7: Text and Characters

Introduction to Computer Science

• Course numbers: 252-0032, 252-0047, 252-0058
• Authors: Manuela Fischer and Felix Friedrich
• Department: Computer Science, ETH Zurich
• Semester: Fall 2024

Characters and Texts

• Characters: Basic building blocks of text, represented by the char data type. Printable characters (e.g., 'a') and control characters (e.g., '\n')
• Strings: Sequences of characters, represented by the std::string type. Act like vectors of char elements.
• ASCII: Standard code for representing characters. Converts characters to numerical values. Letters and digits have specific numerical assignments. Supports only 128 (or 256) characters which is too few for representing a lot of characters from different scripts today (like Chinese, Cyrillic, Greek, Hebrew, Latin, etc.)
• UTF-8: Variable-width encoding used for representing wider character sets (internationalization). Handles multiple scripts and more than 137,000 characters. More common characters use fewer bytes, while uncommon ones use more bytes. UTF-8 is a popular variable width encoding that encodes characters using 1-6 bytes, providing more space to add more characters.

Example: Caesar Cipher

• Caesar Cipher: Simple substitution cipher that shifts each character by a fixed number (e.g. positive or negative integer) in the alphabet. Wraps around at the end of the alphabet.

Text Manipulation

• Declaration and Initialization: Methods for defining std::string variables and assigning values like literals or specific characters.
• Comparison: Comparing strings using equality operator (==).
• Character Access: Accessing individual characters in strings by their index or using the at() function (which checks bounds for safety). This check is important to avoid accessing outside of valid array indices.
• Iteration: Traversing through characters in strings using loops (iterating over individual characters).
• Concatenation: Joining strings together using the += operator (e.g. text += "!";).
• Size: Determining the length of a string using the .size() method. This is also helpful for the for loop when correctly iterating through text characters.

Additional Information

• Input Handling: The std::cin >> std::noskipws; directive is used to read characters including whitespace from user input.
• Modulo Function: A function mod(dividend, divisor) is defined for calculating the mathematical modulo remainder (correct for negative numbers as well). This helps ensure the correct mathematical operations.
• Character Shifting: A function shift(c, s) is used to cyclically shift a printable character c by s positions in the ASCII character set. This is useful in the Caesar Cipher to shift characters. Characters outside the printable range [32,126] are not shifted.
• std::noskipws: Used in input to not skip whitespace characters.

This quiz explores the fundamental concepts of characters and strings in computer science. It covers character representation, ASCII encoding, UTF-8, and includes an example of a Caesar cipher. Perfect for students looking to strengthen their understanding of text processing.