Python – Strings & Conditional Statements (Chapter 2)

Strings – Core Concepts

• A string is a sequence of characters.
  • May contain 0, 1 or many characters; can store a word, a sentence, even a paragraph.
  • Belongs to Python’s sequence data-types (already encountered in Chapter 1).

Creating strings (3 syntaxes)

• Double quotes – most common and preferred:

  s1 = "This is a string"

• Single quotes – equally valid:

  s2 = 'hello'

• Triple quotes – for multi-line text or docstrings:

  s3 = '''This is
a multi-line
string.'''

Why 3 quoting styles?

• To embed one type of quote inside another without escaping.
  • Want an apostrophe? Use double quotes: "This is Shraddha's notebook".
  • Want embedded double quotes? Wrap the whole text in single quotes or triple quotes.

Escape-sequence characters (formatting helpers)

Example – insert a newline and a tab:

s = "This is a string\n\twritten on next line with a tab"
print(s)

Fundamental Operations on Strings

1. Concatenation

• Combine two strings via +.

  first = "अपना"
  second = "college"
  final = first + " " + second    # space deliberately added
  print(final)          # अपना college

• Operation is called concatenation. Works only between strings.

2. Length

• Built-in function $\text{len}(\cdot)$ gives total number of characters including spaces & special symbols.

  n = len(final)        # 12, because space counts as 1 char

3. Indexing (random access)

• Characters auto-labelled with indices starting at $0$ from the left.
  • final[0] → first char,
  • final[4] → the space in “अपना college”.

• Attempting to assign: final[4] = '-' ⇒ error “str object does not support item assignment” (immutability!).

4. Negative Indexing

• Python also allows counting from right: last char = $-1$, second-last = $-2$ …

  word = "apple"
  word[-3]    # 'p'

5. Slicing

Extract a sub-string via $[\text{start} : \text{end}]$
• start index inclusive, end exclusive.
• Either bound may be omitted (defaults to 0 or len).

  text = "अपना college"
  text[1:4]   # 'pna'
  text[:4]    # 'अपना'
  text[5:]    # 'college'
  text[-3:-1] # 'le'

Essential String Methods (all return new strings, originals unchanged unless re-assigned)

1. str.endswith(substr)
   • Returns True/False whether str finishes with given substring.

   sentence.endswith("ege")   # True

2. str.capitalize()
   • Makes first character upper-case; rest lower.

   s = "hello world"
   s_cap = s.capitalize()      # 'Hello world'
   s = s.capitalize()          # overwrite if you need permanent change

3. str.replace(old, new)
   • Replaces all occurrences of old with new.

   "room 101".replace("o", "0")   # 'r00m 101'

4. str.find(substr)
   • Returns first starting index of substr; $-1$ if not found.

5. str.count(substr)
   • Returns number of (possibly overlapping) occurrences.

Practical tip

Conditional Statements

Syntax skeleton

if CONDITION1:
    stmt_block_1
elif CONDITION2:
    stmt_block_2
elif CONDITION3:
    ...
else:
    default_block

Rules

• if block executes when its condition is $\text{True}$.

• As soon as one block runs, interpreter skips all remaining elif/else siblings.

• else (optional) must be last; contains no condition – executes when none of the earlier tests passed.

• Indentation (4 spaces or a tab) defines code blocks; Python does not use {}.

Example 1 – Age & licence

age = 21
if age >= 18:
    print("Can vote and apply for licence")
else:
    print("Cannot vote")

Example 2 – Traffic light

light = "green"
if light == "red":
    print("STOP")
elif light == "green":
    print("GO")
elif light == "yellow":
    print("LOOK / WAIT")
else:
    print("Light is broken!")

Logical operators inside conditions

• and, or, not – build compound expressions.

  if marks >= 80 and marks < 90:
      grade = 'B'

Nesting

• One if (or loop) inside another. Completely legal:

  if age >= 18:
      if age >= 80:
          print("Cannot drive (too old)")
      else:
          print("Can drive")
  else:
      print("Too young to drive")

End-to-End Program – Grading System

marks = int(input("Enter marks: "))
if marks >= 90:
    grade = 'A'
elif marks >= 80 and marks < 90:
    grade = 'B'
elif marks >= 70 and marks < 80:
    grade = 'C'
else:
    grade = 'D'
print("Grade:", grade)

Flow

1. Input → integer marks.

2. Cascade through ranges using if/elif.

3. Output corresponding letter grade.

Practice Problems & Approaches

1. Odd or Even

Logic: a number is even if remainder after division by 2 is 0.
Formula: $\text{num} \% 2 == 0$
Code (minimal):

n = int(input())
print("Even" if n % 2 == 0 else "Odd")

2. Largest of Three Numbers

Steps

1. Read $a,b,c$.

2. Compare hierarchically using if-elif-else or nested if.

   if a >= b and a >= c:
       largest = a
   elif b >= c:
       largest = b
   else:
       largest = c

Extension homework → generalise to n numbers (hint: loop or use max() built-in).

3. Multiple of 7 Checker

Criterion: $\text{num} \% 7 == 0$.

x = int(input())
if x % 7 == 0:
    print("Multiple of 7")
else:
    print("Not a multiple of 7")

Programming Tips Mentioned

• Immutability of strings ⇒ you must build new strings for modifications.

• Google / docs during development is normal; focus on logical understanding, not brute memorisation.

• Indentation is syntactically mandatory in Python; always maintain consistent four-space blocks.

Real-World & Future Relevance

• String slicing & indexing are critical for data-cleaning in Machine Learning / Data Science.

• Conditional statements form the decision-making backbone in every non-trivial program – from simple CLI apps to production servers.

(End of Chapter 2 study notes)