Loops

Q: When should I use for vs while ?

Use for when you know the number of iterations (iterating over lists/ranges/strings). Use while when you know the stopping condition (waiting for input, approaching a value). They're often interchangeable, but choosing the right one makes code clearer. ⚠️ Q: Is for i in range(len(list)) Pythonic? A: Not really. Python prefers direct iteration: for item in list . Only use range(len(...)) when you specifically need indices — and even then, enumerate() is more elegant. Q: What if I have too many nested loops? A: Nesting beyond 3 levels hurts readability. Solutions: extract inner loops into functions, use itertools for combinations, or rethink the algorithm.

When writing code, you often encounter scenarios where you need to repeat the same action — printing numbers 1 to 100, processing every element in a list, or continuously asking for user input until you get a valid answer.

Writing 100 lines of print() for 100 numbers is impractical. That's where loops come in. Python provides two types of loops:

for loop — for "iterating" over a sequence (list, string, range, etc.)
while loop — for "execute while condition is true"

`for` Loop Basics

The for loop is the most commonly used loop in Python. Its core idea: take elements from a sequence one by one, executing the same code block each time.

Basic syntax:

PYTHON

for variable in sequence:
    code to repeat

The variable is assigned each element in the sequence, and the indented block is executed for each one.

Example: Iterating Over Lists and Strings (⭐)

PYTHON

# Iterate over a list
fruits = ["apple", "banana", "orange", "grape"]
for fruit in fruits:
    print(f"I like {fruit}")

print("---")

# Iterate over a string — characters one by one
for char in "Python":
    print(char)

▶ Try it Yourself

Output:

TEXT

I like apple
I like banana
I like orange
I like grape
---
P
y
t
h
o
n

💡 Naming tip: Name the loop variable in singular form. If the list is fruits, use fruit; if students, use student. The code reads like natural language.

The `range()` Function — Generate Number Sequences

When you need to repeat a fixed number of times, range() is your best friend.

range() has three forms:

Syntax	Meaning	Example	Result
`range(stop)`	0 → stop-1	`range(5)`	0,1,2,3,4
`range(start, stop)`	start → stop-1	`range(2, 6)`	2,3,4,5
`range(start, stop, step)`	With step	`range(1, 10, 2)`	1,3,5,7,9

Note: range() does not include the stop value — same as slicing, it's "up to but not including."

Example: Controlling Loop Count with range (⭐)

PYTHON

# Print 1 to 5
for i in range(1, 6):
    print(f"Loop {i}")

print("---")

# Step of 2: print even numbers from 0 to 10
for i in range(0, 11, 2):
    print(i, end=" ")  # end=" " prevents newlines
# Output: 0 2 4 6 8 10

▶ Try it Yourself

Example: Iterating with Index (⭐⭐)

PYTHON

students = ["Alice", "Bob", "Charlie", "Diana"]

# Get both index and element
for i in range(len(students)):
    print(f"Student {i + 1}: {students[i]}")

▶ Try it Yourself

Output:

TEXT

Student 1: Alice
Student 2: Bob
Student 3: Charlie
Student 4: Diana

💡 More elegant way: In Python, if you need both index and element, use enumerate(). We'll cover it in a later lesson.

`while` Loop

The difference between while and for: for knows how many times to loop; while only knows when to stop.

Basic syntax:

PYTHON

while condition:
    code to repeat

Before each iteration, the condition is checked. If it's True, the code block runs, then the condition is checked again... until it becomes False.

Example: Countdown (⭐)

PYTHON

count = 5
while count > 0:
    print(f"Countdown: {count}")
    count -= 1  # Decrement by 1 each time — very important!

print("Launch!")

▶ Try it Yourself

Output:

TEXT

Countdown: 5
Countdown: 4
Countdown: 3
Countdown: 2
Countdown: 1
Launch!

⚠️ Infinite loop warning: If you forget count -= 1, the condition count > 0 is always True, and the program runs forever until you force-stop it. When writing while loops, always ensure the condition can become False at some point.

Example: Typical while Usage — Until a Specific Value (⭐⭐)

PYTHON

# Keep halving until less than 1
num = 100
while num >= 1:
    print(num, end=" → ")
    num /= 2
# Output: 100 → 50.0 → 25.0 → 12.5 → 6.25 → 3.125 → 1.5625 → 0.78125

▶ Try it Yourself

This shows the strength of while: we don't know in advance how many iterations are needed — we only know when to stop.

`break` and `continue` — Brake and Skip

`break` — Exit the Loop Immediately

When a condition is met, break jumps out of the entire loop, disregarding any remaining iterations.

`continue` — Skip the Current Iteration

When a condition is met, continue skips the rest of the current iteration and moves to the next one.

