Python数据科学：NumPy Cheat Sheet

Key and Imports

In this cheat sheet, we use the following shorthand:

arr | A NumPy Array object

You’ll also need to import numpy to get started:

import numpy as np

Importing/exporting

np.loadtxt(‘file.txt’) | From a text file np.genfromtxt(‘file.csv’,delimiter=’,’) | From a CSV file np.savetxt(‘file.txt’,arr,delimiter=’ ‘) | Writes to a text file np.savetxt(‘file.csv’,arr,delimiter=’,’) | Writes to a CSV file

Creating Arrays

np.array([1,2,3]) | One dimensional array np.array([(1,2,3),(4,5,6)]) | Two dimensional array np.zeros(3) | 1D array of length 3 all values 0 np.ones((3,4)) | 3x4 array with all values 1 np.eye(5) | 5x5 array of 0 with 1 on diagonal (Identity matrix) np.linspace(0,100,6) | Array of 6 evenly divided values from 0 to 100 np.arange(0,10,3) | Array of values from 0 to less than 10 with step 3 (eg [0,3,6,9]) np.full((2,3),8) | 2x3 array with all values 8 np.random.rand(4,5) | 4x5 array of random floats between 0-1 np.random.rand(6,7)*100 | 6x7 array of random floats between 0-100 np.random.randint(5,size=(2,3)) | 2x3 array with random ints between 0-4

Inspecting Properties

arr.size | Returns number of elements in arr arr.shape | Returns dimensions of arr (rows,columns) arr.dtype | Returns type of elements in arr arr.astype(dtype) | Convert arr elements to type dtype arr.tolist() | Convert arr to a Python list np.info(np.eye) | View documentation for np.eye

Copying/sorting/reshaping

np.copy(arr) | Copies arr to new memory arr.view(dtype) | Creates view of arr elements with type dtype arr.sort() | Sorts arr arr.sort(axis=0) | Sorts specific axis of arr two_d_arr.flatten() | Flattens 2D array two_d_arr to 1D arr.T | Transposes arr (rows become columns and vice versa) arr.reshape(3,4) | Reshapes arr to 3 rows, 4 columns without changing data arr.resize((5,6)) | Changes arr shape to 5x6 and fills new values with 0

Adding/removing Elements

np.append(arr,values) | Appends values to end of arr np.insert(arr,2,values) | Inserts values into arr before index 2 np.delete(arr,3,axis=0) | Deletes row on index 3 of arr np.delete(arr,4,axis=1) | Deletes column on index 4 of arr

Combining/splitting

np.concatenate((arr1,arr2),axis=0) | Adds arr2 as rows to the end of arr1 np.concatenate((arr1,arr2),axis=1) | Adds arr2 as columns to end of arr1 np.split(arr,3) | Splits arr into 3 sub-arrays np.hsplit(arr,5) | Splits arr horizontally on the 5th index

Indexing/slicing/subsetting

arr[5] | Returns the element at index 5 arr[2,5] | Returns the 2D array element on index [2][5] arr[1]=4 | Assigns array element on index 1 the value 4 arr[1,3]=10 | Assigns array element on index [1][3] the value 10 arr[0:3] | Returns the elements at indices 0,1,2 (On a 2D array: returns rows 0,1,2) arr[0:3,4] | Returns the elements on rows 0,1,2 at column 4 arr[:2] | Returns the elements at indices 0,1 (On a 2D array: returns rows 0,1) arr[:,1] | Returns the elements at index 1 on all rows arr