Numpy Tutorial_4

 

Introduction to NumPy (Tutorial 4): NumPy Array Slicing

L = [1,2,3,3]

print(L[1:4])

[2, 3, 3]

import numpy as np

L = [[1,2,3],[6,7,8],[22,33,44]]

N = np.array([[1,2,3],[6,7,8],[22,33,44]])

print(N)

print(N[:1,:1])

print(N[1:2, :]) # return just the second row and all the columns

print(N[:,:]) # print all

print(N[-1,-1]) # print the last element in tha numpy array

print(N[:1,:1])# return the first element in the row with the first element in the columnes 

[[ 1  2  3]
 [ 6  7  8]
 [22 33 44]]
[[1]]
[[6 7 8]]
[[ 1  2  3]
 [ 6  7  8]
 [22 33 44]]
44
[[1]]

L = ['first','second','third','fourth','fifth','sixth','seventh','eight','ninth','Ten'] 

print(L[:6]) # return all first six elements (To six)

print(L[6:]) # return just sixth element exactly and all after (from six)

['first', 'second', 'third', 'fourth', 'fifth', 'sixth']
['seventh', 'eight', 'ninth', 'Ten']

print(L[-1]) # return last element

Ten

print(L[-1:-3])

[]

print(L[-1:])

['Ten']

print(L[-3:-1])

['eight', 'ninth']

print(L[1:9:3]) # [begin, end, step] jump equal to 3 

['second', 'fifth', 'eight']

print(L[::4])

['first', 'fifth', 'ninth']

print(L[::-1]) # reverse the list 

['Ten', 'ninth', 'eight', 'seventh', 'sixth', 'fifth', 'fourth', 'third', 'second', 'first']

print(L[::-2]) # just type the even number for example and reverse it

['Ten', 'eight', 'sixth', 'fourth', 'second']

arr = np.array([[1,2,3,4,5], [6,7,8,9,10]])

print(arr)

print(arr[1,1])

print(arr[0,0])

[[ 1  2  3  4  5]
 [ 6  7  8  9 10]]
7
1

print(arr[:1, 1:4])

​

[[2 3 4]]

print(arr[:1,:1])

[[1]]

print(arr[1:,1:])

[[ 7  8  9 10]]

# let's change the first element :

arr[:1,:1]= 11 #change 

​

print(arr)

[[11 99 99 99 99]
 [99 99 99 99 99]]

arr[:] = 99  # change all values

 L = np.array([[1,2,3,4,5,6],[3,4,5,6,7,9],[45,33,22,77,66,98]])

print(L)

[[ 1  2  3  4  5  6]
 [ 3  4  5  6  7  9]
 [45 33 22 77 66 98]]

print(L[2:, 4:]) # we make two line to full understand , 

#first horizontal line for row wich is third line , second vertical for column which is 5 and the contact is the sol.

​

[[66 98]]

L[1:, 4:] = 888

​

print(L)

[[  1   2   3   4   5   6]
 [  3   4   5   6 888 888]
 [ 45  33  22  77 888 888]]

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