Loops

Topic — Repeating a block of code — a fixed number of times with for, or until a condition changes with while.

Loop	Use when
for	you know what to iterate over — a range, string, list, tuple, dict
while	you know when to stop, but not how many steps that takes

---

for — iterating over a sequence

A for loop takes each item of a sequence in turn and runs the block with it.

for i in range(5):
    print(i)

Output

0
1
2
3
4

range() — three forms

range() generates numbers on demand. The stop value is always excluded.

Form	Means	list(...)
range(5)	0 up to but not including 5	[0, 1, 2, 3, 4]
range(1, 6)	start to stop-1	[1, 2, 3, 4, 5]
range(2, 10, 3)	with a step	[2, 5, 8]
range(10, 0, -2)	negative step counts down	[10, 8, 6, 4, 2]
range(5, 1)	start past stop, no step	[] — empty, no error

print(list(range(5)))
print(list(range(1, 6)))
print(list(range(2, 10, 3)))
print(list(range(10, 0, -2)))
print(list(range(5, 1)))

Output

[0, 1, 2, 3, 4]
[1, 2, 3, 4, 5]
[2, 5, 8]
[10, 8, 6, 4, 2]
[]

warning

range(5, 1) produces nothing rather than raising an error — a loop over it silently does zero iterations. If a loop mysteriously never runs, check the direction of your range.

To count 1 to 5, write range(1, 6). Off-by-one here is the most common range() slip.

Iterating a string

Each character, in order:

word = "CAT"
for letter in word:
    print(letter)

Output

C
A
T

Iterating a tuple or list

numbers = (10, 20, 30)
for n in numbers:
    print(n)

Output

10
20
30

Iterating a dictionary

Looping a dict directly gives you the keys. Use .items() for both.

student = {"name": "Vettri", "age": 28}

for key in student:
    print(key)

for key, value in student.items():
    print(f"{key} = {value}")

Output

name
age
name = Vettri
age = 28

enumerate() and zip()

You rarely need range(len(...)). enumerate() gives you the index alongside the item:

for i, fruit in enumerate(["apple", "banana", "cherry"], start=1):
    print(i, fruit)

Output

1 apple
2 banana
3 cherry

zip() walks two sequences together:

for name, score in zip(["Alice", "Bob"], [95, 87]):
    print(name, score)

Output

Alice 95
Bob 87

---

while — looping until a condition changes

i = 1
while i <= 5:
    print(i)
    i += 1

Output

1
2
3
4
5

Three parts, and all three are your responsibility:

1. initialise — i = 1 before the loop
2. test — while i <= 5:
3. update — i += 1 inside the loop

Forget the update and the loop never ends

i = 1
while i <= 5:
    print(i)
    # no i += 1  →  i stays 1 forever

i never reaches 6, so the condition never turns false. Press Ctrl+C to stop a runaway loop. Whenever you write a while, check that something inside the body moves the condition toward false.

---

break — leave the loop early

for i in range(5):
    if i == 3:
        break
    print(i)

Output

0
1
2

break abandons the whole loop the moment it runs — 3 and 4 are never printed.

continue — skip to the next iteration

for i in range(5):
    if i == 2:
        continue
    print(i)

Output

0
1
3
4

2 is skipped, but the loop carries on. That's the difference: break exits, continue skips.

---

The loop else clause

Python lets a loop have an else. It's unusual, and the rule is easy to state:

note

A loop's else runs only if the loop finished without break.

No break, so else runs:

for x in range(3):
    print(x)
else:
    print("Loop completed")

Output

0
1
2
Loop completed

break vs continue against else

This pair is worth tracing carefully — it's the classic exam question.

# Block A — break
for i in range(1, 6):
    if i == 3:
        break
    print(i)
else:
    print("Completed")
print("Done")

Output

1
2
Done

# Block B — continue
for i in range(1, 6):
    if i == 3:
        continue
    print(i)
else:
    print("Completed")
print("Done")

Output

1
2
4
5
Completed
Done

Block A never prints Completed — the break cancelled the else. Block B does, because continue isn't break: the loop still ran to the end.

while loops take an else too, with the same rule:

i = 1
while i < 6:
    print(i)
    i += 1
else:
    print("Loop completed")

Output

1
2
3
4
5
Loop completed

Where the loop else earns its keep

Searching. "I looked at everything and never found it" is exactly what for...else says:

for number in [29, 21]:
    for divisor in range(2, int(number ** 0.5) + 1):
        if number % divisor == 0:
            print(f"{number} is not prime (divisible by {divisor})")
            break
    else:
        print(f"{number} is prime")

Output

29 is prime
21 is not prime (divisible by 3)

Without else you'd need a found = False flag and a check after the loop.

