Advanced Text Processing

COS102: Advanced Text Processing

Mastering String Manipulation in Python

This module focuses on advanced text processing techniques in Python, covering string slicing, formatting with f-strings, and essential string methods for efficient text manipulation.

Learning Outcomes

• Master text extraction using String Slicing boundaries.
• Implement negative indexing to safely traverse text backwards.
• Dynamically inject values into strings using modern f-strings.
• Leverage native string methods to clean, parse, and validate text.
• Handle structural data extractions like email and matric numbers.

What is String Slicing?

While indexing allows you to access a single character, Slicing enables you to extract an entire substring or segment from a string.

The Bread Analogy

Instead of inspecting just one crumb, slicing lets you specify where to start and stop "cutting" to pull out a clean "slice" of text.

Slicing Syntax Matrix

Slicing uses square brackets with colons to define the target limits: string[start:stop]

P y t h o n
0 1 2 3 4 5

word = "Python"
print(word[0:2]) # Output: 'Py'
print(word[2:5]) # Output: 'tho'

▲ The Off-By-One Reminder:

Python includes the start index, but completely excludes the stop index!

Slicing Shortcuts

Parameters can be left blank to trigger automatic default values:

• Omit Start ([:stop]): Starts from index 0.

  "Python"[:4] → 'Pyth'

• Omit Stop ([start:]): Goes all the way to the end.

  "Python"[2:] → 'thon'

• Omit Both ([:]): Clones the entire string.

Negative Slicing Limits

Negative indexing allows you to pull characters from the end of a string without counting from zero.

P  y  t  h  o  n
-6 -5 -4 -3 -2 -1

filename = "report.txt"
# Extract the extension using negative steps
ext = filename[-4:]
print(ext) # Output: '.txt'

String Formatting (f-Strings)

Old concatenation using + is clunky and error-prone. Modern Python uses f-strings (Formatted String Literals) to easily embed variables inside strings.

name = "Tolani"
score = 85

# Put an 'f' before the quotes, and wrap variables in curly braces {}
msg = f"Hello {name}, your score is {score}."
print(msg) # Output: Hello Tolani, your score is 85.

Inline f-String Math

f-strings can also evaluate active expressions and format numbers on the fly.

price = 2500
discount = 0.10

print(f"Total: #{price * (1 - discount)}")
# Output: Total: #2250.0

val = 22 / 7
print(f"Pi to 2 decimals: {val:.2f}")
# Output: Pi to 2 decimals: 3.14

Essential String Methods

String methods are built-in operations that can be run on text variables using the dot (.) operator.

• .upper() / .lower(): Converts character casing completely.

  "Hi".upper() → "HI"

• .strip(): Removes leading/trailing whitespaces.

  " x ".strip() → "x"

• .replace(old, new): Swaps character sub-segments.

  "abc".replace("b", "z") → "azc"

• .split(separator): Breaks text into a list of chunks.

  "a,b".split(",") → ["a", "b"]

Strings are Immutable!

Immutable means a string cannot be altered once it's created in memory.

name = "Python"
name[0] = "J" # CRASH! TypeError!

String methods do not modify the original text; instead, they return a brand new string value that you have to save into a variable.

Lab: Email Domain Extractor (Code Challenge)

Write a program that takes a user's input email address, extracts the domain name using string methods, and prints a clean greeting.

email = " student@unilag.edu.ng "
# 1. Clean the outer whitespace
# 2. Extract everything after the "@" symbol
# 3. Print a message using an f-string

Questions?

Text processing transforms raw data into structured information inputs.

Next Chapter: Error and Exception Handling (Try / Except)