🔁 Loops

The Factory Worker Analogy

Imagine you're a factory worker boxing products:

• Without automation: You manually pack one box, seal it, label it. Then do the exact same thing for the next box. And the next. And the next. 1000 times.

• With automation: You program a machine: "Repeat these steps until all products are boxed."

Loops are your automation.

Instead of writing the same code 100 times, you write it ONCE and tell the computer: "Keep doing this until I say stop."

Why Loops Matter

Without loops:

print("Hello Student 1")
print("Hello Student 2")
print("Hello Student 3")
... 997 more lines ...
print("Hello Student 1000")

That's 1000 lines of repetitive code!

With loops:

for each student from 1 to 1000:
    print("Hello Student " + number)

Two lines. Same result. MUCH easier to maintain.

Loops power everything:

The Two Main Types

1. FOR Loop: "Do This a Specific Number of Times"

Use when you know HOW MANY times to repeat.

FOR counter from 0 to 5:
    do something

"Run 5 times, counting from 0 to 4"

Real-world examples:

• Print 10 receipts
• Show 20 products per page
• Retry an operation 3 times

2. WHILE Loop: "Keep Going Until a Condition Changes"

Use when you DON'T know how many times, but know WHEN to stop.

WHILE user hasn't typed "quit":
    get input
    process input

"Keep going until user says stop"

Real-world examples:

• Keep asking for password until correct
• Read file until end is reached
• Keep game running until player loses

How FOR Loops Work

A for loop has three parts:

for (START; CONDITION; UPDATE)
     │        │         │
     │        │         └── "After each round, do this"
     │        └── "Keep going while this is true"
     └── "Before starting, set this up"

Example (step-by-step):

for (let i = 0; i < 5; i++)

let i = 0     → Start at 0
i < 5         → Keep going while i is less than 5
i++           → After each round, add 1 to i

Rounds: 0, 1, 2, 3, 4 (5 total iterations)

Think of it like stairs:

Step 0: Do the task
Step 1: Do the task
Step 2: Do the task
Step 3: Do the task
Step 4: Do the task
Step 5: STOP (condition i < 5 is now false)

How WHILE Loops Work

A while loop checks the condition FIRST:

WHILE condition is true:
    do something
    (hopefully something changes the condition!)

Important: Make sure something inside the loop eventually makes the condition false, or it runs FOREVER!

attempts = 0
WHILE attempts < 3:
    try to connect
    if failed:
        attempts = attempts + 1  ← This changes the condition!
    else:
        break  ← Exit early if successful

For vs While: Quick Decision Guide

Examples:

Looping Through Collections

Most commonly, you loop through lists of things:

Loop through array with for...of:

const fruits = ["apple", "banana", "orange"];

for (const fruit of fruits) {
  console.log(fruit);
}
// "apple", "banana", "orange"

Loop through object keys with for...in:

const person = { name: "Alice", age: 30 };

for (const key in person) {
  console.log(key + ": " + person[key]);
}
// "name: Alice", "age: 30"

Break and Continue: Loop Control

Break: "Stop the entire loop NOW"

Looking for Alice in a list of 1000 people...

Loop through each person:
    If this is Alice:
        BREAK!  ← Found her, no need to check remaining 999!

Continue: "Skip this one, move to next"

Processing orders, skip cancelled ones...

Loop through each order:
    If order is cancelled:
        CONTINUE  ← Skip this, check next order
    Process the order

Common Mistakes

Mistake 1: Infinite Loops

The loop can run forever if the condition doesn't become false:

counter = 0
WHILE counter < 5:
    print(counter)
    // Forgot to increase counter!
    // counter stays at 0 forever!

Fix: Make sure something changes the condition:

counter = 0
WHILE counter < 5:
    print(counter)
    counter = counter + 1  ← Now it will eventually reach 5!

Mistake 2: Off-by-One Errors

The most common loop bug! You go one step too far or one step short.

Array has 5 items: [0, 1, 2, 3, 4]
Valid indices: 0, 1, 2, 3, 4

❌ for i from 0 to 5:  ← Index 5 doesn't exist!
✓ for i from 0 to 4:  ← Correct range

Mistake 3: Modifying While Looping

Don't change a list while you're looping through it:

❌ Loop through items:
    If item is bad, remove it  ← Indices shift! You'll skip items!

✓ Create new list without bad items

FAQ

Q: Which loop is faster?

In modern code, the difference is negligible. Choose based on readability, not micro-optimizations.

Q: Can I nest loops?

Yes, but be careful! A loop inside a loop multiplies iterations:

• Outer loop: 100 times
• Inner loop: 100 times
• Total: 10,000 iterations!

Q: When should I use forEach vs for...of?

Use for...of when you need to break early or use async/await. Use forEach for simple iterations.

Q: How do I loop backwards?

Start at the end and decrease:

for i from length-1 down to 0:
    print item[i]

Q: Should I use map/filter instead of loops?

For transforming data, yes! They're clearer for specific purposes:

• map: Transform each item
• filter: Keep matching items
• reduce: Combine all items into one value

Q: What's an iterator?

An object that knows how to access items one at a time. for...of works with any iterator (arrays, strings, maps, etc).

Summary

Loops automate repetitive tasks. Instead of writing the same code 100 times, write it once and loop.

Key Takeaways:

• FOR: Use when you know how many times
• WHILE: Use when you know when to stop, but not how many times
• for...of: Loop through array values
• for...in: Loop through object keys
• break: Exit loop immediately
• continue: Skip to next iteration
• Watch for infinite loops (make sure something changes the condition!)
• Watch for off-by-one errors (check your boundaries)

Master loops and you can automate any repetitive task!