---

Where the update sits changes everything

In a while loop, moving i += 1 above or below the if changes the output. Trace both:

i = 1
while i < 6:
    print(i)        # print BEFORE the check
    if i == 3:
        break
    i += 1          # update LAST

print("---")

i = 0
while i < 6:
    i += 1          # update FIRST
    if i == 3:
        continue
    print(i)        # print AFTER the check

Output

1
2
3
---
1
2
4
5
6

The first loop prints 3 and then breaks, because the print comes before the test. The second starts at 0 but increments first, so it prints 1 through 6 — skipping 3.

---

Nested loops

A loop inside a loop. The inner loop runs completely for each iteration of the outer one.

adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]

for x in adj:
    for y in fruits:
        print(x, y)

Output

red apple
red banana
red cherry
big apple
big banana
big cherry
tasty apple
tasty banana
tasty cherry

Three outer × three inner = nine lines. Nesting multiplies the work, so two nested loops over 1,000 items each is a million iterations.

note

break only exits the innermost loop it sits in. To leave both, you need a flag, or to move the inner loop into a function and return.

Pattern printing

Nested loops with a range that depends on the outer variable:

for row in range(1, 6):
    print("* " * row)

Output

* 
* * 
* * * 
* * * * 
* * * * * 

---

Worked examples

Even numbers, two ways

Either filter with if, or step by 2 and skip the test entirely:

for i in range(1, 21):
    if i % 2 == 0:
        print(i, end=" ")
print()

for i in range(2, 21, 2):
    print(i, end=" ")
print()

Output

2 4 6 8 10 12 14 16 18 20 
2 4 6 8 10 12 14 16 18 20 

Same result; the second does half the iterations. See Input and Output for what end=" " is doing.

Multiplication table

n = 5
for i in range(1, 11):
    print(f"{n} x {i} = {n * i}")

Output

5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
5 x 4 = 20
5 x 5 = 25
5 x 6 = 30
5 x 7 = 35
5 x 8 = 40
5 x 9 = 45
5 x 10 = 50

Factorial

An accumulator starting at 1, multiplied each pass:

n = 5
factorial = 1
for i in range(1, n + 1):
    factorial *= i
print(f"{n}! = {factorial}")

Output

5! = 120

range(1, n + 1) — the + 1 is what includes n itself.

Sum of digits

% 10 peels off the last digit, // 10 removes it. A while loop, because you don't know how many digits there are:

number = 1234
total = 0
temp = number
while temp > 0:
    total += temp % 10
    temp //= 10
print(f"digits of {number} sum to {total}")

Output

digits of 1234 sum to 10

Note temp — the original number is kept intact for the message.

Stop at 6, skip 5

for i in range(1, 11):
    if i == 6:
        break
    print(i, end=" ")
print()

for i in range(1, 11):
    if i == 5:
        continue
    print(i, end=" ")
print()

Output

1 2 3 4 5 
1 2 3 4 6 7 8 9 10 

---

Common Mistakes

#	Mistake	Wrong	Right
1	Forgetting the update in a while	loop never ends	i += 1 inside the body
2	Off-by-one in range()	range(1, 5) for 1–5	range(1, 6)
3	Expecting range(5, 1) to count down	gives [], loop never runs	range(5, 0, -1)
4	break where continue was meant	exits the whole loop	continue skips one pass
5	Expecting else to run after a break	it doesn't	that's the whole point of loop else
6	break inside nested loops	only exits the inner one	use a flag, or a function with return
7	for i in range(len(items))	verbose	for item in items: or enumerate()
8	Modifying a list while looping over it	items get skipped	loop over a copy: for x in items[:]

