Numpy - appending to an empty array the column means of an existing array

Question

I'm trying to use the numpy module (which I imported as np) to find the means of the column vectors and then substract the means from the columns. However, when I try to get the means and append it to an empty array, it starts with 0 and has the wrong values. Any ideas what I could be doing wrong?

rating2 = np.array(student_data.values[:,1:])
print(rating2)
means = np.empty([1,25])
for index in range(0,24):
    b = np.mean(rating2[:,index], axis = 0)
    print("b is", b)
    np.append(means,b)
A = np.array(means)
print(A)

I checked each mean that's being calculated, and it doesnt start with 0. I checked my indexing, and it seems right.

My output:

[[3 2 3 3 3 3 3 3 4 3 4 4 3 4 3 2 2 3 4 2 2 3 3 4 4]
 [2 3 4 3 2 3 4 3 3 4 5 3 4 3 3 1 1 3 3 2 1 3 3 4 3]
 [2 5 4 2 3 2 4 4 1 4 3 1 4 2 2 3 3 2 2 4 3 2 1 3 2]
 [3 4 3 3 3 3 3 3 3 3 3 2 3 3 3 3 3 3 3 4 4 3 3 3 3]
 [4 1 1 5 4 4 2 2 4 1 1 4 1 4 4 4 4 4 4 2 4 4 5 2 4]
 [4 2 2 4 3 3 2 3 4 2 2 4 2 4 4 3 3 4 4 2 3 4 4 3 4]
 [2 5 5 1 2 2 4 4 2 5 5 2 5 2 2 2 2 2 2 4 2 2 1 4 2]
 [3 4 3 3 3 3 3 3 2 3 3 2 3 2 3 4 4 3 2 4 4 3 2 2 2]
 [4 1 2 4 3 4 2 2 4 2 3 5 3 4 4 3 3 4 4 2 2 4 4 4 4]
 [3 4 3 3 3 3 3 3 2 3 2 2 3 2 3 4 4 3 2 4 4 3 3 2 2]]
b is 3.0
b is 3.1
b is 3.0
b is 3.1
b is 2.9
b is 3.0
b is 3.0
b is 3.0
b is 2.9
b is 3.0
b is 3.1
b is 2.9
b is 3.1
b is 3.0
b is 3.1
b is 2.9
b is 2.9
b is 3.1
b is 3.0
b is 3.0
b is 2.9
b is 3.1
b is 2.9
b is 3.1
[[0.  3.  3.1 3.  3.1 2.9 3.  3.  3.  2.9 3.  3.1 2.9 3.1 3.  3.1 2.9 2.9
  3.1 3.  3.  2.9 3.1 2.9 3.1]]

In general, loops and appending should be avoided when using NumPy. Have you looked at the numpy docs? — AMC
– AMC, Commented Apr 15, 2020 at 19:11
np.append is not a list.append clone. Don't use it in the same way. — hpaulj
– hpaulj, Commented Apr 15, 2020 at 19:26

MachineLearner · Accepted Answer · 2020-04-15 18:42:45Z

1

This should work

import numpy as np

# your numpy array
matrix = np.random.random((10, 10))

# this is the column wise mean
mean = np.mean(matrix, axis=0)

# this line is broadcasting the mean to the matrix
matrix_without_mean = matrix - mean

answered Apr 15, 2020 at 18:42

MachineLearner

9772 gold badges8 silver badges23 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Numpy - appending to an empty array the column means of an existing array

1 Answer 1

Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Related