✂️ Data Structures

Strings

स्ट्रिंग्स

⏱ 3 hr4 topicsInteractive

🎯 By the end: You can index and slice strings (including negative indices), use the common string methods, and explain why strings are immutable.

A string is a sequence of characters — text — written in quotes: "hello" or 'Vidaara'. You've already printed strings; now you'll learn to take them apart, search them, and transform them. Strings are everywhere in real programs, so this chapter pays off immediately — and it ends with you building a real Password Strength Checker.

1Indexing and immutability

Each character in a string has a position called its index. Indexing starts at 0, and you can also count backwards from the end with negative indices:

  P  y  t  h  o  n
  0  1  2  3  4  5     (positive index)
 -6 -5 -4 -3 -2 -1     (negative index)

So for word = "Python": word[0] is 'P' and word[-1] is 'n'. The len() function gives the length (here, 6).

Strings are immutable — once created, you cannot change a character inside them. word[0] = 'J' causes an error. Instead, you build a new string from the old one. This is a favourite exam point.

Indexing▶ runs real Python

main.py

word = "Python"
print(word[0])     # P (first)
print(word[-1])    # n (last)
print(len(word))   # 6
# word[0] = 'J'   # would ERROR: strings are immutable

Output

Click Run to execute.

Key points

Indexing starts at 0; negative indices count from the end (-1 is the last character).
len(s) gives the number of characters.
Strings are immutable — you can't change a character in place; you build a new string instead.

2Slicing

Slicing takes a piece of a string with the syntax string[start:stop:step]. As with range(), the stop index is excluded:

word[0:3] → characters 0, 1, 2 (not 3).
word[2:] → from index 2 to the end.
word[:4] → from the start up to (not including) index 4.
word[::-1] → the whole string reversed (a neat trick).

Slicing always returns a new string (the original is untouched, because strings are immutable).

Slicing▶ runs real Python

main.py

word = "Programming"
print(word[0:4])    # Prog
print(word[4:])     # ramming
print(word[:4])     # Prog
print(word[::-1])   # gnimmargorP (reversed)

Output

Click Run to execute.

Key points

Slice with s[start:stop:step]; the stop index is excluded.
Omit start (s[:n]) or stop (s[n:]) to go from the beginning or to the end.
s[::-1] reverses a string; slicing returns a new string.

3String methods

Strings come with many built-in methods (functions called with a dot). The ones you must know:

Method	Does
`upper()` / `lower()`	Returns the string in upper / lower case
`strip()`	Removes spaces from both ends
`split()`	Splits into a list of words
`join()`	Joins a list of strings into one
`replace(a, b)`	Replaces every a with b
`find(x)`	Returns the index of x (or -1 if absent)
`isalpha()` / `isdigit()`	True if all letters / all digits

You can also join strings with + (concatenation) and repeat with *: "ab" * 3 gives "ababab". Remember — every method returns a new string; the original never changes.

String methods▶ runs real Python

main.py

name = "  vidaara  "
print(name.strip().upper())     # VIDAARA
print("a,b,c".split(","))       # ['a', 'b', 'c']
print("data science".replace("data", "computer"))
print("Python".find("th"))      # 2
print("ab" * 3)                 # ababab

Output

Click Run to execute.

Key points

Key methods: upper/lower, strip, split, join, replace, find, isalpha/isdigit.
+ concatenates strings; * repeats them.
Every string method returns a NEW string — the original is unchanged (immutability).

4Project: Password Strength Checker

Time to combine everything — strings, methods, loops and conditions — into a real, useful program. This Password Strength Checker scores a password on three rules: length, having a digit, and having a capital letter. Run it, then change the password and the rules:

Password Strength Checker▶ runs real Python

main.py

password = "Vidaara123"

has_digit = False
has_upper = False
for ch in password:
    if ch.isdigit():
        has_digit = True
    if ch.isupper():
        has_upper = True

score = 0
if len(password) >= 8:
    score = score + 1
if has_digit:
    score = score + 1
if has_upper:
    score = score + 1

print("Password:", password)
print("Score:", score, "/ 3")
if score == 3:
    print("Strength: Strong")
elif score == 2:
    print("Strength: Medium")
else:
    print("Strength: Weak")

Output

Click Run to execute.

Key points

Real programs combine strings, loops and conditions.
Loop over each character and use isdigit()/isupper() to inspect it.
Build a score with if-statements, then decide the result.

★ Project: extend the Password Strength Checker

Using the playground above, improve the checker:

Add a fourth rule: the password must contain a lowercase letter (use islower()).
Change the strength bands so a score of 4 prints 'Very Strong'.
Test it with three different passwords and check the results make sense.
Bonus: reject any password shorter than 6 characters with a clear message.

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