2. Strings

💡 Why this matters for DSA Strings appear in ~30% of DSA problems — palindromes, anagrams, pattern matching, substring searches. Mastering string operations is essential.

2.1 Creating & Indexing

Strings are immutable — you cannot change a character in place.

python

s = "hello"
print(s[0])    # 'h'
print(s[-1])   # 'o' (negative index = from end)
print(s[4])    # 'o'

# s[0] = 'H'   # ❌ Error! Strings are immutable

✅ Tip — String ↔ List conversion:

python

s = "hello"
chars = list(s)      # ['h', 'e', 'l', 'l', 'o']
chars[0] = 'H'
s = "".join(chars)   # "Hello"

2.2 Slicing

One of Python's most powerful features for DSA.

python

s = "abcdef"

s[1:4]       # "bcd"   (start:end — end exclusive)
s[:3]        # "abc"   (start defaults to 0)
s[3:]        # "def"   (end defaults to end)
s[::2]       # "ace"   (step)
s[::-1]      # "fedcba" (reverse — extremely common!)
s[4:1:-1]    # "edc"   (negative step)

⚠️ Warning: Slicing creates a new string (O(n) time & space). For large strings, be mindful.

2.3 Useful Methods

python

s = "  Hello World!  "

s.lower()           # "  hello world!  "
s.upper()           # "  HELLO WORLD!  "
s.strip()           # "Hello World!"     (removes leading/trailing whitespace)
s.split()           # ["Hello", "World!"] (splits on whitespace)
s.split(",")        # split on specific delimiter
",".join(["a","b","c"])  # "a,b,c"
s.replace("World", "Python")  # "  Hello Python!  "
"hello" in s        # True (membership check — O(n) but very fast in C)
s.count("l")        # 3
s.startswith("  He") # True
s.endswith("!  ")   # True

✅ Tip — .split() vs .join() in DSA:

python

# Splitting a sentence into words
words = "hello world python".split()  # ["hello", "world", "python"]

# Joining back
sentence = " ".join(words)  # "hello world python"

2.4 f-strings (Formatting)

python

name = "Manik"
age = 25

print(f"My name is {name} and I am {age} years old")
print(f"Sum of {a} + {b} = {a + b}")    # expressions inside {}
print(f"{value:.2f}")                    # 2 decimal places

2.5 Character Operations

python

ord('a')       # 97   (char → ASCII)
chr(97)        # 'a'  (ASCII → char)

# Check character properties
"abc".isalpha()    # True
"123".isdigit()    # True
"  ".isspace()    # True

✅ Tip — Common DSA pattern:

python

# Convert lowercase letter to 0-indexed position
pos = ord('c') - ord('a')  # 2

Practice Drill

python

# 1. Reverse a string
# 2. Check if a string is a palindrome
# 3. Count vowels in a string
# 4. Remove all spaces from a string

💡 Click for Solutions

python

# 1. Reverse
s = "hello"
print(s[::-1])  # "olleh"

# 2. Palindrome
s = "racecar"
print(s == s[::-1])  # True

# 3. Count vowels
s = "hello world"
vowels = "aeiou"
count = sum(1 for ch in s if ch in vowels)
print(count)  # 3

# 4. Remove spaces
s = "hello world python"
print(s.replace(" ", ""))  # "helloworldpython"

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