How to Select and Transform Numpy Arrays?

In previous post , we learnt how to create arrays with numpy. Today we’re going to talk about how to index and transform numpy arrays.

When it comes to arrays, we usually use quare brackets [] to index and slice:

arr = np.arange(6).reshape(3,2)

.arange method specified arr array with 6 elements, and .reshape method specified tha array with 3 rows and 2 colonms. Print arr:

print(arr)

Feedback:

array([[0, 1],
          [2, 3],
          [4, 5]])

How to select elements in array?

Select a column

If you want to select a column, say, the rightmost column 1,3,5:

arr[:, 1]

The colon : here means to specify the start and end place you need from the origal array.

Feedback:

array([1, 3, 5])

Select multiple columns

arr[:, [0,1]]

Feedback:

array([[0, 1],
         [2, 3],
         [4, 5]])

Select a row

arr[1, :]

Feedback:

array([2, 3])

Select multiple rows

arr[[0,1], :]

Feedback:

array([[0, 1],
          [2, 3]])

Select a specific element

arr[1,1]

Feedback:

3

Select in single condition

Select a row in which the rightmost elment is greater than 2:

arr[arr[:, 1]>2,]

Feedback:

array([[2, 3],
          [4, 5]])

Select in multiple conditions

Select a row that meets both conditions, which are the rightmost elment is greater than 2 and the rightmost elment is less than 4:

arr[(arr[:,1]>2) & (arr[:, 1]<4),]

Feedback:

array([[2, 3]])

How to transform the array?

In practice, we often need to change the form of the array. There are basically four method:

Change array dimensions

arr.reshape(2,3)

Feedback:

array([[0, 1, 2],
        [3, 4, 5]])

Notice: arr is a new array with a different set of dimensions.

Transposition of arrays

arr.T

or

np.transpose(arr)

.T method or np.transpose() function reversed the look of the array compeletely. We see following below:

array([[0, 2, 4],
        [1, 3, 5]])

arr has not changed yet. When we input print(arr) in the python console, we’ll get the feedback:

  [[0 1]
  [2 3]
  [4 5]]

Flatten the array

arr.flatten()

or:

arr.ravel()

Feedback the same as below:

array([0, 1, 2, 3, 4, 5])

.flatten() and .ravel() functions take us the same one-dimension array.

Notice: ravel usually returns a view into the existing array (sometimes it returns a copy). Flatten returns a new array.

Have a good time, and see you next article.