Example: break vs continue (⭐)

PYTHON

print("=== break example ===")
for i in range(1, 10):
    if i == 5:
        break  # End the loop when reaching 5
    print(i, end=" ")
# Output: 1 2 3 4

print("\n=== continue example ===")
for i in range(1, 10):
    if i == 5:
        continue  # Skip 5, continue with the rest
    print(i, end=" ")
# Output: 1 2 3 4 6 7 8 9

▶ Try it Yourself

Example: break in Practice — Search and Stop (⭐⭐)

PYTHON

# Search for the first even number in a list
numbers = [3, 7, 9, 2, 5, 8, 1]

for num in numbers:
    if num % 2 == 0:
        print(f"Found first even number: {num}")
        break  # Stop searching once found
else:
    print("No even numbers in the list")  # This else belongs to the for loop

▶ Try it Yourself

Example: continue in Practice — Process Only Valid Data (⭐⭐)

PYTHON

# Process only positive numbers, skip negatives and zero
data = [3, -5, 0, 8, -2, 7, 0, 4]
positive_sum = 0

for num in data:
    if num <= 0:
        continue  # Skip invalid data
    positive_sum += num

print(f"Sum of positive numbers: {positive_sum}")  # Output: 22 (3+8+7+4)

▶ Try it Yourself

`for-else` and `while-else` — "Normal Completion Bonus"

This is a unique Python syntax feature, not found in many other languages.

The else block executes when the loop completes normally (without being interrupted by break). Think of it as: "if the loop wasn't interrupted, run this bonus code."

Example: Prime Number Check (⭐⭐)

PYTHON

num = 17

for i in range(2, num):
    if num % i == 0:
        print(f"{num} is not prime, divisible by {i}")
        break
else:
    print(f"{num} is prime!")  # Executes only if no break occurred

▶ Try it Yourself

In this example:

If a divisor is found, break is triggered, else doesn't run
If no divisor is found after checking all possibilities, the loop ends normally, and else runs

💡 Many beginners find for-else counterintuitive. Remember it as: else means "if no break happened, then...".

Nested Loops

A loop inside another loop is a nested loop. For each iteration of the outer loop, the inner loop runs a full round.

Example: Multiplication Table (⭐⭐)

PYTHON

for i in range(1, 10):         # Outer loop: rows
    for j in range(1, i + 1):  # Inner loop: columns
        print(f"{j}×{i}={i*j}", end="\t")
    print()  # Newline after each row

▶ Try it Yourself

Output:

TEXT

1×1=1
1×2=2    2×2=4
1×3=3    2×3=6    3×3=9
1×4=4    2×4=8    3×4=12   4×4=16
1×5=5    2×5=10   3×5=15   4×5=20   5×5=25
1×6=6    2×6=12   3×6=18   4×6=24   5×6=30   6×6=36
1×7=7    2×7=14   3×7=21   4×7=28   5×7=35   6×7=42   7×7=49
1×8=8    2×8=16   3×8=24   4×8=32   5×8=40   6×8=48   7×8=56   8×8=64
1×9=9    2×9=18   3×9=27   4×9=36   5×9=45   6×9=54   7×9=63   8×9=72   9×9=81

⚠️ Performance tip: Total executions = outer count × inner count. N-level nesting means exponential growth. Be careful with 3+ levels of nesting when dealing with large datasets.

Example: break Only Affects the Current Level (⭐⭐)

PYTHON

for i in range(1, 4):
    for j in range(1, 10):
        if j > i:
            break  # Only breaks the inner loop
        print(f"({i},{j})", end=" ")
    print()

▶ Try it Yourself

Output:

TEXT

(1,1)
(2,1) (2,2)
(3,1) (3,2) (3,3)

Comprehensive Examples

Example 1: Guess the Number Game (⭐⭐)

The system generates a random number between 1 and 100. The player guesses, and the system gives hints ("too high" or "too low") until the player guesses correctly.

PYTHON

import random

# Generate a random target
target = random.randint(1, 100)
attempts = 0

print("🎯 Guess the Number! I'm thinking of a number between 1 and 100.")

while True:  # Infinite loop, exit via break
    try:
        guess = int(input("Enter your guess: "))
    except ValueError:
        print("Please enter a valid number!")
        continue

    attempts += 1

    if guess < target:
        print("Too low, try higher!")
    elif guess > target:
        print("Too high, try lower!")
    else:
        print(f"🎉 Congratulations! It's {target}!")
        print(f"You guessed it in {attempts} attempts.")
        break  # Exit the loop on correct guess

▶ Try it Yourself

Key points:

while True with break is a very common pattern in Python
try-except catches non-numeric input, continue prompts the user again
break exits the loop when guessed correctly

Example 2: Password Strength Checker (⭐⭐⭐)

Check the user's password and score its strength.

