Python Loops

What is a loop?

A loop repeats a block of code while following a defined iteration rule or condition.

for number in range(1, 4):
    print(number)

Try your first loop →

The for loop

Python for loop an iterable se values one by one leta hai.

courses = ["HTML", "CSS", "Python"]

for course in courses:
    print(course)

Har iteration me course next item ko reference karta hai.

Loop over a string

Strings iterable hoti hain, so characters one by one iterate kiye ja sakte hain.

for character in "Python":
    print(character)

Generate number sequences with range()

range() integer sequence represent karta hai and loops me commonly use hota hai.

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

for number in range(2, 6):
    print(number)

for number in range(2, 11, 2):
    print(number)

• range(stop) — starts at 0.
• range(start, stop) — stop excluded.
• range(start, stop, step) — custom step.

Count backwards

for number in range(5, 0, -1):
    print(number)

print("Go!")

Negative step use karte waqt start/stop direction compatible honi chahiye.

The while loop

while tab tak repeat karta hai jab tak condition truthy rahe.

count = 1

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

Try while loop →

for vs while

• for: iterable ke items ya known sequence par iterate karna ho.
• while: repetition kisi changing condition par depend kare.

Known collection ke liye manual index-based while ke bajay direct for often clearer hota hai.

Stop early with break

break nearest loop ko immediately exit karta hai.

for number in range(1, 10):
    if number == 5:
        break
    print(number)

Output 1 to 4 tak hoga.

Skip an iteration with continue

continue current iteration ka remaining block skip karke next iteration par chala jata hai.

for number in range(1, 7):
    if number % 2 == 0:
        continue
    print(number)

Only odd numbers print honge.

pass inside loops

pass no-op placeholder hai; unlike continue, it next iteration par jump nahi karta.

for item in range(3):
    pass

print("Loop finished")

Loop else

Python loops optional else support karte hain. else tab run hota hai jab loop normal completion kare; break se exit hone par nahi.

target = 7

for number in range(1, 6):
    if number == target:
        print("Found")
        break
else:
    print("Not found")

Nested loops

Ek loop ke andar another loop ho sakta hai.

for row in range(1, 4):
    for column in range(1, 4):
        print(row, column)

Outer loop ki each iteration ke liye inner loop fully run hota hai. Large nested loops expensive ho sakte hain.

Running totals with an accumulator

total = 0

for number in range(1, 6):
    total += number

print(total)

Accumulator variable each iteration me updated result store karta hai. Built-in sum() bhi many simple cases me better choice ho sakta hai.

Counting matches

scores = [82, 45, 91, 67, 38]
passed = 0

for score in scores:
    if score >= 40:
        passed += 1

print("Passed:", passed)

Get index and value with enumerate()

enumerate() manual index counter ke bajay readable index + value pairs deta hai.

courses = ["HTML", "CSS", "Python"]

for position, course in enumerate(courses, start=1):
    print(position, course)

Manual index += 1 se cleaner hota hai jab index actually needed ho.

Loop over multiple iterables with zip()

names = ["Aman", "Riya", "Kabir"]
scores = [82, 91, 74]

for name, score in zip(names, scores):
    print(name, score)

Default zip() shortest iterable khatam hote hi stop karta hai, so unequal lengths silently ignore extra items kar sakti hain.

Repeated input with while

attempts = 0

while attempts < 3:
    answer = input("Type yes: ").strip().lower()
    if answer == "yes":
        print("Accepted")
        break
    attempts += 1
else:
    print("No attempts left")

Run retry example →

Infinite loops

while True intentionally useful ho sakta hai when loop explicitly break se terminate hota ho, but accidental infinite loops avoid karo.

while True:
    command = input("Command (quit to stop): ").strip().lower()
    if command == "quit":
        break
    print("You typed:", command)

Changing a collection while iterating

Same collection ko iterate karte waqt uski structure mutate karna surprising behavior create kar sakta hai.

numbers = [1, 2, 3, 4]

for number in numbers.copy():
    if number % 2 == 0:
        numbers.remove(number)

print(numbers)

Often better approach new collection build karna ya later comprehensions use karna hota hai.

Common beginner mistakes

• range() stop value ko included assume karna.
• while condition ki state update bhoolkar infinite loop banana.
• break aur continue ka difference confuse karna.
• pass ko continue samajhna.
• Unnecessary manual index counters use karna instead of enumerate().
• Deep nested loops without need.
• Loop ke andar repeated expensive work karna jab outside ho sakta ho.
• Same list ko unsafe way se mutate karte hue iterate karna.
• zip() unequal lengths ka behavior ignore karna.
• Simple built-ins like sum() available hone par manual loop overuse karna.

Beginner best practices

• Collection iteration ke liye direct for prefer karo.
• Condition-driven repetition ke liye while use karo.
• Clear loop variable names rakho.
• Boundary values and empty iterables test karo.
• break/continue ko readability improve karne ke liye use karo, control flow hide karne ke liye nahi.
• Index required ho to enumerate() dekho.
• Parallel data iterate karna ho to zip() consider karo.
• Loop body ko short aur understandable rakho.

Chapter checklist

• for loop se iterable traverse kar sakte ho?
• range() ke start, stop, step clear hain?
• while loop safely write kar sakte ho?
• break, continue, pass ka difference clear hai?
• Loop else kab run hota hai samajh aaya?
• Nested loop aur accumulator pattern use kar sakte ho?
• enumerate() aur zip() ka purpose clear hai?

Open practice starter →