---

Summary

Statement	Effect
for x in seq:	run the block once per item
range(a, b, c)	numbers from a to b-1, stepping c
while cond:	repeat while the condition stays true
break	exit the loop immediately
continue	skip the rest of this pass, carry on
else on a loop	runs only if the loop ended without break
enumerate(seq)	index and item together
zip(a, b)	walk two sequences in step

Key takeaways

• for when you know what to iterate; while when you only know the stop condition
• range() excludes its stop — range(1, 6) gives 1 to 5
• Every while needs something in the body that moves the condition toward false
• break exits the loop, continue skips one iteration
• A loop's else runs only if no break fired — perfect for "searched and found nothing"
• In a while, whether the update sits before or after the test changes the output
• break escapes only the innermost loop
• Prefer for item in items: and enumerate() over range(len(items))

See also: Conditional Statements for the conditions loops test · Data Types for the sequences you iterate

---

Run It Yourself

loops.py

# 1. for + range
print("1 to 5:      ", end="")
for i in range(1, 6):
    print(i, end=" ")
print()

# 2. while, same result
print("while 1 to 5:", end="")
i = 1
while i <= 5:
    print(i, end=" ")
    i += 1
print()

# 3. break and continue
print("break at 4:  ", end="")
for i in range(1, 8):
    if i == 4:
        break
    print(i, end=" ")
print()

print("skip 4:      ", end="")
for i in range(1, 8):
    if i == 4:
        continue
    print(i, end=" ")
print()

# 4. loop else — searching
target = 7
for n in [2, 5, 7, 9]:
    if n == target:
        print(f"found {target}")
        break
else:
    print(f"{target} not in the list")

# 5. nested loop pattern
for row in range(1, 5):
    print("* " * row)

Output

1 to 5:      1 2 3 4 5 
while 1 to 5:1 2 3 4 5 
break at 4:  1 2 3 
skip 4:      1 2 3 5 6 7 
found 7
* 
* * 
* * * 
* * * * 

---

Practice Questions

From the Unit 1 question bank. Tags and marks are explained on the Python index.

Unit 1 § I — Looping Statements

I1. [OUT] [4]

# Block A
for i in range(1, 6):
    if i == 3:
        break
    print(i)
else:
    print("Completed")
print("Done")

# Block B
for i in range(1, 6):
    if i == 3:
        continue
    print(i)
else:
    print("Completed")
print("Done")

I2. [OUT] The three-argument form of range(). [4]

print(list(range(5)))
print(list(range(1, 6)))
print(list(range(2, 10, 3)))
print(list(range(10, 0, -2)))
print(list(range(5, 1)))

I3. [PROG] Print the numbers 1 to 20 using a while loop. [3]

I4. [PROG] Print all even numbers between 1 and 20 using a for loop. Do it twice — once with if, once using range()'s step. [4]

I5. [PROG] Print 1 to 10, but stop the loop when the number becomes 6. [2]

I6. [PROG] Print 1 to 10, but skip the number 5. [2]

I7. [OUT] [4]

for x in ["red", "big"]:
    for y in ["apple", "banana"]:
        print(x, y)

I8. [PROG] Accept a number n and print its multiplication table from 1 to 10, formatted as 5 x 3 = 15. [3]

I9. [PROG] Accept a number and compute its factorial using a loop. [3]

I10. [PROG] Accept a number and compute the sum of its digits. (e.g. 1234 → 10) [4]

I11. [PROG] Print this pattern using nested loops: [4]

*
* *
* * *
* * * *
* * * * *

I12. [THEORY] This program never stops. Find the bug and fix it. [2]

i = 1
while i <= 5:
    print(i)

I13. [OUT] Trace carefully — note where the increment sits in each loop. [5]

i = 1
while i < 6:
    print(i)
    if i == 3:
        break
    i += 1

print("---")

i = 0
while i < 6:
    i += 1
    if i == 3:
        continue
    print(i)

I14. [PROG] Accept a number and print whether it is prime, using a loop with break and the loop's else. [4]

Unit 1 § L — Mini Challenges

L3. [PROG] Build a small menu-driven program using a while True loop that offers: [8]

1. Add a student
2. Show all students
3. Show the topper
4. Exit

Store the students as a list of tuples (name, marks). Keep looping until the user chooses 4.

Viva

1. When does a loop's else block not run?
2. What is the difference between break and continue?