for Loop and while Loop

Why Loops?

A loop repeats a block of code many times without writing it again and again. Python has two loops: for (repeat a known number of times) and while (repeat until a condition becomes false).

The for Loop & range()

for repeats over a sequence. range(start, stop, step) generates numbers (stop is not included).

for i in range(1, 6):
    print(i, end=" ")

• range(1, 6) gives 1,2,3,4,5 (6 is excluded).
• i takes each value one by one.

The while Loop

while keeps running as long as its condition is True. You must change something inside, or it loops forever.

count = 1
while count <= 3:
    print("Count:", count)
    count += 1   # this line ends the loop eventually

Program 1: Print 1 to 10

for i in range(1, 11):
    print(i)

range(1, 11) covers 1 to 10. Each pass prints one number.

Program 2: Multiplication Table

n = int(input("Table of: "))
for i in range(1, 11):
    print(n, "x", i, "=", n * i)

The loop runs 10 times; each time i grows from 1 to 10 and we print n * i.

Program 3: Sum & Average of N Numbers

n = int(input("How many numbers? "))
total = 0
for i in range(n):
    num = int(input("Enter number: "))
    total += num

print("Sum     =", total)
print("Average =", total / n)

• total = 0 starts an accumulator before the loop.
• total += num adds each input to the running total.

Program 4: Factorial using while

n = int(input("Enter n: "))
fact = 1
while n > 0:
    fact *= n
    n -= 1
print("Factorial =", fact)

5! = 5x4x3x2x1 = 120. Each pass multiplies fact by n, then reduces n.

Program 5: Star Pattern (nested loop)

rows = 5
for i in range(1, rows + 1):
    for j in range(i):
        print("*", end="")
    print()

• The outer loop controls rows; the inner loop prints stars in each row.
• Row i prints i stars, then print() moves to a new line.

Program 6: Number Guess with break & continue

secret = 7
while True:
    g = int(input("Guess (1-10): "))
    if g == secret:
        print("Correct!")
        break          # exit the loop
    if g < 1 or g > 10:
        print("Out of range, try again")
        continue       # skip to next loop
    print("Wrong, try again")

break, continue and loop-else

• break — exit the loop immediately.
• continue — skip the rest of this pass, go to the next.
• else on a loop — runs once if the loop finished without break.

Common Mistakes

• Forgetting to update the variable in a while → infinite loop.
• Off-by-one with range() (remember stop is excluded).
• Wrong indentation for the loop body.

Practice Tasks

1. Print all even numbers from 1 to 50.
2. Print the table of any number entered by the user.
3. Find the sum of digits of a number using a while loop.
4. Print an inverted star triangle (5 stars down to 1).
5. Count how many numbers in a list are positive.

Summary

• for repeats a known number of times (often with range()).
• while repeats until its condition is false — update the variable inside.
• break exits, continue skips a pass.
• Nested loops make patterns and tables.