Print only the column of a matrix

Question

I’m a beginner in python and I’m having doubts about the use of list and tuple and its operators. I’m trying to print only the column of an array but I’m not getting it. I’ve tried the following to print the second column:

matriz = [[1, 5], 
          [7, 4], 
          [8, 3]]

print matriz[0:3][1]

But he is printing only the second line, as if he were print matriz[1] only. The [0:3] does not indicate that it must traverse all lines and the [1] what should print the second column? I would like to know how these operators work. I have also seen in other codes using something like matriz[0, 1] instead of matriz[0][1], however if I do not separate rows from columns with brackets, my program gives error.

Answer 1

That’s not what that expression does. lista[inicio:fim] creates a sublist started (including) in inicio and terminated (exclusively) in fim:

>>> [0,1,2,3,4,5][1:4]
[1, 2, 3]

>>> [0,1,2][0:3]
[0, 1, 2]

Note that in this second example the list was the same (because you started at its beginning and went all the way. Similarly, in your code the returned sublist is being equal to the original matrix, and by accessing your second element you are actually taking the second line:

>>> matriz = [[1,5],
...           [7,4],
...           [8,3]]
>>> matriz[0:3]
[[1, 5], [7, 4], [8, 3]]
>>> matriz[0:3][1]
[7, 4]

If you want the second element of each matrix row, you can use an understanding of lists:

>>> [x[1] for x in matriz]
[5, 4, 3]

Then just add a loop to print them one by one:

>>> for v in [x[1] for x in matriz]:
...     print v
...
5
4
3

(although in this case it is silly to do this, just iterate on the original list and print v[1]...)

By the way, if instead of a list you have an object, you can use other complex objects (such as tuples) as keys. Then it would make sense something like matriz[0, 1], but it would be accessing a specific property of the object and not two distinct values of a nested list:

>>> x = []
>>> x[1, 2] = 10
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list indices must be integers, not tuple

>>> x = {}
>>> x[1, 2] = 10
>>> x
{(1, 2): 10}

But if what you have are lists of lists, in fact it takes two uses of the brackets to access an internal element, you cannot (unfortunately...) use tuples as indexes.

---

Note: if it is sorely needed, you could put the print within the very understanding of lists, avoiding a loop. In Python 3 at least, I don’t know if you can do this in Python 2:

>>> [print(x[1]) for x in matriz]
5
4
3
[None, None, None]

(this last line is the list created, where each element is the return of the print - None; normally it will be ignored)

Just be careful that this creates a new list, the same size as the original (1st level), which increases memory consumption. This technique would only be recommended if you were in a context where only one expression fits (within a lambda, for example) and - concisely - you wanted to avoid creating a new function.

Answer 2

Imagine your matrix is a table and you have an Axis: x and y:

     0 1
   .-----> x
0  | 1 5
1  | 7 4
2  | 8 3
   V
   y

If you want to access the position 7 (x=>0,y=>1). Just do it through the indexes:

matriz[1][0];

That in theory would be something represented by:

 matriz[y][x]

Answer 3

To work with matrices, it’s nice to use numpy.

With it you can do:

>>> import numpy as np
>>> a = np.array([[1, 5], [7, 4], [8, 3]])
>>> print a.transpose()[0]
[1 7 8]

Answer 4

Save Andre you want something like this:

>>> matriz = [[1, 5], 
...           [7, 4], 
...           [8, 3]]
>>> for m in matriz:
...     print m[1]
... 
5
4
3
>>> 

If you want the first column just change m[0]