Hello,

For my current project, I am trying to figure out how to check to see if a string typed into the text area is a palindrome. For this project, we are given most of the code, but I am supposed to fill in the code for the is_palindrome function that is given to me. I am not allowed to use the strrev function, recursion, or regular expressions and I am not allowed to change any other other code. I spent a few good hours trying to figure this out and after some research and trying to figure out it on my own, I got it to the point were it tells me if it is a palindrome or not, but I think the way I did it is recursion. I was never taught how to do recursion code, but after looking online what it is, I think my function has recursion. My questions for you guys are, is the code in the is_palindrome function recursion code, and if so, how could I put a loop (for or while) into that function that includes the strlen() and substr() function without making it recursion code?

Thanks so much.

<?php
//declare variables to store the string to be checked and the message to be displayed.
$string = $message = "";

//retrieve the string to be checked and then strip any invalid characters
if (filter_has_var(INPUT_GET, "string")) {
    $string = trim(filter_input(INPUT_GET, 'string', FILTER_DEFAULT));

//remove all non-alphanumeric characters and then convert all characcters to lower cases
$string_stripped = strtolower(preg_replace("/[^A-Za-z0-9]/", "", $string));
}

if ($string != "") {
    $result = is_palindrome($string_stripped);

if ($result)
    $message = "\"" . $string . "\" is a palindrome.";
else
    $message = "\"" . $string . "\" is NOT a palindrome.";
}



//this is the  function that I am needed to fill out

function is_palindrome($str) {

    if (strlen($str) == 0) {
        return true;
    }
    if ($str[0] == $str[strlen($str) - 1]) {
        return true and is_palindrome(substr($str, 1, strlen($str) - 2));
    } else {
        return false;
    }
}
?>
<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>Checking Palindromes</title>
        <link rel="stylesheet" href="styles.css" type="text/css" />
    </head>
    <body>
        <h2>Checking Palindromes</h2>
        <p>Enter a string and then click the <strong>Check</strong> button to see the result.</p>
        <form action="palindrome.php" method="get" enctype="text/plain">
            <table>
                <tr>
                    <th>Enter a string: </th>
                    <td><input type="text" name="string" size="20" value="<?php echo $string ?>" required /></td>
                </tr>
                <tr>
                    <th>Result: </th>
                    <td><?php echo $message ?></td>
                </tr>
            </table>
            <br>
            <input type="submit" value="Check" />
        </form>
    </body>
</html>

    My first thought is:

    function is_palindrome($text)
{
  $justWordChars= strtolower(preg_replace('/\W/', '', $text));
  return $justWordChars == strrev($justWordChars;
}

    (Untested)

      That might work, but... I am not allowed to use the strrev function as well, so your code would not work for my project. :/

        Since PHP strings can use array-style notation to access any one character, you could maybe do something like:

        function is_palindrome($text)
{
  // any validation, stripping of white-space if desired, etc., then...
  $len = strlen($text);
  for($a=0, $b=$len-1; $a++, $b--; $a<$len)
  {
    if($text[$a] != $text[$b]) {
      return false;
    }
  }
  return true;
}

        (also untested)

          I just tested it and it works. I was not aware that you could have 2 different expressions and counter updates in the for loop. I learned something there.

          Thank you so much!

            Yeah, the multiple expressions like that in for() is very useful at times -- and most of us weren't aware it existed at first. 🙂

              NogDog;11061271 wrote:

              Yeah, the multiple expressions like that in for() is very useful at times -- and most of us weren't aware it existed at first. 🙂

              Honestly, it doesn't look odd or out of place when I see it. Yet everytime I see it I think "that's right, you can do that" lol

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