Lesson 07

for Loop and for..else — Iterate Over Collections

Use Python for loops over strings, lists, range, and dicts. Compare while vs for, build multiplication tables, find factors, and check perfect numbers.

Beginner40 min

for loop / for..else loop

Syntax 1 — basic for loop

python

for varname in iterable_object:
    ----------------------------------------
    indentation block of statements
    ----------------------------------------
# Other statements in program

Syntax 2 — for..else loop

python

for varname in iterable_object:
    ----------------------------------------
    indentation block of statements
    ----------------------------------------
else:
    ----------------------------------------
    else block of statements
    ----------------------------------------
# Other statements in program

Explanation

for, in, and else are keywords.
iterable_object can be:
- Sequence: bytes, bytearray, range, str
- List types: list, tuple
- Set types: set, frozenset
- Mapping: dict (iterates keys by default)
Each element of the iterable is selected, placed in varname, and the block executes. This repeats until all elements are processed.
When all elements are completed, PVM exits the for loop and runs the else block (if written).
Writing the else block is optional.

Example 1 — while vs for over a string

python

s = "PYTHON"

print("By using while loop")
i = 0
while i < len(s):
    print(s[i])
    i = i + 1

print("By using for loop")
for ch in s:
    print(ch)

Example 2 — Multiplication table with range

python

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

if n <= 0:
    print("{} is Invalid Input".format(n))
else:
    for i in range(1, 11):
        print("{} x {} = {}".format(n, i, n * i))
    else:
        print("-" * 50)

Example 3 — Find factors of a number

python

num = int(input("Enter a Number for Finding Its Factors:"))

if num <= 0:
    print("{} is Invalid Input".format(num))
else:
    for i in range(1, (num // 2) + 1):
        if num % i == 0:
            print(i)
    print(num)

Example 4 — Perfect number check (for..else)

python

num = int(input("Enter a Number for Finding Its Factors:"))

if num <= 0:
    print("{} is Invalid Input".format(num))
else:
    s = 0
    for i in range(1, (num // 2) + 1):
        if num % i == 0:
            s = s + i
            print(i)
    else:
        res = "Perfect" if num == s else " Not Perfect"
        print("{} is {}".format(num, res))

Example 5 — Sum of digits (while inside for-style logic)

python

num = int(input("Enter a Number for Finding Digits Sum:"))

if num <= 0:
    print("{} is Invalid input".format(num))
else:
    s = 0
    while num > 0:
        digit = num % 10
        s = s + digit
        num = num // 10
    else:
        print("Sum of Digits=", s)

Knowledge check

Q1: What types can a for loop iterate over?
Answer: Any iterable — str, list, tuple, set, frozenset, dict, range, bytes, bytearray.

Q2: When does for..else execute else?
Answer: After all items are processed without break.

Next: File 08 — break Statement

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