Question

For this assignment, you will create a program that tests a string to see if it is a palindrome. A palindrome is a string such as “madam”, “radar”, “dad”, and “I”, that reads the same forwards and backwards. The empty string is regarded as a palindrome. Write a recursive function: bool isPalindrome(string str, int lower, int upper) that returns true if and only if the part of the string str in positions lower through upper (inclusive at both ends) is a palindrome. Test your function by writing a main function that repeatedly asks the user to enter strings terminated by the ENTER key. These strings are then tested for palindromicity. The program terminates when the user presses the ENTER key without typing any characters before it. I'm using XCODE.

Answer 1

Below is the complete program as per the requirements. It is well explained inside the code using comments. If still, you find any difficulty in understanding this, let me know.

#include <iostream>
#include <string>

using namespace std;

// returns true if and only if the part of string str in positions
// lower through upper inclusive both is a Palindrome, otherwise
// it returns false
bool isPalindrome(string str, int lower, int upper)
{
// if lower and upper are equal, it means there is ony 1 element
// return true as one character is a palindrome
if (lower == upper)
return true;

// now, check the first and last character and check if they are equal
// if they are not, it can't be a palindrome, return false
if (str[lower] != str[upper])
return false;

// if there are more charcters apart from first and last
// call the recursive function again, and pass the middle indexes
if (upper - lower > 1)
{
// call function again with increasing lower and decreasing upper by 1
return isPalindrome(str, lower + 1, upper - 1);
}
}

// main function
int main()
{
string str; //to store user entered string

// ask user to enter the string
cout << "Enter the string: ";

// loop to ask user and input repeatedly until user enters an empty string by pressing enter key only
while (getline(cin, str) && !str.empty())
{
// check if it is palindrome by calling the function
// for the first call of the function, lower will be starting index, 0
// and upper would be length of str - 1 which is last index
if (isPalindrome(str, 0, str.length() - 1))
{
cout << "It is a Palindrome" << endl;
}
else
{
cout << "It is not a Palindrome" << endl;
}

// ask user again to enter the string
cout << "\nEnter the string: ";
}

return 0;
}