Can anybody explain me the numpy.indices()?

Question

I've read documentation several times about np.indices() but I can't seem to grasp what is it about. I've used it numerous times on things to see what it does, but I still can't really get it. Maybe the thing is I'm a beginner in programming so I can't understand the idea behind the words describing it. In addition I'm not a native English speaker (though I have no problems with it). I would be very grateful for kind of easier explanation, possibly on some example. Thanks.

Answer 1

Suppose you have a matrix M whose (i,j)-th element equals

M_ij = 2*i + 3*j 

One way to define this matrix would be

i, j = np.indices((2,3)) M = 2*i + 3*j 

which yields

array([[0, 3, 6],        [2, 5, 8]]) 

In other words, np.indices returns arrays which can be used as indices. The elements in i indicate the row index:

In [12]: i Out[12]:  array([[0, 0, 0],        [1, 1, 1]]) 

The elements in j indicate the column index:

In [13]: j Out[13]:  array([[0, 1, 2],        [0, 1, 2]]) 

Answer 2

The already posted answers are still complex so here a the simplest way to understand this.

---

Step 1: Let's create a 2x2 grid

ids = np.indices((2,2)) 

Step 2: Now let's unpack the i,j indices

i, j = ids  

These are the indices i,j:

print(i) [[0 0]  [1 1]]  print(j) [[0 1]  [0 1]] 

Step 3: Understand what i,j represent

The easy way to think of it is to make pairs as (i0,j0), (i1,j1), (i2,j2), (i3,j3) i.e. match each element of i with the corresponding element of j.

So we get: (0,0), (0,1), (1,0), (1,1).

These are exactly the indices of a 2x2 grid: