NumPy Cheat Sheet

Question 1

A NumPy Array object shorthand

Answer 1

arr

Question 2

Import NumPy

Answer 2

import numpy as np

Question 3

Import from a text file

Answer 3

np.loadtxt(‘file.txt’)

Question 4

Import from a CSV file

Answer 4

np.genfromtxt(‘file.csv’,delimiter=‘,’)

Question 5

Writes to a text file

Answer 5

np.savetxt(‘file.txt’,arr,delimiter=‘ ‘)

Question 6

Writes to a CSV file

Answer 6

np.savetext(‘file.csv’,arr,delimiter=‘,’)

Question 7

Create one dimensional array

Answer 7

np.array([1,2,3])

Question 8

Create two dimensional array

Answer 8

np.array([(1,2,3),(4,5,6)])

Question 9

Create 1D array of length 3 all values 0

Answer 9

np.zeros(3)

Question 10

3 x 4 array with all values 1

Answer 10

np.ones((3,4)

Question 11

5 x 5 array of 0 with 1 on diagonal (identity matrix)

Answer 11

np.eye(5)

Question 12

Array of 6 evenly divided values from 0 to 100

Answer 12

np.linspace(0,100,6)

Question 13

Array of values from 0 to less than 10 with step 3

Answer 13

np.arange(0,10,3)

Question 14

2 x 3 array with all values 8

Answer 14

np.full((2,3),8)

Question 15

4 x 5 array of random floats between 0-1

Answer 15

np.random.rand(4,5)

Question 16

6 x 7 array of random floats between 0-100

Answer 16

np.random.rand(6,7)*100

Question 17

2 x 3 array with random ints between 0-4

Answer 17

np.random.randint(5,size=2,3))

Question 18

Returns number of elements in arr

Answer 18

arr.size

Question 19

Returns dimensions of arr

Answer 19

arr.shape

Question 20

Returns the type of elements in arr

Answer 20

arr.dtype

Question 21

Convert arr elements to type dtype

Answer 21

arr.astype(dtype)

Question 22

Convert arr to a Python list

Answer 22

arr.tolist()

Question 23

View documentation for np.eye

Answer 23

np.info(np.eye)

Question 24

Copies arr to new memory

Answer 24

np.copy(arr)

Question 25

Creates view of arr elements with type dtype

Answer 25

arr.view(dtype)

Question 26

Sorts arr

Answer 26

arr.sort()

Question 27

Sorts specific axis of arr

Answer 27

arr.sort(axis=0)

Question 28

Flattens 2D array two_d_arr to 1D

Answer 28

two_d_arr.flatten()

Question 29

Transposes arr (rows become columns and vice versa)

Answer 29

arr.T

Question 30

Reshapes arr to 3 rows, 4 columns without changing data

Answer 30

arr.reshape(3,4)

Question 31

Changes arr shape to 5 x 6 and fills new values with 0

Answer 31

arr.resize((5,6))

Question 32

Appends values to the end of arr

Answer 32

np.append(arr,values)

Question 33

Inserts values into arr before index 2

Answer 33

np.insert(arr,2,values)

Question 34

Deletes rows on index 3 of arr

Answer 34

np.delete(arr,3,axis=0)

Question 35

Deletes column on index 4 of arr

Answer 35

np.delete(arr,4,axis=1)

Question 36

Adds arr2 as rows to the end of arr1

Answer 36

np.concatenate((arr1,arr2),axis=0)

Question 37

Adds arr2 as columns to end of arr1

Answer 37

np.concatenate((arr1,arr2),axis=1)

Question 38

Splits arr into 3 sub-arrays

Answer 38

np.split(arr,3)

Question 39

Splits arr horizontally on the 5th index

Answer 39

np.hsplit(arr,5)

Question 40

Returns the element at index 5

Answer 40

arr[5]

Question 41

Returns the 2D array element on index [2] [5]

Answer 41

arr[2,5]

Question 42

Assigns array element on index 1 the value 4

Answer 42

arr[1]=4

Question 43

Assigns array element on index [1] [3] the value 10

Answer 43

arr[1,3]=10

Question 44

Returns the elements at indices 0, 1, 2 (for 2D array, returns rows 0, 1, 2)

Answer 44

arr[0:3]

Question 45

Returns the elements on rows 0, 1, 2 at column 4

Answer 45

arr[0:3,4]

Question 46

Returns the elements at indices 0, 1 (on a 2D array: returns rows 0,1)

Answer 46

arr[:2]

Question 47

Returns the elements at index 1 on all rows

Answer 47

arr[:,1]

Question 48

Returns an array with boolean values

Answer 48

arr<5 OR (arr1<3) & (arr2>5)

Question 49

Inverts a boolean array

Answer 49

~arr

Question 50

Returns array elements smaller than 5

Answer 50

arr[arr<5]

Question 51

Add 1 to each array element

Answer 51

np.add(arr,1)

Question 52

Subtract 2 from each array element

Answer 52

np.subtract(arr,2)

Question 53

Multiply each array element by 3

Answer 53

np.mulitply(arr,3)

Question 54

Divide each array element by 4

Answer 54

np.divide(arr,4)

Question 55

Raise each array element to the 5th power

Answer 55

np.power(arr,5)

Question 56

Elementwise add arr2 to arr1

Answer 56

np.add(arr1,arr2)

Question 57

Elementwise subtract arr2 from arr1

Answer 57

np.subtract(arr1,arr2)

Question 58

Elementwise multiply arr1 by arr2

Answer 58

np.multiply(arr1,arr2)

Question 59

Elementwise divide arr1 by arr2

Answer 59

np.divide(arr1,arr2)

Question 60

Elementwise raise arr1 raised to the power of arr2

Answer 60

np.power(arr1,arr2)

Question 61

Returns true if the arrays have the same elements and shape

Answer 61

np.array_equal(arr1,arr2)

Question 62

Square root of each element in the array

Answer 62

np.sqrt(arr)

Question 63

Sine of each element in the array

Answer 63

np.sin(arr)

Question 64

Natural log of each element in the array

Answer 64

np.log(arr)

Question 65

Absolute value of each element in the array

Answer 65

np.abs(arr)

Question 66

Rounds up to the nearest int

Answer 66

np.ceil(arr)

Question 67

Rounds down to the nearest int

Answer 67

np.floor(arr)

Question 68

Rounds to the nearest int

Answer 68

np.round(arr)

Question 69

Returns mean along specific axis

Answer 69

np.mean(arr,axis=0)

Question 70

Returns sum of arr

Answer 70

arr.sum()

Question 71

Returns minimum value of arr

Answer 71

arr.min()

Question 72

Returns maximum value of specific axis

Answer 72

arr.max(axis=0)

Question 73

Returns the variance of array

Answer 73

np.var(arr)

Question 74

Returns the standard deviation of specific axis

Answer 74

np.std(arr,axis=1)

Question 75

Returns correlation coefficient of array

Answer 75

arr.corrcoef()