3.7 · Strings

Goal: manipulate strings using slicing, methods and concatenation.

Creating strings

greeting = "Hello"
name = 'Alice'             # single or double quotes both fine
multi = """Line 1
Line 2"""

Length and indexing

s = "HKDSE"
print(len(s))      # 5
print(s[0])        # 'H'
print(s[-1])       # 'E'
print(s[1:4])      # 'KDS'   slicing exclusive on right

Concatenation and repetition

first = "Hello"
last  = "World"
combined = first + " " + last       # "Hello World"
echoed   = "abc" * 3                # "abcabcabc"

Common string methods

Method	Purpose	Example
s.lower()	All lowercase	"HKDSE".lower() → "hkdse"
s.upper()	All uppercase
s.strip()	Remove leading/trailing whitespace	" hi ".strip() → "hi"
s.replace(a, b)	Replace all a with b	"aaa".replace("a","b") → "bbb"
s.split(sep)	Split into list	"a,b,c".split(",") → ["a","b","c"]
sep.join(list)	Join list with separator	",".join(["a","b"]) → "a,b"
s.startswith(p) / s.endswith(p)	Boolean
s.find(sub)	Index of first occurrence or -1
s.count(sub)	Count occurrences
s.isdigit() / s.isalpha()	Boolean checks

Iterating over a string

for ch in "HKDSE":
    print(ch)

String formatting

name = "Alice"
score = 86
print(f"{name} scored {score}")        # f-string
print("{} scored {}".format(name, score))  # older .format

Worked example · Count vowels

def count_vowels(s):
    count = 0
    for ch in s.lower():
        if ch in "aeiou":
            count += 1
    return count

print(count_vowels("Hong Kong DSE ICT"))   # 5

Worked example · Reverse a string

s = "HKDSE"
reversed_s = s[::-1]   # 'ESDKH'

Or step by step:

result = ""
for ch in s:
    result = ch + result

Worked example · Validate password

pw = input("Password: ")
ok = len(pw) >= 8 and any(c.isdigit() for c in pw) and any(c.isupper() for c in pw)
print("Strong" if ok else "Weak")

Common student mistakes

• Strings are immutable — s[0] = "Z" raises an error. Build a new string instead.
• Forgetting that slicing is exclusive on the right (s[1:4] doesn't include index 4).
• Using == to compare strings instead of is (use ==; is checks identity).

Exam-style question

Q (5 marks): Write a Python function is_palindrome(s) that returns True if s reads the same forwards and backwards (case-insensitive, ignoring spaces).

Sample answer:

def is_palindrome(s):
    cleaned = s.replace(" ", "").lower()
    return cleaned == cleaned[::-1]

print(is_palindrome("Race car"))   # True
print(is_palindrome("Hello"))      # False

Key takeaways

• Strings: index, slice, methods.
• Immutable — create new strings rather than modifying.
• Format strings (f"…") for clean output.

➡️ Next: 3.8 Standard Algorithms