3.1 Strings revisited

Strings in Python let you store and manipulate text. They can be sliced, indexed, and processed with built-in methods, which makes them useful for handling text data in programs.

Special characters and escape sequences add flexibility to string representation. Unicode conversion functions allow working with a wide range of characters, while string operations and formatting techniques enable complex text processing and presentation.

String Manipulation and Special Characters

String character indexing

• Strings are sequences of characters that can be accessed and manipulated using indexing
  • Indexing starts at 0 for the first character and increments by 1 for each subsequent character (string[0] accesses the first character)
  • Negative indexing starts at -1 for the last character and decrements by 1 for each preceding character (string[-1] accesses the last character)
• Individual characters can be accessed using square bracket notation string[index]
• Substrings can be extracted using slicing string[start:end]
  • start is the index of the first character to include (inclusive)
  • end is the index of the last character to exclude (exclusive)
  • If start is omitted, it defaults to the beginning of the string (string[:5] extracts the first 5 characters)
  • If end is omitted, it defaults to the end of the string (string[2:] extracts from the 3rd character to the end)
• Strings are immutable, meaning individual characters cannot be directly modified
  • To modify a string, create a new string with the desired changes (concatenate, slice, etc.)

Escape sequences in strings

• Escape sequences are used to represent special characters within a string (escape characters)
• Escape sequences start with a backslash \ followed by a specific character or sequence
• Common escape sequences include:
  • \n inserts a newline
  • \t inserts a tab
  • \\ inserts a literal backslash
  • \' inserts a single quote
  • \" inserts a double quote
• Escape sequences are treated as a single character within the string
• Raw strings (prefixed with r or R) treat backslashes as literal characters, ignoring escape sequences (r"C:\newfile" preserves the backslashes)

Unicode conversion functions

• Unicode is a standard for representing a wide range of characters from various scripts and symbols
• Each character is assigned a unique code point, which is an integer value
• The ord() function takes a single character as an argument and returns its Unicode code point
  • ord('A') returns 65
  • ord('€') returns 8364
• The chr() function takes a Unicode code point as an argument and returns the corresponding character
  • chr(65) returns 'A'
  • chr(8364) returns '€'
• These functions allow for conversion between characters and their numerical representations
• Unicode code points can be used for character comparisons and manipulations
  • ord('A') < ord('B') evaluates to True
  • chr(ord('A') + 1) returns 'B'

String Operations and Formatting

• String operations allow for manipulation and combination of strings
  • Concatenation: Joining strings using the + operator
  • Repetition: Repeating strings using the * operator
• String comparison uses relational operators to compare strings lexicographically
  • <, >, <=, >=, ==, != can be used to compare strings
• String methods provide built-in functionality for string manipulation
  • Common methods include lower(), upper(), strip(), split(), join()
• String formatting allows for creating formatted strings with placeholders
  • F-strings: f"Hello, {name}!" for inline variable substitution
  • .format() method: "Hello, {}!".format(name) for more complex formatting