1) == [1]


# make sure it works with mixed width numbers

class TestWithMixedWidth3x3Matrix:

    matrix = Matrix("1 23 456\n789 10 2\n34 5678 90120")

    def test_extract_first_col(self):
        assert self.matrix.column(1) == [1, 789, 34]

    def test_extract_first_row(self):
        assert self.matrix.row(1) == [1, 23, 456]

    def test_extract_middle_col(self):
        assert self.matrix.column(2) == [23, 10, 5678]

    def test_extract_middle_row(self):
        assert self.matrix.row(2) == [789, 10, 2]

    def test_extract_last_col(self):
        assert self.matrix.column(3) == [456, 2, 90120]

    def test_extract_last_row(self):
        assert self.matrix.row(3) == [34, 5678, 90120]


# Now some bulletproofing.  Not necessary for normal academic exercise, but I'm
# using Exercism to lead some juniors through learning Python and testing, and
# we're doing security-related software so we should have more than usual.


class TestWithInvalidIndex:

    matrix = Matrix("1 2\n3 4")

    def test_extract_row_too_high(self):
        assert self.matrix.row(3) is None

    def test_extract_row_too_low(self):
        assert self.matrix.row(0) is None

    def test_extract_col_too_high(self):
        assert self.matrix.column(3) is None

    def test_extract_col_too_low(self):
        assert self.matrix.column(0) is None


class TestWithEmptyMatrix:

    matrix = Matrix("")

    def test_extract_row(self):
        assert self.matrix.row(1) is None

    def test_extract_col(self):
        assert self.matrix.column(1) is None

    # Having these return [] would be OK too, so long as they're
    # consistent between them.


# NOT BOTHERING TO TEST:
# - matrices that are not rectangular/square
# - numbers other than positive integers
# - scientific notation
# - non-numbers, tho it should be easy to adapt this to any unbroken string
# - matrices of multiple blank rows (e.g, "\n\n\n" -> [[],[],[],[]]

matrix.py

class Matrix:

    def __init__(self, matrix_string):
        # use splitlines() instead of split("\n") because
        # "".splitlines() is [] but "".split("\n") is [""],
        # which is not totally empty.
        rows = matrix_string.splitlines()
        self.rows = [list(map(int, row.split())) for row in rows]

    def row(self, index):
        if not self.rows: return None
        if index < 1: return None
        if index > len(self.rows): return None
        return self.rows[index-1]

    def column(self, index):
        if not self.rows: return None
        if index < 1: return None
        if index > len(self.rows[0]): return None
        return [row[index-1] for row in self.rows]

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?