Python Loops — while, for, and Loop Control

Repetition is at the heart of computing. Whether you are processing every item in a dataset, waiting for user input, or counting to a billion, loops give Python the ability to execute a block of code multiple times without duplicating a single line.

Python Loops — while, for, and Loop Control

Switch to Watch Mode to follow along

Key Takeaways

• while loops repeat as long as a condition remains True — ideal when the number of iterations is unknown upfront
• for loops iterate over a known sequence (list, string, range) — the workhorse of Python iteration
• The range() function generates numeric sequences for counted loops
• break, continue, and pass give you fine-grained control over loop execution
• Nested loops let you work with multi-dimensional data but increase complexity quickly

While Loops — Indefinite Iteration

A while loop checks its condition before each iteration. If the condition is True, the body executes; if False, the loop ends.

count = 0

while count < 5:
    print(f"Count is {count}")
    count += 1
# Output: Count is 0, 1, 2, 3, 4

Summing Numbers 1 through 10

total = 0
number = 1

while number <= 10:
    total += number
    number += 1

print(f"Sum of 1–10: {total}")  # 55

Mini-Project — Password Matcher

A practical use of while: keep prompting until the user enters the correct password.

correct_password = "python123"
attempt = ""

while attempt != correct_password:
    attempt = input("Enter the password: ")

print("Access granted!")

🖼️ Visual Suggestion: A flowchart showing the while-loop cycle — condition check → body execution → condition re-check → exit.

Avoiding Infinite Loops

If the condition never becomes False, the loop runs forever and your program hangs. The most common cause is forgetting to update the loop variable.

# DANGER — this loop never ends because i is never incremented
i = 0
while i < 5:
    print(i)
    # i += 1  ← missing this line creates an infinite loop

Always verify that something inside the loop moves the condition toward False.

For Loops — Definite Iteration

A for loop iterates directly over items in a sequence. You don't manage a counter variable — Python handles it for you.

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)
# Output: apple, banana, cherry

Filtering with a Condition

Combine a for loop with if to process only items that match a criterion.

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

even_numbers = []
for n in numbers:
    if n % 2 == 0:
        even_numbers.append(n)

print(even_numbers)  # [2, 4, 6, 8, 10]

Iterating Over Strings

Strings are sequences of characters, so for works on them naturally.

for char in "Python":
    print(char, end=" ")
# Output: P y t h o n

Iterating Over Dictionaries

By default, iterating a dictionary yields its keys. Use .items() to get both keys and values.

student = {"name": "Alice", "age": 22, "major": "CS"}

for key, value in student.items():
    print(f"{key}: {value}")
# name: Alice
# age: 22
# major: CS

🖼️ Visual Suggestion: A diagram showing a for loop pointer moving through each element in a list, one step per iteration.

The range() Function

range() generates a sequence of integers on the fly — essential for counted loops.

# Print the first five squares
for i in range(1, 6):
    print(f"{i}² = {i ** 2}")
# 1² = 1, 2² = 4, 3² = 9, 4² = 16, 5² = 25

Loop Control Statements

Python provides three keywords to alter the normal flow of a loop.

break — Exit Immediately

break terminates the loop entirely, regardless of the condition.

for num in range(1, 100):
    if num == 7:
        print(f"Found it: {num}")
        break
# Only prints "Found it: 7" — the loop stops at 7

continue — Skip to Next Iteration

continue skips the rest of the current iteration and jumps back to the loop's condition check.

for num in range(1, 11):
    if num % 3 == 0:
        continue  # skip multiples of 3
    print(num, end=" ")
# Output: 1 2 4 5 7 8 10

pass — Do Nothing (Placeholder)

pass is a syntactic no-op. Use it when a block is required by Python's grammar but you have nothing to execute yet.

for item in range(5):
    pass  # implementation pending — loop body cannot be empty

🖼️ Visual Suggestion: Three side-by-side flowcharts illustrating how break, continue, and pass alter the normal loop flow.

Nested Loops

A loop inside another loop creates a nested loop. The inner loop runs to completion for every single iteration of the outer loop.

for row in range(1, 4):
    for col in range(1, 4):
        print(f"({row},{col})", end="  ")
    print()  # newline after each row
# (1,1)  (1,2)  (1,3)
# (2,1)  (2,2)  (2,3)
# (3,1)  (3,2)  (3,3)

Multiplication Table

A classic nested-loop exercise:

size = 5
for i in range(1, size + 1):
    for j in range(1, size + 1):
        print(f"{i * j:4}", end="")
    print()

Output:

   1   2   3   4   5
   2   4   6   8  10
   3   6   9  12  15
   4   8  12  16  20
   5  10  15  20  25

Keep in mind that nested loops multiply the iteration count. Two loops of n each produce n² total iterations — something to watch when working with large datasets.

Quick Reference

Up next: we'll combine loops with functions — learning how to package reusable logic and keep your code clean.