R Program to Check if a String is a Palindrome

In this article, we explore a simple yet essential task in programming: checking whether a given string is a palindrome. A palindrome is a sequence of characters that reads the same forwards and backwards, making it a common problem in text processing and string manipulation. We’ll delve into the logic and implementation of a program in the R programming language to efficiently determine if a string exhibits this intriguing property, offering practical insights for both beginners and experienced programmers.

Palindrome

Palindrome, well it’s a word, phrase, number, or sequence that reads the same forwards and backwards. For instance, “racecar” and “madam” are palindromes, but “hello” and “12345” are not.

Examples:

Input: ‘madam’

Output: YES

Input : ‘hello’

Output: NO

Concepts related to the topic:

• Case Insensitivity: In many palindrome checks, it’s essential to make the comparison case-insensitive to consider uppercase and lowercase characters as equal.
• Whitespace Removal: Removing spaces from the input string is often necessary to check palindromes accurately.
• String Reversal: Palindrome checking involves reversing a string and comparing it to the original.

Native Approach

One of the simplest ways to check for a palindrome in R is by comparing the original string with its reverse.

R

# Function to check if a string is a palindrome
isPalindrome <- function(s) {
  s <- tolower(s)
  s <- gsub(" ", "", s)
  rev_s <- paste(rev(unlist(strsplit(s, ""))), collapse = "")
  return(s == rev_s)
}

# Predefined input (change this string as needed)
user_input <- "malayalam"
# user_input<- "mala yalam"  # this also give same output as previous because both are plaindrome

# Check if the user input is a palindrome and print YES or NO
if (isPalindrome(user_input)) {
  cat("YES\n")
} else {
  cat("NO\n")
}

Output:

YES

We convert the string to lowercase and remove spaces for a case-insensitive and space-agnostic comparison. Then, we check if the string is equal to its reverse.

Iterative Method

This method involves comparing characters from the start and end of the string, working towards the center. If all characters match, the string is a palindrome. Here’s the code:

R

# Function to check if a string is a palindrome
isPalindrome <- function(s) {
  s <- tolower(s)
  s <- gsub(" ", "", s)
  n <- nchar(s)

  for (i in 1:(n %/% 2)) {
    if (substr(s, i, i) != substr(s, n - i + 1, n - i + 1)) {
      return(FALSE)
    }
  }
  return(TRUE)
}

# Predefined input (change this string as needed)
user_input <- "nitin"

# Check if the user input is a palindrome and print YES or NO
if (isPalindrome(user_input)) {
  cat("YES\n")
} else {
  cat("NO\n")
}

Output:

YES

• Function isPalindrome:
  • The function takes a string s as input.
  • It converts the input string to lowercase using tolower to ensure case insensitivity.
  • It removes spaces from the string using gsub.
  • It calculates the length of the processed string and stores it in n
• The function then begins an iterative loop with n/2 repetitions. Due to the symmetry of the comparison (from both ends towards the centre), this loop only needs to verify the first half of the string.
• If the loop completes without finding any non-matching characters, the function returns YES, indicating that the string is a palindrome.

Using an Extra Variable

This method involves storing characters in an empty variable and comparing it with the original string. We reverse the string by building it character by character in reverse order and then compare it to the original.

R

# Function to check if a string is a palindrome
isPalindrome <- function(s) {
  s <- tolower(s)
  s <- gsub(" ", "", s)
  w <- ""
  for (i in s) {
    w <- paste(i, w, sep = "")
  }
  return(s == w)
}

# Predefined input (change this string as needed)
user_input <- "madam"

# Check if the user input is a palindrome and print YES or NO
if (isPalindrome(user_input)) {
  cat("YES\n")
} else {
  cat("NO\n")
}

Output:

YES

• The isPalindrome function:
  • Converts the input string s to lowercase using tolower to make the comparison case-insensitive.
  • Removes spaces from the string using gsub.
  • Initializes an empty string w.
  • Iterates through each character in the modified string, reversing the characters and storing them in w.
• The code then compares the original modified string s with the reversed string w to determine if they are equal.
• If they are equal we get our output as YES.

Using Inbuilt Functions

R provides convenient inbuilt functions to reverse strings. We use rev() and strsplit() to reverse the string and then compare it with the original.

R

# Function to check if a string is a palindrome
isPalindrome <- function(s) {
  s <- tolower(s)
  s <- gsub(" ", "", s)
  rev_s <- paste(rev(strsplit(s, NULL)[[1]]), collapse = "")
  return(s == rev_s)
}

# Predefined input (change this string as needed)
user_input <- "13431"

# Check if the user input is a palindrome and print YES or NO
if (isPalindrome(user_input)) {
  cat("YES\n")
} else {
  cat("NO\n")
}

Output:

YES

• The isPalindrome function starts by converting the input string s to lowercase using tolower(s) to ensure a case-insensitive comparison.
• It then uses gsub(” “, “”, s) to remove spaces from the string, making it space-agnostic.
• Next, the code reverses the processed string s using rev and paste, storing the result in rev_s.
• Finally, it checks if the original string s is equal to the reversed string rev_s and returns YES if they match, indicating a palindrome, or NO otherwise.