Lesson 06

while Loop and while..else — Iterative Statements

Write Python while loops with initialization, condition, and updation. Generate number series, multiplication tables, and sums using while..else.

Beginner35 min

Looping (Iterative / Repetitive) statements

The purpose of looping statements is to perform a certain operation repeatedly for a finite number of times until the test condition becomes false.

Python has 2 types of looping statements:

while loop OR while..else loop
for loop OR for..else loop

When you write any loop program, remember 3 points:

Part	Role
Initialization	Where to start
Conditional	Where to stop
Updation	Increment or decrement

Loop three parts — init condition update

while loop syntax

python

while test_condition:
    ----------------------------------------
    indentation block of statements
    ----------------------------------------
# Other statements in program

while..else syntax

python

while test_condition:
    ----------------------------------------
    indentation block of statements
    ----------------------------------------
else:
    ----------------------------------------
    else block of statements
    ----------------------------------------
# Other statements in program

The else block runs when the loop exits normally (condition becomes false). It does not run if break terminates the loop.

Example 1 — Generate 1 to n

python

n = int(input("Enter How Many Numbers u want to Generate:"))

if n <= 0:
    print("{} is Invalid input".format(n))
else:
    print("-" * 50)
    print("Numbers within:{}".format(n))
    i = 1          # Initialization Part
    while i <= n:  # Conditional Part
        print(i)
        i += 1     # Updation Part
    else:
        print("*" * 50)

Example 2 — Generate n down to 1

python

n = int(input("Enter How Many Numbers u want to Enter:"))

if n <= 0:
    print("{} is Invalid".format(n))
else:
    print("=" * 50)
    print("Numbers within :{}".format(n))
    i = n              # Initialization
    while i >= 1:      # Condition
        print("\t\t{}".format(i))
        i = i - 1      # Updation
    else:
        print("=" * 50)

Example 3 — Multiplication table

python

n = int(input("Enter a Number for Generating Mul table:"))

if n <= 0:
    print("{} is Invalid Input".format(n))
else:
    i = 1
    while i <= 10:
        print("\t\t{} x {} = {}".format(n, i, n * i))
        i += 1
    else:
        print("=" * 50)

Example 4 — Sum of n natural numbers

python

n = int(input("Enter How Many natural nums u want:"))

if n <= 0:
    print("{} is Invalid Input".format(n))
else:
    s = 0
    i = 1
    while i <= n:
        print("\t{}".format(i))
        s = s + i
        i = i + 1
    else:
        print("sum={}".format(s))

Knowledge check

Q1: What three parts must every while loop have?
Answer: Initialization, condition, updation.

Q2: When does while..else run the else block?
Answer: When the loop condition becomes false without break.

Next: File 07 — for Loop

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