Revision Notes on Strings for Python (Class 12 CS)

  Revision Notes on Strings for Python (Class 12 CS). Strings are a high-weightage topic, appearing frequently in "Predict the Output" and coding questions.

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1. The Basics

• Definition: A sequence of characters enclosed in quotes (' ', " ", or """ """).

• Immutability: Strings are Immutable. You cannot change individual characters in place.

  Python

  s = "Hello"
  s[0] = "M"  # ❌ TypeError!
  s = "M" + s[1:] # ✅ Correct (Creates a NEW string)
  

2. Indexing & Traversal

Each character has a unique position (index).

• Forward Indexing: Starts from 0 (Left to Right).

• Backward Indexing: Starts from -1 (Right to Left).

• Traversal: Iterating through a string.

  Python

  # Method 1: By Element
  for ch in "Code":
      print(ch) 
  
  # Method 2: By Index
  S = "Code"
  for i in range(len(S)):
      print(S[i])
  

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3. String Slicing (The Most Important Part)

Syntax: String[Start : Stop : Step]

• Start: Inclusive (default 0).

• Stop: Exclusive (default Length).

• Step: Jump value (default 1).

Cheat Sheet Examples: | Code | Output | Logic | | :--- | :--- | :--- | | S = "COMPUTER" | | | | S[1:5] | "OMPU" | Index 1 to 4. | | S[:3] | "COM" | Start to Index 2. | | S[4:] | "UTER" | Index 4 to End. | | S[::2] | "CMUE" | Every 2nd character. | | S[::-1] | "RETUPMOC" | Reverses the string. | | S[-4:] | "UTER" | Last 4 characters. |

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4. String Operators

1. Concatenation (+): Joins strings.

   • "Hi" + "Bye" → "HiBye"

   • Note: Cannot add Number to String ("Hi" + 5 → Error).

2. Repetition (*): Repeats string.

   • "No" * 3 → "NoNoNo"

3. Membership (in, not in): Checks presence.

   • "a" in "Apple" → False (Case sensitive).

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5. Must-Know String Methods

These are crucial for "Output" questions.

A. Case Conversion

• s.upper(): Returns uppercase.

• s.lower(): Returns lowercase.

• s.capitalize(): First char capital, rest lower.

• s.title(): First char of every word capital.

• s.swapcase(): Swaps case (lower ↔ upper).

B. Validation (Returns True/False)

• s.isalpha(): True if all chars are alphabets (No digits/spaces).

• s.isdigit(): True if all chars are numbers.

• s.isalnum(): True if alphabets OR numbers (No symbols/spaces).

• s.islower() / s.isupper(): Checks case.

• s.isspace(): True if string contains only whitespace.

C. Searching & Editing

• s.count(sub): Counts occurrences of substring.

  • "banana".count("a") → 3

• s.find(sub): Returns index of first occurrence. Returns -1 if not found.

• s.index(sub): Same as find, but returns Error if not found.

• s.replace(old, new): Replaces all occurrences.

  • "Beep".replace("e", "o") → "Boop"

D. Splitting & Joining (Very Important)

1. split(sep): Converts String $\to$ List.

   Python

   s = "Python is fun"
   L = s.split() 
   # Result: ['Python', 'is', 'fun']
   

2. join(iterable): Converts List $\to$ String.

   Python

   L = ['Python', 'is', 'fun']
   s = "-".join(L)
   # Result: "Python-is-fun"
   

E. Cleaning

• s.strip(): Removes leading and trailing whitespace.

• s.lstrip(): Removes left whitespace.

• s.rstrip(): Removes right whitespace.

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6. Common Exam Errors to Avoid

1. Difference between find() and index():

   • Question: "What happens if the substring is not found?"

   • find() returns -1 (Safe). index() crashes (Error).

2. Strings are Immutable:

   • You cannot write s.upper(). You must write s = s.upper() to save the change.

3. Split Logic:

   • "A,,B".split(",") produces ['A', '', 'B']. (Empty string between commas).

Quick Test

Predict the output:

Python

txt = "Exam 2025"
print(txt.replace("20", "!!").upper().split())

(Answer: ['EXAM', '!!25'])