Question

#include <iostream>
#include <ctype.h>

using namespace std;

void isPalindrome();

int main()
{
    char response;
    isPalindrome();
    cout << "Input another string(y/n)?" << endl;
    cin >> response;
    response = toupper(response);
    if (response == 'Y')
        isPalindrome();

    return 0;
}

void isPalindrome()
{
    char str[80], str2[80];
    int strlength;
    int j = 0;
    int front, back;
    bool flag = 1;

    cout << "Input a string:" << endl;
    cin.getline(str, 80);
    strlength = strlen(str);

    for (int i = 0; i < strlength; i++)
    {
        if (islower(str[i]))
            str[i] = toupper(str[i]);
    }

    for (int i = 0; i < strlength; i++)
    {
        if (isalpha(str[i]))
        {
            str2[j] = str[i];
            j++;

        }
    }

    str2[j] = '\0';
    front = 0;
    back = strlength - 1;

    for (int i = 0; i < j / 2; i++)
    {
        if (str2[front] != str2[back])
        {
            flag = 0;

            break;
        }
    }

    if (!(flag))
        cout << "It is not a palindrome" << endl;
    else
        cout << "It's a palindrome" << endl;

    cout << "str: " << str << " str2: " << str2 << " strlength: " << strlength << " j: " << j << endl;
    cout << "front: " << front << " back: " << back << " flag: " << flag << endl;
}

I was just wondering if anybody could help explain to me why my code isn’t working.

I can run it once just fine and I get the right answer, but when the prompt asks if I want to input another string and I type 'y', the prompt just skips over the input and terminates on it’s own.

I tried cin.ginore('\n', 80), but that just gave me a bunch of blank lines. I added the bit of code at the end to check the values and they all go to 0 and drop the strings.

Maybe a link to a proper explanation of how the system handles memory?

edit:
I keep getting the same problem when running the input sequence a second time. The output looks like this:

    Input a string:
    Radar
    It's a palindrome
    Input another string(y/n)?
    y


    _ <- this being my cursor after pressing enter 3 times

I’ll just re-build the program from scratch and try to do it without a function. I’d still appreciate a link to a page that explains how to process user input using modern c++.

Answer 1

The problem is with:

cin >> response;

This reads the user input y/n into the variable response but a newline is left in the input buffer which is picked by the getline function the isPalindrome function.

To fix this you need to remove the newline from the input buffer after you read the user response. You do it by using:

cin >> response;
std::cin.ignore(INT_MAX);

With the above fix you can retry the palindrome check just once. To make multiple retries possible you’ll need a loop. I would recommend a do-while loop in your main as:

char response;
do {
        isPalindrome();
        cout << "Input another string(y/n)?" << endl;
        cin >> response;
        std::cin.ignore(INT_MAX);
        response = toupper(response);
} while(response == 'Y');