CODEWITHSUNDEEP
python
Adnan
Adnan

Reputation: 3347

How To Think Like A Computer Scientist (Chapter 9): I don't understand the following

I'm finding difficulting what exactly the doctest in the following code is looking for in the function.

Can someone make it more clear for me, please?

Question 14:

  1. Write functions row_times_column and matrix_mult:
def row_times_column(m1, row, m2, column):
    """
      >>> row_times_column([[1, 2], [3, 4]], 0, [[5, 6], [7, 8]], 0)
      19
      >>> row_times_column([[1, 2], [3, 4]], 0, [[5, 6], [7, 8]], 1)
      22
      >>> row_times_column([[1, 2], [3, 4]], 1, [[5, 6], [7, 8]], 0)
      43
      >>> row_times_column([[1, 2], [3, 4]], 1, [[5, 6], [7, 8]], 1)
      50
    """
def matrix_mult(m1, m2):
   """
      >>> matrix_mult([[1, 2], [3,  4]], [[5, 6], [7, 8]])
      [[19, 22], [43, 50]]
      >>> matrix_mult([[1, 2, 3], [4,  5, 6]], [[7, 8], [9, 1], [2, 3]])
      [[31, 19], [85, 55]]
      >>> matrix_mult([[7, 8], [9, 1], [2, 3]], [[1, 2, 3], [4, 5, 6]])
      [[39, 54, 69], [13, 23, 33], [14, 19, 24]]
    """

Add your new functions to matrices.py and be sure it passes the doctests above.

Upvotes: 1

Views: 656

Answers (3)

Gary Kerr
Gary Kerr

Reputation: 14420

The first function, first example in docstring: take the zeroth row from the first matrix ([1, 2]) and multiply it by the zeroth column of the second matrix ([5, 7])

1 x 5 + 2 x 7 = 19

The second function is just standard matrix multiplication. You can use the first function to answer this problem.

Upvotes: 3

murgatroid99
murgatroid99

Reputation: 20277

The first function, row_times_column multiplies the nth row of the first matrix by the mth column of the second matrix. In the first doctest, for example, n = 0 and m = 0, so we multiply the row matrix [1, 2] by the column matrix [5, 7] to get 1 * 5 + 2 * 7 which is equal to 19 as specified.

The second function is the generalization, and you have to multiply the first matrix by the second. You are probably supposed to use the first function. The linked article shows how to get matrix multiplication from a combination of row by column multiplications.

Upvotes: 1

Will
Will

Reputation: 871

Its asking you to implement some matrix multiplication methods.

In the first one, given a matrix, multiply the row of m1 x the column of m2. The first one given is:

1 2   times   5 6
3 4           7 8

but we only want row 0 times col 0, so it would be

[1 2] x 5 
        7

= 5+14 = 19. And so on for the others...

The second function wants a full matrix multiplication. See http://en.wikipedia.org/wiki/Matrix_multiplication

Upvotes: 3

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