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<5 - Returns an array with boolean values
(arr1<3) & (arr2>5) - Returns an array with
boolean values
~arr - Inverts a boolean array
arr[arr<5] - Returns array elements smaller than 5
SCALAR MATH
np.add(arr,1) - Add 1 to each array element
np.subtract(arr,2) - Subtract 2 from each array
element
np.multiply(arr,3) - Multiply each array
element by 3
np.divide(arr,4) - Divide each array element by
4 (returns np.nan for division by zero)
np.power(arr,5) - Raise each array element to
the 5th power
VECTOR MATH
np.add(arr1,arr2) - Elementwise add arr2 to
arr1
np.subtract(arr1,arr2) - Elementwise subtract
arr2 from arr1
np.multiply(arr1,arr2) - Elementwise multiply
arr1 by arr2
np.divide(arr1,arr2) - Elementwise divide arr1
by arr2
np.power(arr1,arr2) - Elementwise raise arr1
raised to the power of arr2
np.array_equal(arr1,arr2) - Returns True if the
arrays have the same elements and shape
np.sqrt(arr) - Square root of each element in the
array
np.sin(arr) - Sine of each element in the array
np.log(arr) - Natural log of each element in the
array
np.abs(arr) - Absolute value of each element in
the array
np.ceil(arr) - Rounds up to the nearest int
np.floor(arr) - Rounds down to the nearest int
np.round(arr) - Rounds to the nearest int
STATISTICS
np.mean(arr,axis=0) - Returns mean along
specific axis
arr.sum() - Returns sum of arr
arr.min() - Returns minimum value of arr
arr.max(axis=0) - Returns maximum value of
specific axis
np.var(arr) - Returns the variance of array
np.std(arr,axis=1) - Returns the standard
deviation of specific axis
arr.corrcoef() - Returns correlation coefficient
of array
Data Science Cheat Sheet
NumPy
KEY
We’ll use shorthand in this cheat sheet
arr - A numpy Array object
IMPORTS
Import these to start
import numpy as np