broncoAbierto - 5 months ago 42

Python Question

I was growing confused during the development of a small Python script involving matrix operations, so I fired up a shell to play around with a toy example and develop a better understanding of matrix indexing in Numpy.

This is what I did:

`>>> import numpy as np`

>>> A = np.matrix([1,2,3])

>>> A

matrix([[1, 2, 3]])

>>> A[0]

matrix([[1, 2, 3]])

>>> A[0][0]

matrix([[1, 2, 3]])

>>> A[0][0][0]

matrix([[1, 2, 3]])

>>> A[0][0][0][0]

matrix([[1, 2, 3]])

As you can imagine, this has

What is, then, the logic to the output I obtained? Why would the first element of a matrix object be itself?

PS: I know how to obtain the first entry of the matrix. What I am interested in is the logic behind this design decision.

EDIT: I'm not asking how to access a matrix element, or why a matrix row behaves like a matrix. I'm asking for a definition of the behavior of a matrix when indexed with a single number. It's an action typical of arrays, but the resulting behavior is nothing like the one you would expect from an array. I would like to know how this is implemented and what's the logic behind the design decision.

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Answer Source

Look at the shape after indexing:

```
In [295]: A=np.matrix([1,2,3])
In [296]: A.shape
Out[296]: (1, 3)
In [297]: A[0]
Out[297]: matrix([[1, 2, 3]])
In [298]: A[0].shape
Out[298]: (1, 3)
```

The key to this behavior is that `np.matrix`

is always 2d. So even if you select one row (`A[0,:]`

), the result is still 2d, shape `(1,3)`

. So you can string along as many `[0]`

as you like, and nothing new happens.

What are you trying to accomplish with `A[0][0]`

? The same as `A[0,0]`

?
For the base `np.ndarray`

class these are equivalent.

Note that `Python`

interpreter translates indexing to `__getitem__`

calls.

```
A.__getitem__(0).__getitem__(0)
A.__getitem__((0,0))
```

`[0][0]`

is 2 indexing operations, not one. So the effect of the second `[0]`

depends on what the first produces.

For an array `A[0,0]`

is equivalent to `A[0,:][0]`

. But for a matrix, you need to do:

```
In [299]: A[0,:][:,0]
Out[299]: matrix([[1]]) # still 2d
```

=============================

"An array of itself", but I doubt anyone in their right mind would choose that as a model for matrices in a scientific library.

What is, then, the logic to the output I obtained? Why would the first element of a matrix object be itself?

In addition, A[0,:] is not the same as A[0]

In light of these comments let me suggest some clarifications.

`A[0]`

does not mean 'return the 1st element'. It means select along the 1st axis. For a 1d array that means the 1st item. For a 2d array it means the 1st row. For `ndarray`

that would be a 1d array, but for a `matrix`

it is another `matrix`

. So for a 2d array or matrix, `A[i,:]`

is the same thing as `A[i]`

.

`A[0]`

does not just return itself. It returns a new matrix. Different `id`

:

```
In [303]: id(A)
Out[303]: 2994367932
In [304]: id(A[0])
Out[304]: 2994532108
```

It may have the same data, shape and strides, but it's a new object. It's just as unique as the `ith`

row of a many row matrix.

Most of the unique `matrix`

activity is defined in: `numpy/matrixlib/defmatrix.py`

. I was going to suggest looking at the `matrix.__getitem__`

method, but most of the action is performed in `np.ndarray.__getitem__`

.

`np.matrix`

class was added to `numpy`

as a convenience for old-school MATLAB programmers. `numpy`

arrays can have almost any number of dimensions, 0, 1, .... MATLAB allowed only 2, though a release around 2000 generalized it to 2 or more.

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