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Learn Python By Example – Indexing And Slicing NumPy Arrays

By on Friday, May 28, 2021

Indexing And Slicing NumPy Arrays

Slicing Arrays

Explanation Of Broadcasting

Unlike many other data types, slicing an array into a new variable means that any chances to that new variable are broadcasted to the original variable. Put other way, a slice is a hotlink to the original array variable, not a separate and independent copy of it.


/* Import Modules */
import numpy as np
/* Create an array of battle casualties from the first to the last battle */
battleDeaths = np.array([1245, 2732, 3853, 4824, 5292, 6184, 7282, 81393, 932, 10834])
/* Divide the array of battle deaths into start, middle, and end of the war */
warStart = battleDeaths[0:3]; print('Death from battles at the start of war:', warStart)
warMiddle = battleDeaths[3:7]; print('Death from battles at the middle of war:', warMiddle)
warEnd = battleDeaths[7:10]; print('Death from battles at the end of war:', warEnd)

Death from battles at the start of war: [1245 2732 3853]
Death from battles at the middle of war: [4824 5292 6184 7282]
Death from battles at the end of war: [81393   932 10834]

/* Change the battle death numbers from the first battle */
warStart[0] = 11101
/* View that change reflected in the warStart slice of the battleDeaths array */
warStart
array([11101,  2732,  3853])

/* View that change reflected in (i.e. "broadcasted to) the original battleDeaths array */

battleDeaths
array([11101,  2732,  3853,  4824,  5292,  6184,  7282, 81393,   932, 10834])

Indexing Arrays

Note: This multidimensional array behaves like a dataframe or matrix (i.e. columns and rows)

/* Create an array of regiment information */
regimentNames = ['Nighthawks', 'Sky Warriors', 'Rough Riders', 'New Birds']
regimentNumber = [1, 2, 3, 4]
regimentSize = [1092, 2039, 3011, 4099]
regimentCommander = ['Mitchell', 'Blackthorn', 'Baker', 'Miller']

regiments = np.array([regimentNames, regimentNumber, regimentSize, regimentCommander])
regiments
array([['Nighthawks', 'Sky Warriors', 'Rough Riders', 'New Birds'],
       ['1', '2', '3', '4'],
       ['1092', '2039', '3011', '4099'],
       ['Mitchell', 'Blackthorn', 'Baker', 'Miller']], 
      dtype='<U12')

/* View the first column of the matrix */
regiments[:,0]
array(['Nighthawks', '1', '1092', 'Mitchell'], 
      dtype='<U12')

/* View the second row of the matrix */
regiments[1,]
array(['1', '2', '3', '4'], 
      dtype='<U12')

/* View the top-right quarter of the matrix */
regiments[:2,2:]
array([['Rough Riders', 'New Birds'],
       ['3', '4']], 
      dtype='<U12')

 

Python Example for Beginners

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