Code-Memo

Strings

A string is a sequence of characters enclosed in quotes. Strings are immutable, meaning once created, they cannot be changed. Strings support indexing, slicing, and a rich set of built-in methods for manipulation.

Initialization

s = "hello"
s2 = 'world'
s3 = """multi
line"""

Basic Operations

1. Access Characters

print(s[0])     # 'h'
print(s[-1])    # 'o' (last character)

2. Slicing

print(s[1:4])   # 'ell'
print(s[:3])    # 'hel'
print(s[::-1])  # 'olleh' (reverse string)

3. Concatenation

s = "hello"
s += " world"
print(s)  # 'hello world'

4. Repetition

print("ha" * 3)  # 'hahaha'

5. Length

print(len(s))  # Number of characters

6. Iteration

for char in s:
    print(char)

7. Search

• Check substring

"ell" in s  # True

• Find index

s.find("l")       # Returns 2 (first occurrence)
s.rfind("l")      # Returns 3 (last occurrence)
s.index("l")      # Like find(), but raises error if not found

8. Modification (via new string)

• Replace

s = s.replace("world", "there")  # 'hello there'

• Upper/Lower

s.upper()     # 'HELLO THERE'
s.lower()     # 'hello there'

• Strip

"  hello  ".strip()   # 'hello'
"hello\n".rstrip()    # 'hello'

9. Splitting and Joining

words = s.split()       # ['hello', 'there']
joined = "-".join(words)  # 'hello-there'

10. Formatting

• f-Strings

name = "Ortie"
f"Hello, {name}!"  # 'Hello, Ortie!'

• Old style

"Hello, %s!" % name

• str.format()

"Hello, {}!".format(name)

11. String Comparison

"apple" < "banana"   # True (lexicographical)

12. Useful String Methods

s.isalpha()     # All letters?
s.isdigit()     # All digits?
s.startswith("he")  # True
s.endswith("e")     # True