CODEWITHSUNDEEP
pythonnumpy
user12551
user12551

Reputation: 151

What is the built-in function A of numpy array.A?

Recently I read a line of code that confused me:

pointsInCurrCluster = dataSet[nonzero(clusterAssment[:, 0].A == i)[0], :]

The author did not define the function for A, so I assume that .A is some kind of built-in function. Does anyone know what it is?

Upvotes: 1

Views: 6442

Answers (4)

Shreejit Ray
Shreejit Ray

Reputation: 11

So, in python you can get result of a conditional check on each element of an array by writing a statement like: arr > 3. What it does is, for an array arr = [[1,2,3],[3,4,5]] the output you will have is [[False,False,False],[False,True,True]]. Now having said that you need an array to do this. That is what a .A does in python, it gives you an array representation of the matrix. Now, clusterAssment[:, 0].A == i, gives you conditional check answer for every row and first column against the value i. nonzero(clusterAssment[:, 0].A == i) converts the conditional check to index of rows and columns which Satisfy the condition. More details here: nonZero . Now, since clusterAssment is a 2-D array the nonzero(~)[0] gives the rows which have the value i as 1st element, and dataSet[nonzero(clusterAssment[:, 0].A == i)[0], :] gives all those respective tuples from dataset.

Upvotes: 0

user6767234
user6767234

Reputation:

.A means 'change the data type from a matrix to an array'

For example:
enter image description here

Upvotes: 0

hpaulj
hpaulj

Reputation: 231510

In https://github.com/skodali1/python-machine-learning/blob/master/kmeansclusteringalgo.py (found by google search for 'python clusterAssment'

from numpy import *
clusterAssment = matrix(zeros((m,2)))
...
ptsInClust = dataSet[nonzero(clusterAssment[:,0].A==cent)[0]]

In this case clusterAssment is a numpy.matrix object. This is like a numpy.ndarray, except it is always 2d, and has MATLAB like matrix operators.

clusterAssment.A

just turns the matrix into a regular numpy.array, probably so it can be passed to numpy.nonzero.

scipy.sparse implements sparse matrices, which also have this .A property. But based on this code, I don't think that applies here.

Upvotes: 1

Marcin
Marcin

Reputation: 238487

Ok, so in this way I think its is proparty of scipy matrix returning nparray:

In [115]: mtx = sp.matrix([1,2,3])    
In [116]: mtx.A
Out[116]: array([[1, 2, 3]])

A is getter/shortcut for getA.

Upvotes: 1

Related Questions