Check if a string is palindrome - c++

Question

I need to create a program that allows a user to input a string and my program will check to see if that string they entered is a palindrome (word that can be read the same backwards as it can forwards).

Answer 1

Note that reversing the whole string (either with the rbegin()/rend() range constructor or with std::reverse) and comparing it with the input would perform unnecessary work.
It's sufficient to compare the first half of the string with the latter half, in reverse:
#include <string>
#include <algorithm>
#include <iostream>
int main()
{
std::string s;
std::cin >> s;
if( equal(s.begin(), s.begin() + s.size()/2, s.rbegin()) )
std::cout << "is a palindrome.\n";
else
std::cout << "is NOT a palindrome.\n";
}
demo: http://ideone.com/mq8qK

Answer 2

Just compare the string with itself reversed:
string input;
cout << "Please enter a string: ";
cin >> input;
if (input == string(input.rbegin(), input.rend())) {
cout << input << " is a palindrome";
}
This constructor of string takes a beginning and ending iterator and creates the string from the characters between those two iterators. Since rbegin() is the end of the string and incrementing it goes backwards through the string, the string we create will have the characters of input added to it in reverse, reversing the string.
Then you just compare it to input and if they are equal, it is a palindrome.
This does not take into account capitalisation or spaces, so you'll have to improve on it yourself.

Answer 3

bool IsPalindrome(const char* psz)
{
int i = 0;
int j;
if ((psz == NULL) || (psz[0] == '\0'))
{
return false;
}
j = strlen(psz) - 1;
while (i < j)
{
if (psz[i] != psz[j])
{
return false;
}
i++;
j--;
}
return true;
}
// STL string version:
bool IsPalindrome(const string& str)
{
if (str.empty())
return false;
int i = 0; // first characters
int j = str.length() - 1; // last character
while (i < j)
{
if (str[i] != str[j])
{
return false;
}
i++;
j--;
}
return true;
}

Answer 4

// The below C++ function checks for a palindrome and
// returns true if it is a palindrome and returns false otherwise
bool checkPalindrome ( string s )
{
// This calculates the length of the string
int n = s.length();
// the for loop iterates until the first half of the string
// and checks first element with the last element,
// second element with second last element and so on.
// if those two characters are not same, hence we return false because
// this string is not a palindrome
for ( int i = 0; i <= n/2; i++ )
{
if ( s[i] != s[n-1-i] )
return false;
}
// if the above for loop executes completely ,
// this implies that the string is palindrome,
// hence we return true and exit
return true;
}

Answer 5

#include <iostream>
#include <string>
bool isPalindrome(const std::string& str){
if(str.empty()) return true;
std::string::const_iterator itFirst = str.begin();
std::string::const_iterator itLast = str.end() - 1;
while(itFirst < itLast) {
if (*itFirst != *itLast)
return false;
++itFirst;
--itLast;
}
return true;
}
int main(){
while(1){
std::string input;
std::cout << "Eneter a string ...\n";
std::cin >> input;
if(isPalindrome(input)){
std::cout << input << " is palindrome.\n";
} else {
std::cout << input << " is not palindrome.\n";
}
}
return 0;
}

Answer 6

Check the string starting at each end and meet in the middle. Return false if there is a discrepancy.
#include <iostream>
bool palidromeCheck(std::string str) {
for (int i = 0, j = str.length()-1; i <= j; i++, j--)
if (str[i] != str[j])
return false;
return true;
}
int main(){
std::cout << palidromeCheck("mike");
std::cout << palidromeCheck("racecar");
}

Answer 7

Reverse the string and check if original string and reverse are same or not

Answer 8

I'm no c++ guy, but you should be able to get the gist from this.
public static string Reverse(string s) {
if (s == null || s.Length < 2) {
return s;
}
int length = s.Length;
int loop = (length >> 1) + 1;
int j;
char[] chars = new char[length];
for (int i = 0; i < loop; i++) {
j = length - i - 1;
chars[i] = s[j];
chars[j] = s[i];
}
return new string(chars);
}