~/Strings

16.1 Creating a String

Python accepts three kinds of quotes, so you can always pick one that avoids clashing with quotes inside the text.

a = 'Aarav'
b = "O'Brien"                   # single quote inside → use "
c = """This string
spans two lines."""
empty = ""                        # empty string — length 0, perfectly valid

16.1.1 Escape characters

Some characters are hard to type directly — a newline, a tab, a quote inside a quote. Python uses escape sequences starting with \:

path = r"C:\Users\Aarav\file.txt"     # no escaping needed

16.2 Basic Operations

Every sequence operation you met in Chapter 9 works on strings.

16.2.1 Concatenation +

first = "Aarav"
last  = "Kumar"
full  = first + " " + last
print(full)               # Aarav Kumar

16.2.2 Repetition *

print("ab" * 3)          # ababab
print("-" * 20)          # --------------------
print("🎉" * 5)          # 🎉🎉🎉🎉🎉

16.2.3 Membership in / not in

s = "Python Programming"
print("Py" in s)          # True
print("java" in s)        # False
print("XYZ" not in s)     # True

16.2.4 Length — len()

print(len("Python"))      # 6
print(len(""))            # 0  — empty string
print(len("a b c"))       # 5  — spaces count

16.3 Indexing

Each character has two indices — a positive one counted from the left (starting at 0) and a negative one counted from the right (starting at −1). Picking a single character uses square brackets.

s = "Python"
print(s[0])          # 'P'  — first
print(s[5])          # 'n'  — last
print(s[-1])         # 'n'  — same character, counted from the right
print(s[-3])         # 'h'
# print(s[10])      ❌ IndexError: string index out of range

16.4 Slicing [start:stop:step]

Slicing extracts a sub-string — zero or more consecutive characters. The recipe is identical to range():

• start — first index, included. Default 0.
• stop — one past the last index, excluded. Default len(s).
• step — stride between characters. Default 1. Negative values reverse direction.

s = "PROGRAMMING"       # indexes 0–10, length 11

print(s[0:4])          # PROG       — chars 0, 1, 2, 3
print(s[4:])           # RAMMING    — from 4 to end
print(s[:3])           # PRO        — start to 3 (exclusive)
print(s[:])             # PROGRAMMING  — whole string (a copy)
print(s[-4:])          # MING       — last 4 characters
print(s[::2])          # PORMIG     — every 2nd character
print(s[::-1])         # GNIMMARGORP  — reversed!
print(s[6:2:-1])       # MARG       — from 6 back to (but not) 2

16.5 Strings Are Immutable

You can read any character with indexing, but you cannot write to a character. Python raises TypeError if you try.

s = "Python"
# s[0] = "J"       ❌ TypeError: 'str' object does not support item assignment

# The way to "change" a string is to make a new one:
s = "J" + s[1:]     # "Jython"
print(s)

16.6 Traversing a String

Traversal means visiting each character of a string one at a time. Two common idioms:

s = "Hello"
for ch in s:
    print(ch, end=" ")        # H e l l o

s = "Hello"
for i in range(len(s)):
    print(i, "→", s[i])

s = "Hello"
i = len(s) - 1
while i >= 0:
    print(s[i], end=" ")       # o l l e H
    i -= 1

16.7 String Methods — Overview

Every string object has dozens of built-in methods — functions attached to the object itself. Call them with the dot notation: s.method(arguments). Because strings are immutable, every method that looks like it modifies the string actually returns a new string; the original is untouched.

16.8 Case-conversion methods

16.9 Search-and-count methods

16.10 Classification methods — is…()

All classification methods return a Boolean. They check whether the entire string satisfies the property. "" (empty) makes every one of them return False.

otp = input("Enter 6-digit OTP: ")
if len(otp) == 6 and otp.isdigit():
    print("OTP accepted")
else:
    print("Invalid — must be exactly 6 digits")

16.11 Trimming and replacing

16.12 Splitting and joining

line = "Aarav,Kavya,Ishan,Priya"
names = line.split(",")               # ['Aarav','Kavya','Ishan','Priya']
print(names)

csv_back = ",".join(names)             # 'Aarav,Kavya,Ishan,Priya'
print(csv_back)

# Split on whitespace when no argument is given:
sentence = "Python is fun"
words = sentence.split()                 # ['Python','is','fun']
print(len(words))                       # 3

",".join(["a", "b"])       # ✓ correct — separator calls join
["a", "b"].join(",")          # ❌ AttributeError — list has no join

16.13 Beyond the Syllabus — Other Useful str Methods

Python's str class has around 45 methods. The 23 covered so far are those named in the CBSE Class XI syllabus. You may meet the following extras in real-world code — they are not required for the exam, but worth knowing.

16.13.1 More string methods

16.13.2 Built-in FUNCTIONS that work with strings

These are functions (called as f(x)), not methods (called as x.f()):

16.14 Worked Programs

16.14.1 Count vowels in a string

s = input("Enter a string: ").lower()
vowels = "aeiou"
count = 0
for ch in s:
    if ch in vowels:
        count += 1
print("Vowels found:", count)

16.14.2 Palindrome check

A palindrome reads the same forwards and backwards: MADAM, nitin, racecar, 12321.

s = input("Enter a word: ").lower()
if s == s[::-1]:              # reverse using slicing
    print(s, "is a palindrome")
else:
    print(s, "is not a palindrome")

16.14.3 Count upper-case, lower-case, digits, spaces

s = input("Enter a string: ")
u = l = d = sp = 0
for ch in s:
    if   ch.isupper():  u  += 1
    elif ch.islower():  l  += 1
    elif ch.isdigit():  d  += 1
    elif ch.isspace():  sp += 1
print(f"Upper={u}  Lower={l}  Digits={d}  Spaces={sp}")

16.14.4 Convert case (every letter)

s = input("Enter a string: ")
print("UPPER:",     s.upper())
print("lower:",     s.lower())
print("Capitalized:", s.capitalize())
print("Title:",     s.title())
print("Swapped:",   s.swapcase())