Recursive functions in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple factorial recursion

What is the output of this R code when calling factorial(4)?

R Programming

factorial <- function(n) {
  if (n == 0) {
    return(1)
  } else {
    return(n * factorial(n - 1))
  }
}
factorial(4)

A 120

B Error: object 'factorial' not found

C 10

D 24

💡 Hint

Remember factorial of 4 is 4 * 3 * 2 * 1.

The function calls itself multiplying n by factorial of n-1 until n is 0, where it returns 1. So factorial(4) = 4*3*2*1 = 24.

❓ Predict Output

intermediate

Output of Fibonacci recursive function

What is the output of fib(5) given this R function?

R Programming

fib <- function(n) {
  if (n <= 1) {
    return(n)
  } else {
    return(fib(n - 1) + fib(n - 2))
  }
}
fib(5)

A 5

B 8

C 3

D Error: recursive call limit reached

💡 Hint

Fibonacci sequence starts 0,1,1,2,3,5...

fib(5) = fib(4) + fib(3) = 3 + 2 = 5. The function returns the 5th Fibonacci number starting from 0.

🔧 Debug

advanced

Identify the error in this recursive sum function

What error will this R code produce when calling sum_recursive(3)?

R Programming

sum_recursive <- function(n) {
  if (n == 0) {
    return(0)
  } else {
    return(n + sum_recursive)
  }
}
sum_recursive(3)

A 6

B Error: argument is missing, with no default

C Error: object 'sum_recursive' not found

D Error: recursive call limit reached

💡 Hint

Check how the recursive call is made inside the function.

The recursive call is missing the argument n-1, so R tries to call sum_recursive() without arguments causing an error about missing argument.

❓ Predict Output

advanced

Output of recursive string reversal

What is the output of reverse_str('abc') with this R function?

R Programming

reverse_str <- function(s) {
  if (nchar(s) == 0) {
    return("")
  } else {
    return(paste0(substr(s, nchar(s), nchar(s)), reverse_str(substr(s, 1, nchar(s) - 1))))
  }
}
reverse_str('abc')

A "cba"

B "abc"

C Error: non-numeric argument to binary operator

D "bac"

💡 Hint

The function takes last character and appends reversed rest.

The function takes last character 'c' and appends reverse of 'ab' which is 'ba', so output is 'cba'.

🧠 Conceptual

expert

Maximum recursion depth error cause

Which option best explains why this recursive R function causes a 'maximum recursion depth exceeded' error?

R Programming

infinite_recursion <- function(n) {
  if (n == 0) {
    return(0)
  } else {
    return(infinite_recursion(n + 1))
  }
}
infinite_recursion(1)

A The base case is missing so recursion never stops.

B The function has a syntax error causing infinite calls.

C The function never reaches the base case because n increases instead of decreasing.

D The function calls itself with the same argument causing infinite loop.

💡 Hint

Check how the argument changes in recursive calls.

The function increases n each call, so it never reaches n == 0 base case, causing infinite recursion until limit is reached.