For loop in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple for loop with sequence

What is the output of this R code?

result <- c()
for (i in 1:4) {
  result <- c(result, i * 2)
}
print(result)

R Programming

result <- c()
for (i in 1:4) {
  result <- c(result, i * 2)
}
print(result)

A [1] 1 2 3 4

B [1] 2 3 4 5

C [1] 4 8 12 16

D [1] 2 4 6 8

💡 Hint

Remember the loop multiplies each number by 2 before adding to the result.

The loop runs from 1 to 4, multiplying each number by 2 and adding it to the result vector. So the output is 2, 4, 6, 8.

❓ Predict Output

intermediate

Output of nested for loops with sum

What is the output of this R code?

total <- 0
for (i in 1:2) {
  for (j in 1:3) {
    total <- total + i + j
  }
}
print(total)

R Programming

total <- 0
for (i in 1:2) {
  for (j in 1:3) {
    total <- total + i + j
  }
}
print(total)

A [1] 18

B [1] 21

C [1] 12

D [1] 15

💡 Hint

Add all pairs of i and j from the loops.

The sums are: (1+1)+(1+2)+(1+3)+(2+1)+(2+2)+(2+3) = 2+3+4+3+4+5 = 21. But the code adds i+j each iteration to total, so total is 21.

🔧 Debug

advanced

Identify the error in this for loop

What error does this R code produce?

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

R Programming

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

A SyntaxError: unexpected symbol 'i'

B Error: object 'i' not found

C Error: unexpected '{' in "for i in 1:5 {"

D No error, prints numbers 1 to 5

💡 Hint

Check the syntax of the for loop header in R.

In R, the for loop syntax requires parentheses around the loop variable and sequence: for (i in 1:5) {...}. Missing parentheses cause a syntax error at '{'.

❓ Predict Output

advanced

Output of for loop with conditional break

What is the output of this R code?

result <- c()
for (i in 1:10) {
  if (i > 4) {
    break
  }
  result <- c(result, i)
}
print(result)

R Programming

result <- c()
for (i in 1:10) {
  if (i > 4) {
    break
  }
  result <- c(result, i)
}
print(result)

A [1] 1 2 3 4

B [1] 1 2 3 4 5

C [1] 1 2 3

D [1] 5 6 7 8 9 10

💡 Hint

The loop stops when i is greater than 4.

The loop adds numbers 1 to 4 to result. When i becomes 5, the break stops the loop, so 5 is not added.

🧠 Conceptual

expert

Number of iterations in a for loop with sequence and step

How many times will this R for loop run?

count <- 0
for (i in seq(2, 10, by=3)) {
  count <- count + 1
}
print(count)

R Programming

count <- 0
for (i in seq(2, 10, by=3)) {
  count <- count + 1
}
print(count)

A 3

B 2

C 5

D 4

💡 Hint

Check the sequence generated by seq(2, 10, by=3).

seq(2, 10, by=3) generates 2, 5, 8. So the loop runs 3 times.

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