Flow of Control

Unlike languages like C++ or Java that use curly braces `{}` to define code blocks, Python enforces significant indentation (typically 4 spaces) following a colon (`:`):

A. The `if-elif-else` Multi-Way Selection

marks = float(input("Enter percentage: "))

if marks >= 90:
    grade = "A+"
elif marks >= 75:
    grade = "A"
elif marks >= 60:
    grade = "B"
elif marks >= 40:
    grade = "C"
else:
    grade = "F"

print(f"Verified Grade: {grade}")

B. Short-Circuit Logical Evaluation

Python evaluates compound Boolean expressions using short-circuit logic:

• In `A and B`: If `A` evaluates to `False`, Python immediately returns `False` without evaluating `B` (saves execution time and prevents runtime crashes like division by zero: `x != 0 and (10 / x) > 2`).
• In `A or B`: If `A` evaluates to `True`, Python immediately returns `True` without evaluating `B`.

The `for` statement in Python is not a traditional counting loop—it is a Sequence Iterator that traverses through each item of an iterable collection (such as a string, list, tuple, or range):

The Anatomy of `range(start, stop[, step])`

• `range(stop)`: Starts at $0$, increments by $1$, stops strictly at $\text{stop} - 1$.
• `range(start, stop)`: Starts at $\text{start}$, increments by $1$, stops at $\text{stop} - 1$.
• `range(start, stop, step)`: Increments by $\text{step}$.
  • If $\text{step} > 0$: Generates increasing sequence up to $\text{stop} - 1$.
  • If $\text{step} < 0$: Generates decreasing countdown sequence down to $\text{stop} + 1$.
  • If $\text{step} == 0$: Raises a `ValueError: range() arg 3 must not be zero`.

Range Mechanics Examples:

list(range(5))             # [0, 1, 2, 3, 4]
list(range(2, 10, 2))       # [2, 4, 6, 8]
list(range(10, 0, -2))      # [10, 8, 6, 4, 2]
list(range(5, 5))           # [] (Empty sequence: start == stop)
list(range(10, 2, 1))       # [] (Empty: positive step cannot count down)

A `while` loop executes as long as a governing condition remains `True`. It is preferred when the number of required iterations cannot be determined prior to runtime (e.g., waiting for user input or waiting for a calculation to converge):

# Sentinel Loop: Terminates when user enters sentinel value (-1)
total = 0
count = 0

val = int(input("Enter score (-1 to exit): "))
while val != -1:
    total += val
    count += 1
    val = int(input("Enter score (-1 to exit): "))

if count > 0:
    print(f"Average score: {total / count:.2f}")
else:
    print("No scores entered.")

A. Jump Statements

• `break` Statement: Immediately terminates the innermost loop and transfers execution to the first statement outside the loop body.
• `continue` Statement: Skips all remaining statements in the *current iteration* and jumps immediately to the next iteration (re-evaluating loop condition or fetching next item).
• `pass` Statement: A syntactic null statement (NOP - No Operation). Used as an empty placeholder where code is syntactically required but no action is needed (e.g., in empty loops, stub functions, or exception handling).

B. The Python `loop...else` Construct

Python provides a unique and elegant feature: an `else` clause attached to a `for` or `while` loop! The `else` block executes only if the loop completed naturally without being terminated by a `break` statement:

# Prime Number Search demonstrating for...else
num = int(input("Enter integer > 1: "))

for i in range(2, int(num**0.5) + 1):
    if num % i == 0:
        print(f"{num} is composite ({i} is a factor).")
        break
else:
    # Executes ONLY if loop finished without breaking!
    print(f"{num} is a PRIME number!")