PYTHON

password = input("Enter your password: ")
score = 0
feedback = []

# 1. Check length
if len(password) >= 8:
    score += 1
else:
    feedback.append("❌ Password is less than 8 characters")

# 2. Check character types
has_upper = False
has_lower = False
has_digit = False
has_special = False
special_chars = "!@#$%^&*"

for char in password:
    if char.isupper():
        has_upper = True
    elif char.islower():
        has_lower = True
    elif char.isdigit():
        has_digit = True
    elif char in special_chars:
        has_special = True

# Score based on checks
checks = [has_upper, has_lower, has_digit, has_special]
score += sum(checks)

if not has_upper:
    feedback.append("❌ Missing uppercase letter")
if not has_lower:
    feedback.append("❌ Missing lowercase letter")
if not has_digit:
    feedback.append("❌ Missing digit")
if not has_special:
    feedback.append("❌ Missing special character")

# 3. Output results
print(f"\nPassword strength: {score}/5")
if feedback:
    print("Suggestions:")
    for item in feedback:
        print(f"  {item}")
else:
    print("🎉 Strong password!")

▶ Try it Yourself

Sample run:

TEXT

Enter your password: Hello123
Password strength: 3/5
Suggestions:
  ❌ Missing special character

TEXT

Enter your password: P@ssw0rd!
Password strength: 5/5
🎉 Strong password!

Common Use Cases

Scenario	Recommended Loop	Notes
Iterating over list/tuple/set	`for`	Process elements one by one
Iterating over dictionary key-value pairs	`for`	Use `.items()`
Repeat N times	`for` + `range()`	Fixed number of iterations
Until condition is met	`while`	Unknown number of iterations
Read file until EOF	`for` / `while`	File objects are iterable
User input validation	`while` + `break`	Loop until valid input
Infinite loop (server, game loop)	`while True`	Exit via break

❓ FAQ

Q When should I use for vs while?

A Use for when you know the number of iterations (iterating over lists/ranges/strings). Use while when you know the stopping condition (waiting for input, approaching a value). They're often interchangeable, but choosing the right one makes code clearer. ⚠️ Q: Is for i in range(len(list)) Pythonic? A: Not really. Python prefers direct iteration: for item in list. Only use range(len(...)) when you specifically need indices — and even then, enumerate() is more elegant. Q: What if I have too many nested loops? A: Nesting beyond 3 levels hurts readability. Solutions: extract inner loops into functions, use itertools for combinations, or rethink the algorithm.

📖 Summary

for loop iterates over sequences (lists, strings, range, etc.)

range(start, stop, step) generates number sequences; stop is exclusive; step can be negative

while loop is condition-driven; avoid infinite loops

break exits the entire loop; continue skips the current iteration

for-else / while-else — Python-specific; else runs when the loop wasn't exited via break

Nested loops: each outer iteration triggers a full inner loop; total = product of counts

📝 Exercises

⭐ Exercise 1: Sum of Even Numbers

Use a for loop and range() to calculate the sum of all even numbers between 1 and 100.

Hint: range(2, 101, 2) gets you there in one step.

⭐⭐ Exercise 2: Fibonacci Sequence

Definition: the first two numbers are 1, and each subsequent number is the sum of the previous two. Sequence: 1, 1, 2, 3, 5, 8, 13, 21, ...

Use a while loop to output all Fibonacci numbers not exceeding 1000.

Hint: use three variables a, b for the current two numbers, compute a + b for the next, then shift.

⭐⭐⭐ Exercise 3: Simple Calculator

Write a program that repeatedly asks for two numbers and an operator (+, -, *, /), outputs the result. Enter q to quit.

Requirements:

Use while True and break

Handle division by zero

Handle invalid operators (show a hint, don't exit)

Use continue to re-prompt on invalid input

Sample run:

TEXT

Enter first number: 10 Enter operator (+ - * /): / Enter second number: 3 = 3.333... Enter first number: 8 Enter operator (+ - * /): % Invalid operator, please use + - * / Enter first number: 8 Enter operator (+ - * /): / Enter second number: 0 Error: cannot divide by zero Enter first number: q Goodbye!

← Previous: String Advanced

Next: Branching and Loops in Practice →

Loops

for Loop Basics

Example: Iterating Over Lists and Strings (⭐)

The range() Function — Generate Number Sequences

Example: Controlling Loop Count with range (⭐)

Example: Iterating with Index (⭐⭐)

while Loop

Example: Countdown (⭐)

Example: Typical while Usage — Until a Specific Value (⭐⭐)

break and continue — Brake and Skip

break — Exit the Loop Immediately

continue — Skip the Current Iteration

Example: break vs continue (⭐)

Example: break in Practice — Search and Stop (⭐⭐)

Example: continue in Practice — Process Only Valid Data (⭐⭐)

for-else and while-else — "Normal Completion Bonus"

Example: Prime Number Check (⭐⭐)

Nested Loops

Example: Multiplication Table (⭐⭐)

Example: break Only Affects the Current Level (⭐⭐)

Comprehensive Examples

Example 1: Guess the Number Game (⭐⭐)

Example 2: Password Strength Checker (⭐⭐⭐)

Common Use Cases

❓ FAQ

📖 Summary

📝 Exercises

🙏 帮我们做得更好

`for` Loop Basics

The `range()` Function — Generate Number Sequences

`while` Loop

`break` and `continue` — Brake and Skip

`break` — Exit the Loop Immediately

`continue` — Skip the Current Iteration

`for-else` and `while-else` — "Normal Completion Bonus"