For loop in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a for loop used for in R?

A for loop is used to repeat a block of code multiple times, once for each item in a sequence or vector.

beginner

Write the basic syntax of a for loop in R.

The basic syntax is:<br>

for (variable in sequence) {
  # code to repeat
}

beginner

What happens if the sequence in a for loop is empty?

The code inside the loop does not run at all because there are no items to iterate over.

beginner

How can you use a for loop to print numbers 1 to 5 in R?

You can write:<br>

for (i in 1:5) {
  print(i)
}

intermediate

Can you modify the loop variable inside a for loop to affect the next iteration?

No, modifying the loop variable inside the loop does not change the sequence or affect the next iteration. The loop variable takes the next value from the original sequence each time.

What does the for loop variable represent in R?

A The last item in the sequence

B The total number of items in the sequence

C The sum of all items in the sequence

D Each item in the sequence one by one

The loop variable takes each item from the sequence one at a time during each iteration.

Which of the following is the correct way to loop over numbers 1 to 3 in R?

A for (i in 1:3) { print(i) }

B for i in 1 to 3 { print(i) }

C for (i = 1; i <= 3; i++) { print(i) }

D loop (i in 1:3) { print(i) }

Option A uses correct R syntax for a for loop. Options B, C, and D are not valid R syntax.

What will this code print?<br>

for (x in c()) { print(x) }

A Nothing

B An error

C NULL

D 0

The sequence is empty, so the loop body does not run and nothing is printed.

Can you use a for loop to iterate over characters in a string in R?

A Yes, directly on the string without conversion

B No, <code>for</code> loops only work with numbers

C Yes, by converting the string to a vector of characters

D No, you must use <code>while</code> loops for strings

You can split a string into characters using functions like strsplit() and then loop over the resulting vector.

What is the output of this code?<br>

for (i in 1:3) { i <- i + 10; print(i) }

A 1, 2, 3

B 11, 12, 13

C 10, 11, 12

D An error

Inside the loop, i is increased by 10 and printed. The loop variable resets each iteration, so the original sequence is unaffected.

Explain how a for loop works in R and give a simple example.

Think about how the loop variable takes each value from the sequence.

Describe what happens if the sequence in a for loop is empty.

Consider what it means to have nothing to loop over.