How to make matrices in Python?

Question

I've googled it and searched StackOverflow and YouTube.. I just can't get matrices in Python to click in my head. Can someone please help me? I'm just trying to create a basic 5x5 box that displays:

A A A A A
B B B B B
C C C C C
D D D D D
E E E E E

I got

a b c d e
a b c d e
a b c d e
a b c d e
a b c d e

To display but I couldn't even get them to break lines that, instead all they would appear as

[['A', 'B', 'C', 'D', 'E'], ['A', 'B', 'C', 'D', 'E'], ['A', 'B', 'C', 'D', 'E'], ['A', 'B', 'C', 'D', 'E'], ['A', 'B', 'C', 'D', 'E']]

And if I try to add \n to them or print "" etc it just doesn't work.. \n will display as 'A\n' and print will display before the matrix.

Please someone help me, even if you direct me to somewhere that should be really obvious and make me look like an idiot, I just want to learn this.

Answer 1

The answer to your question depends on what your learning goals are. If you are trying to get matrices to "click" so you can use them later, I would suggest looking at a Numpy array instead of a list of lists. This will let you slice out rows and columns and subsets easily. Just try to get a column from a list of lists and you will be frustrated.

Using a list of lists as a matrix...

Let's take your list of lists for example:

L = [list("ABCDE") for i in range(5)]

It is easy to get sub-elements for any row:

>>> L[1][0:3]
['A', 'B', 'C']

Or an entire row:

>>> L[1][:]
['A', 'B', 'C', 'D', 'E']

But try to flip that around to get the same elements in column format, and it won't work...

>>> L[0:3][1]
['A', 'B', 'C', 'D', 'E']

>>> L[:][1]
['A', 'B', 'C', 'D', 'E']

You would have to use something like list comprehension to get all the 1th elements....

>>> [x[1] for x in L]
['B', 'B', 'B', 'B', 'B']

Enter matrices

If you use an array instead, you will get the slicing and indexing that you expect from MATLAB or R, (or most other languages, for that matter):

>>> import numpy as np
>>> Y = np.array(list("ABCDE"*5)).reshape(5,5)
>>> print Y
[['A' 'B' 'C' 'D' 'E']
 ['A' 'B' 'C' 'D' 'E']
 ['A' 'B' 'C' 'D' 'E']
 ['A' 'B' 'C' 'D' 'E']
 ['A' 'B' 'C' 'D' 'E']]
>>> print Y.transpose()
[['A' 'A' 'A' 'A' 'A']
 ['B' 'B' 'B' 'B' 'B']
 ['C' 'C' 'C' 'C' 'C']
 ['D' 'D' 'D' 'D' 'D']
 ['E' 'E' 'E' 'E' 'E']]

Grab row 1 (as with lists):

>>> Y[1,:]
array(['A', 'B', 'C', 'D', 'E'], 
      dtype='|S1')

Grab column 1 (new!):

>>> Y[:,1]
array(['B', 'B', 'B', 'B', 'B'], 
      dtype='|S1')

So now to generate your printed matrix:

for mycol in Y.transpose():
    print " ".join(mycol)


A A A A A
B B B B B
C C C C C
D D D D D
E E E E E