Guide to High School Computer Science
  • 💻Introduction
    • windows & Python Development
    • macOS & Python Development
    • Visual Studio Code Settings
    • Set up Github
    • Author Page
  • 🧠Prerequisite Skills
    • Keyboard Typing
    • Files & Directories
    • Use of Command Line
    • Git & GitHub
    • Markdown
    • Starting Your Python Project
  • 🐍Python Programming
    • 🍎Python Basics
      • What is Python?
      • Procedural Programming & Programming Paradigms
      • String Formatting
      • Data Types
      • Input & Output to Console
      • Working with Numbers
      • Useful Built-in Functions
      • Math & Random Module
      • Boolean Data Object
      • Comparison, Logical, and Membership Operators
      • If Statements
      • Binary Decisions
      • Multiple Decisions
      • Nested Conditions
      • [EXTRA] Bitwise Operators
      • [EXTRA] Python Style Guide
    • ⏮️Iterations
      • Introduction to While Loops
      • Infinite Loop
      • Controlling Your While Loops
      • Introduction to For Loops
      • For Loops w/ Numeric Sequences
      • For Loops w/ Strings & Lists
      • Iterable Functions w/ For Loops
    • 📦Collections
      • Strings
        • String Basics
        • String Indexing
        • String Slicing
        • String Operators & Functions
        • Basic String Methods
        • String Methods Extended
        • String Methods Document
      • Tuples & Lists
        • Tuples
        • List Basics
        • List are Mutable
        • Adding Items to a List
        • Removing Items from a List
        • Search & Reverse a List
        • List Comprehension
        • List Methods Document
      • Sets
      • Dictionary
      • How to Store Multiple Data Items
    • 💡Defining Functions
      • Functions
      • print() vs return
      • Pre-determined Arguments
      • Nested Functions
      • Map & Filter
      • [Extra] Dynamic Arguments
    • 💾File I/O
      • How to Save Text to an External File
      • Reading CSV in Python
      • Reading JSON in Python
    • 🔨Basic Python Projects
      • Basic Calculator
        • Improving the calculator
        • Exercise Set 1
        • Exercise Set 2
        • 💎Streamlit Application #1
      • Basic Password Generator
        • Exercise Set 3
        • Exercises Related to Math
        • 💎Streamlit Application #2
      • A To-Do Task List
    • ⏳Introduction to Algorithmic Thinking
      • Big-O Notation
      • Basic Algorithms
        • Linear Search
        • Binary Search
        • Basic Sorting Algorithms
      • Recursion
      • Brute Force Algorithms
      • Greedy Algorithm
        • Time on Task (CCC 2013 J4)
        • Dijkstra’s Algorithm
      • Divide and Conquer
        • Merge Sort
      • Dynamic Programming
    • 🤯Object Oriented Programming
      • Class & Objects (Definitions)
      • OOP in Python
      • Encapsulation
      • Polymorphism
      • Inheritance & Overriding
      • Override Magic Methods
      • Case Study: 2D Vectors
      • Case Study: Deck of Cards
      • Exercise
      • Abstract Data Types
      • Case Study: Static 1D Array From Java
    • Competitive Programming
      • Is This Sum Possible?
        • Is the dataset sorted?
        • Searching for a value
        • Determine if the difference between an integer from the array and the target value exists
        • Sorting Algorithms
        • Using Two Pointers
      • Two Sum - LeetCode
        • Generate all possible pairs of values
        • Subtract each value from the target, see if the difference exists in the list
      • Longest Common Prefix - LeetCode
        • Compare all possible prefixes
        • Create the longest common prefix with the direct neighbour
      • Length of Last Word - LeetCode
        • Compare all possible prefixes
      • Where can I go from one point to another?
      • Sample Outline
    • IB Recipe Book
  • 💾Python & Databases
    • Intro to Databases & Data Modeling
      • Common Data Types in SQL
      • Introduction to ERDs
      • Primary Keys and Foreign Keys
      • Database Normalization
    • What is SQL?
      • Getting Started
      • SELECT Queries
        • Selection with Conditions
        • Selection with Fuzziness
        • Selection and Sorting in Order
        • Selection without Duplicates
        • Selection with Limited Number of Outputs
      • AGGREGATE Queries
        • Counting Rows
        • Sum, Average, Min/Max Queries
        • Working with Aggregate Queries
        • Power of using Groups
        • Exercise
      • Interacting with Multiple Table
      • Inserting Data
      • External Resource
  • ☕Java Essentials
    • Basics
      • Starting Java
      • Data & Variables
      • Handling User Inputs & Type Conversion
      • Arithmetic
      • IPO Model
      • Basic Built-in Methods
      • Exercise Questions
    • Conditionals
      • Boolean Operators
      • Compare Strings
      • If Statements
      • If Else Statements
      • Making Multiple Decisions
      • Using Switch
      • Flowchart Symbols
      • Exercise Questions
    • Iterations
      • While Loops
      • For Loop
      • Exercises
    • Java Type Casting
    • Strings
      • Common String Practices
      • String Formatting
      • Java Special Characters
    • Collection
      • Arrays
      • For Each Loop
      • ArrayList
      • Exercise Questions
    • Static Methods
      • (Aside) Clearing your Console
    • Randomness in Java
    • Delayed Output in Java
    • Java Output Formatting
    • Java Style Guide
  • 🛠️JavaScript Programming
    • Our Programming Editor & Workflow
      • Hello, world!
      • Commenting & Variables
      • Data in JavaScript
      • Operators
      • String Formatting
      • Getting User Input
    • JavaScript Exercise Set 1
    • Making Decisions
      • Comparing Values
      • Combining Boolean Comparisons
      • Creating Branches
    • JavaScript Exercise Set 2
    • While Loops
      • Infinite While Loop
      • While Loops and Numbers
      • While Loops and Flags
      • While loops w/ Strings
    • JavaScript Exercise Set 3
    • Subprograms & Functions
      • Creating a Function in JavaScript
      • Function with Input and Assignable Output
    • JavaScript Exercise Set 4
  • 💾Topics in CS
    • Computer Environments & Systems
      • Computer Components
        • In-depth Explanations
      • File Maintenance
      • Computer & Safety
      • Software Development
      • Bits & Binary
    • Careers related to Computer Science
    • Postsecondary Opportunities
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  1. Python Programming
  2. Collections
  3. Strings

