🔁 R Loops – For, While, and Repeat Loops in R Programming

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🧲 Introduction – Repeating Tasks with Loops in R

When you’re dealing with repetitive tasks, like processing elements of a vector or calculating values in a sequence, loops in R are your go-to solution. R supports three main types of loops:

• for – iterate over sequences
• while – continue while a condition is true
• repeat – infinite loop with exit conditions

Loops automate tasks and are fundamental in data processing, simulations, and algorithm building.

🎯 In this guide, you’ll learn:

• How to use for, while, and repeat loops
• Loop control using break and next
• Practical examples for iteration and logic

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🔂 For Loop – Iterate Over a Sequence

The for loop is ideal when you know how many times to run the loop in advance.

🔧 Syntax:

for (variable in sequence) {
  # code block
}

✅ Example:

for (i in 1:5) {
  print(paste("Iteration:", i))
}

🧾 Output:

[1] "Iteration: 1"
[1] "Iteration: 2"
[1] "Iteration: 3"
[1] "Iteration: 4"
[1] "Iteration: 5"

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🔁 While Loop – Repeat While a Condition is TRUE

Use while when the number of iterations is not known in advance.

🔧 Syntax:

while (condition) {
  # code block
}

✅ Example:

x <- 1
while (x <= 3) {
  print(x)
  x <- x + 1
}

🧾 Output:

[1] 1
[1] 2
[1] 3

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♾️ Repeat Loop – Infinite Loop with Manual Exit

The repeat loop runs endlessly unless you use break.

🔧 Syntax:

repeat {
  # code block
  if (exit_condition) break
}

✅ Example:

x <- 1
repeat {
  print(x)
  x <- x + 1
  if (x > 3) break
}

🧾 Output:

[1] 1
[1] 2
[1] 3

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🛑 Controlling Loop Execution

⛔ break – Exit the loop early

for (i in 1:5) {
  if (i == 3) break
  print(i)
}

🧾 Output:

[1] 1
[1] 2

⏭️ next – Skip to next iteration

for (i in 1:5) {
  if (i == 3) next
  print(i)
}

🧾 Output:

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

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🎯 Looping Over Vectors and Lists

students <- c("Alice", "Bob", "Charlie")

for (name in students) {
  print(paste("Hello,", name))
}

🧾 Output:

[1] "Hello, Alice"
[1] "Hello, Bob"
[1] "Hello, Charlie"

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📌 Summary – Recap & Next Steps

Loops allow you to repeat tasks, iterate over data, and apply logic conditionally. They’re essential for automating processes in data cleaning, calculations, and simulations.

🔍 Key Takeaways:

• for loops are best for sequences or vectors
• while loops are used when the exit condition is dynamic
• repeat loops run until a break condition is met
• Use break to stop a loop, next to skip an iteration
• Loops can iterate over numbers, vectors, and lists

⚙️ Real-World Relevance:
Loops are crucial in batch data operations, simulation models, and real-time monitoring scripts in R—automating the logic for scalable data analysis.

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❓ FAQs – Loops in R

❓ When should I use a for loop vs while loop in R?
✅ Use for when the number of iterations is known; use while when it’s based on dynamic conditions.

❓ What is the default loop exit strategy in repeat?
✅ There is no default exit—you must include a break condition to stop the loop manually.

❓ Can I loop through both keys and values in a list?
✅ Yes, use names() or seq_along():

mylist <- list(a=10, b=20)
for (i in names(mylist)) {
  print(paste(i, mylist[[i]]))
}

❓ What happens if I forget the break in a repeat loop?
✅ It creates an infinite loop and must be manually stopped (Esc or Stop in RStudio).

❓ Is looping slow in R?
✅ Native loops can be slower than vectorized functions. Prefer functions like apply(), lapply() for large datasets when possible.

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