Loops

🔄 R Loops: Making Things Happen Again and Again!

The Magic of Loops

Imagine you have a toy robot that can do one task. Cool, right? But what if you need it to do the same task 100 times? Would you press the button 100 times? That sounds exhausting!

Loops are like teaching your robot to repeat tasks automatically. You say “do this 10 times” and it just does it. No more pressing buttons!

🎢 The Merry-Go-Round Analogy

Think of loops like a merry-go-round at a playground:

• for loop = A merry-go-round that spins exactly 5 times, then stops
• while loop = A merry-go-round that keeps spinning while kids are still on it
• repeat loop = A merry-go-round that spins forever until someone presses the STOP button
• break = The STOP button - makes everything stop immediately
• next = The “skip this seat” button - skips one turn but keeps going

🎯 The for Loop

What is it?

A for loop says: “Do this thing for each item in my list.”

It’s like a teacher calling each student’s name one by one.

Simple Example

# Say hello to 3 friends
for (friend in c("Anna", "Ben", "Cara")) {
  print(paste("Hello,", friend))
}

Output:

[1] "Hello, Anna"
[1] "Hello, Ben"
[1] "Hello, Cara"

Counting with for

# Count from 1 to 5
for (number in 1:5) {
  print(number)
}

Output:

[1] 1
[1] 2
[1] 3
[1] 4
[1] 5

How It Works

graph TD
    A["Start for loop"] --> B["Pick first item"]
    B --> C["Do the task"]
    C --> D{More items?}
    D -->|Yes| E["Pick next item"]
    E --> C
    D -->|No| F["Done!"]

Real Life Example

Imagine you have a basket of apples. You want to check each one:

apples <- c("red", "green", "red", "yellow")

for (apple in apples) {
  print(paste("This apple is", apple))
}

🔄 The while Loop

What is it?

A while loop says: “Keep doing this WHILE something is true.”

It’s like eating cookies while there are still cookies in the jar!

Simple Example

# Count down from 5
countdown <- 5

while (countdown > 0) {
  print(countdown)
  countdown <- countdown - 1
}
print("Blast off!")

Output:

[1] 5
[1] 4
[1] 3
[1] 2
[1] 1
[1] "Blast off!"

How It Works

graph TD
    A["Start while loop"] --> B{Is condition TRUE?}
    B -->|Yes| C["Do the task"]
    C --> D["Update something"]
    D --> B
    B -->|No| E["Done!"]

Important Warning!

Always make sure your while loop can stop! If not, it runs forever!

# GOOD: This stops when count reaches 0
count <- 3
while (count > 0) {
  print(count)
  count <- count - 1  # Makes count smaller
}

# BAD: This never stops! (Don't run this!)
# while (TRUE) {
#   print("Forever...")
# }

🔁 The repeat Loop

What is it?

A repeat loop says: “Keep doing this forever… until I say STOP!”

It’s like a song on repeat - it plays forever until you press stop.

Simple Example

# Keep asking until we get the magic word
counter <- 1

repeat {
  print(paste("Attempt", counter))

  if (counter == 3) {
    print("Success! Stopping now.")
    break  # This is the STOP button!
  }

  counter <- counter + 1
}

Output:

[1] "Attempt 1"
[1] "Attempt 2"
[1] "Attempt 3"
[1] "Success! Stopping now."

How It Works

graph TD
    A["Start repeat loop"] --> B["Do the task"]
    B --> C{Should I stop?}
    C -->|No| B
    C -->|Yes - break!| D["Done!"]

Key Rule

A repeat loop MUST have a break inside it. Otherwise, it runs forever!

🛑 Loop Control: break

What is it?

break is like an emergency exit. It says: “STOP everything right now and leave the loop!”

Example: Stop at First Problem

numbers <- c(2, 4, 6, -1, 8, 10)

for (num in numbers) {
  if (num < 0) {
    print("Found a negative! Stopping!")
    break
  }
  print(paste("Checked:", num))
}

Output:

[1] "Checked: 2"
[1] "Checked: 4"
[1] "Checked: 6"
[1] "Found a negative! Stopping!"

Notice how 8 and 10 were never checked? The loop stopped!

⏭️ Loop Control: next

What is it?

next is like saying: “Skip this one and move to the next!”

It’s like skipping a song you don’t like but keeping the playlist going.

Example: Skip Odd Numbers

for (num in 1:6) {
  if (num %% 2 != 0) {  # Check if odd
    next  # Skip this number!
  }
  print(paste(num, "is even!"))
}

Output:

[1] "2 is even!"
[1] "4 is even!"
[1] "6 is even!"

The odd numbers (1, 3, 5) were skipped!

break vs next

graph TD
    A["Loop Running"] --> B{Problem found?}
    B -->|Use break| C["EXIT loop completely"]
    B -->|Use next| D["SKIP this round only"]
    D --> E["Continue to next item"]

🎨 Choosing the Right Loop

Quick Decision Tree

graph TD
    A["What do I need?"] --> B{Know how many times?}
    B -->|Yes| C["Use for loop"]
    B -->|No| D{Have a condition?}
    D -->|Yes| E["Use while loop"]
    D -->|No| F["Use repeat loop"]

🌟 Putting It All Together

Here’s a fun example using everything we learned:

# Game: Find the treasure!
boxes <- c("empty", "empty", "treasure",
           "empty", "monster")

for (i in 1:length(boxes)) {
  box <- boxes[i]

  if (box == "empty") {
    print(paste("Box", i, "is empty. Next!"))
    next  # Skip to next box
  }

  if (box == "monster") {
    print("MONSTER! Run away!")
    break  # Stop the game!
  }

  if (box == "treasure") {
    print("TREASURE FOUND! You win!")
    break  # Stop - we won!
  }
}

Output:

[1] "Box 1 is empty. Next!"
[1] "Box 2 is empty. Next!"
[1] "TREASURE FOUND! You win!"

🎉 You Did It!

Now you know:

• for loop - Do something for each item
• while loop - Keep going while condition is true
• repeat loop - Keep going until you say stop
• break - Emergency stop button
• next - Skip and continue

Loops are your superpower for making R do repetitive tasks. No more doing things 100 times by hand! 🚀