String Indexing

In Python, we can access a certain/individual item in a sequence if the data type is indexable.

To index/access a single item from a string, we use [ ] square brackets.

# Example
'''
Looking at: 'Hello!'

 |  H  |  e  |  l  |  l  |  o  |  !  |
 0     1     2     3     4     5     6

'''

word = 'Hello!'

print('word[2]:', word[2])
print('word[1]:', word[1])
print('word[0]:', word[0])
print('word[5]:', word[5])
print('word[6]:', word[6])
word[2]: l
word[1]: e
word[0]: H
word[5]: !



---------------------------------------------------------------------------

IndexError                                Traceback (most recent call last)

<ipython-input-3-f291ab6028d1> in <module>
     14 print('word[0]:', word[0])
     15 print('word[5]:', word[5])
---> 16 print('word[6]:', word[6])


IndexError: string index out of range

NOTE:

  • Indexing always starts at 0 (zero)

  • Indexing always accesses a single item from the sequence, for strings: a character from the targetted string

  • The index value cannot go beyond the limit of the sequence. Mathematically, the index value cannot be greater than length of the sequence subtracted by one.

    • Examine the error for print('word[6]:', word[6]): gives us index out of range error

Looking at: 'Hello!'

 |  H  |  e  |  l  |  l  |  o  |  !  |
 0     1     2     3     4     5     6
  • Notice that our index value in this diagram starts at the left of the item

  • When we index, it will go to the given location and look at the item to the right of the index

    • Therefore, print('word[1]:', word[1]) gives us: e

Negative Indexing

Just like how we can use positive integer-indexes to access values, we can also use negative integer-indexes to access values as well.

# Example
'''
Looking at: 'Hello!'

 |  H  |  e  |  l  |  l  |  o  |  !  |  H  |  e  |  l  |  l  |  o  |  !  |
-6    -5    -4    -3    -2    -1     0     1     2     3     4     5     

'''

word = 'Hello!'

print('word[-1]:', word[-1])
print('word[-3]:', word[-3])
print('word[-5]:', word[-5])
print('word[-6]:', word[-6])
print('word[-7]:', word[-7])
word[-1]: !
word[-3]: l
word[-5]: e
word[-6]: H



---------------------------------------------------------------------------

IndexError                                Traceback (most recent call last)

<ipython-input-6-5d7a5ddd18cd> in <module>
     14 print('word[-5]:', word[-5])
     15 print('word[-6]:', word[-6])
---> 16 print('word[-7]:', word[-7])


IndexError: string index out of range

NOTE:

  • Index of -1 is the quickest way to grab the last value of an indexable sequence

  • Examine that at -6 and 0 we got the value of H for Hello!, they are located at the same place

Looking at: 'Hello!'

 |  H  |  e  |  l  |  l  |  o  |  !   
-6    -5    -4    -3    -2    -1      
  • Much like how we look at the right of positive indexes, we do the same for negative indexes

  • The indexes are laid out like an integer line with the string itself duplicated; however, Python does not actually create the string as a double

    • It is smart enough to know how to handle positive and negative indexes with a single instance of the sequence

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Last updated 1 year ago

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