numpy - What exactly A[:, state] means in the python?

Question

What exactly this the following line means?

# p=T[:, state] what does this means?

# Here is the complete code
import numpy as np

T = np.array([ [ 0.40, 0.56, 0.03, 0.01],
               [0.45, 0.51, 0.04, 0.00],
               [0.25, 0.25, 0.25, 0.25 ],
               [0.00, 0.00, 0.01, 0.99 ]])

xk = np.arange(len(T))

def gen_sample(state):
    return np.random.choice(xk, 1, p=T[:, state])

I understand it takes the transition matrix but what does " : " and "state" mean?

Answer 1

T is a numpy array:

In [38]: T
Out[38]: 
array([[0.4 , 0.56, 0.03, 0.01],
       [0.45, 0.51, 0.04, 0.  ],
       [0.25, 0.25, 0.25, 0.25],
       [0.  , 0.  , 0.01, 0.99]])

T[..] is indexing; in this case is selects a column of the array:

In [39]: T[:,0]
Out[39]: array([0.4 , 0.45, 0.25, 0.  ])
In [40]: T[:,3]
Out[40]: array([0.01, 0.  , 0.25, 0.99])

Spend some time to read the numpy basics. Indexing an array is a very basic operation.