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:

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:

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?

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?

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.

Practice

1. What does a for loop do in R programming?

easy

A. Stops the program immediately

B. Creates a new variable

C. Defines a new function

D. Repeats a block of code for each item in a sequence

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop is used to repeat actions multiple times, once for each item in a sequence.

2. Step 2: Match the description to the options

   Repeats a block of code for each item in a sequence correctly describes this behavior, while others describe unrelated actions.

3. Final Answer:

   Repeats a block of code for each item in a sequence -> Option D

4. Quick Check:

   For loop = repeat for each item [OK]

Hint: For loops repeat code for each item in a list or sequence [OK]

Common Mistakes:

• Confusing for loop with function definition
• Thinking for loop stops the program
• Believing for loop creates variables only

2. Which of the following is the correct syntax for a for loop in R?

easy

A. for (i; i < 5; i++) { print(i) }

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

C. for i = 1 to 5 { print(i) }

D. loop (i in 1 to 5) { print(i) }

Solution

1. Step 1: Recall R for loop syntax

   In R, the for loop uses the format: for (variable in sequence) { code }

2. Step 2: Compare options to correct syntax

   for (i in 1:5) { print(i) } matches the correct syntax. Options A, B, and D use syntax from other languages or incorrect forms.

3. Final Answer:

   for (i in 1:5) { print(i) } -> Option B

4. Quick Check:

   R for loop = for (var in sequence) [OK]

Hint: Use 'for (var in sequence)' syntax in R [OK]

Common Mistakes:

• Using 'for i = 1 to 5' like other languages
• Using C-style for loop syntax
• Writing 'loop' instead of 'for'

3. What will be the output of this R code?

for (i in 1:3) {
  print(i * 2)
}

medium

A. [1] 2 [1] 4 [1] 6

B. [1] 1 [1] 2 [1] 3

C. [1] 3 [1] 6 [1] 9

D. Error: object 'i' not found

Solution

1. Step 1: Understand the loop iterations

   The loop runs with i = 1, 2, 3.

2. Step 2: Calculate print output for each i

   For i=1: print(2), i=2: print(4), i=3: print(6).

3. Final Answer:

   [1] 2 [1] 4 [1] 6 -> Option A

4. Quick Check:

   Multiply i by 2 each loop [OK]

Hint: Multiply loop variable by 2 inside print [OK]

Common Mistakes:

• Printing i instead of i*2
• Confusing output format
• Expecting a single combined print

4. Identify the error in this R for loop:

for i in 1:4 {
  print(i)
}

medium

A. Loop variable should be a string

B. Using curly braces instead of parentheses

C. Missing parentheses around the loop variable and sequence

D. Incorrect print function usage

Solution

1. Step 1: Check for correct for loop syntax in R

   R requires parentheses around the loop variable and sequence: for (i in 1:4) { ... }

2. Step 2: Identify the error in given code

   The code misses parentheses around 'i in 1:4', causing a syntax error.

3. Final Answer:

   Missing parentheses around the loop variable and sequence -> Option C

4. Quick Check:

   For loop needs parentheses ( ) [OK]

Hint: Always put parentheses after 'for' in R [OK]

Common Mistakes:

• Omitting parentheses around 'i in 1:4'
• Confusing braces and parentheses
• Thinking print syntax is wrong

5. You want to create a vector of squares of numbers 1 to 5 using a for loop in R. Which code correctly does this?

hard

A. squares <- c()
for (i in 1:5) {
squares[i] <- i^2
}
squares

B. for (i in 1:5) {
squares <- i^2
}
squares

C. squares <- 1:5
for (i in squares) {
squares[i] = i * 2
}
squares

D. squares <- c()
for i in 1:5 {
squares[i] <- i^2
}
squares

Solution

1. Step 1: Initialize an empty vector to store results

   squares <- c() for (i in 1:5) { squares[i] <- i^2 } squares initializes 'squares' as an empty vector to store squares.

2. Step 2: Use a for loop to assign squares to each position

   Inside the loop, squares[i] <- i^2 correctly stores the square of i at position i.

3. Step 3: Check other options for errors

   for (i in 1:5) { squares <- i^2 } squares overwrites 'squares' each time, losing previous values. squares <- 1:5 for (i in squares) { squares[i] = i * 2 } squares multiplies by 2, not square. squares <- c() for i in 1:5 { squares[i] <- i^2 } squares misses parentheses in for loop syntax.

4. Final Answer:

   squares <- c() for (i in 1:5) { squares[i] <- i^2 } squares -> Option A

5. Quick Check:

   Initialize vector, assign squares in loop [OK]

Hint: Initialize vector before loop, assign squares by index [OK]

Common Mistakes:

• Not initializing vector before loop
• Overwriting vector inside loop
• Wrong loop syntax without parentheses