PHP quadratic equation calculator strange output

Question

Having written a quadratic equation calculator in PHP, I thought I would have most of my problems with the maths. Not so though, as I am getting very strange output. The program is supposed to $_GET the values of the x², x, and other number from a form, calculate the values of x, and then display them. when I first load up the page, the program outputs -10 (even though I have not entered anything into the form) and then does nothing if I enter values. For instance, if I enter 1, 11 and 18 into the text fields, which should output x = -9 and -2, the program outputs -22. What am I doing wrong?

Here is my code (the <body> section of my HTML document):

<body>
<h1>Quadratic equation calculator</h1>
<p>Type the values of your equation into the calculator to get the answer.</p>
<?php
    $xsqrd;
    $x;
    $num;
    $ans1 = null;
    $ans2 = null;
    $errdivzero = "The calculation could not be completed as it attempts to divide by zero.";
    $errsqrtmin1 = "The calculation could not be completed as it attempts to find the square root of a negative number.";
    $errnoent = "Please enter some values into the form.";
?>
<form name = "values" action = "calc.php" method = "get">
<input type = "text" name = "x2"><p>x<sup>2</sup></p>
&nbsp;
<input type = "text" name = "x"><p>x</p>
&nbsp;
<input type = "text" name = "num">
&nbsp;
<input type = "submit">
</form>
<?php
    if ((!empty($_GET['x2'])) && (!empty($_GET['x'])) && (!empty($_GET['num']))) {
        $xsqrd = $_GET['x2'];
        $x = $_GET['x'];
        $num = $_GET['num'];

            $ans1 = (-$x) + (sqrt(pow($x, 2) - (4 * $xsqrd * $num))) / (2 * $xsqrd);
            $ans2 = (-$x) - (sqrt(pow($x, 2) - (4 * $xsqrd * $num))) / (2 * $xsqrd);

    }
?>
<p>
<?php 
    if(($ans1==null) or ($ans2==null))
    {
    print $errnoent;
    }
    else
    {
    print "x = " + $ans1 + "," + $ans2;
    }
?>
    </p>
</body>

score 1 · Accepted Answer · answered Mar 09 '13 at 10:59

1

You have two errors.

First one is mathematical, it should be

$ans1 = ((-$x) + (sqrt(pow($x, 2) - (4 * $xsqrd * $num)))) / (2 * $xsqrd);
$ans2 = ((-$x) - (sqrt(pow($x, 2) - (4 * $xsqrd * $num)))) / (2 * $xsqrd);

Correct formula is (-b+-sqrt(b^2-4ac))/(2a) instead of -b+sqrt(b^2-4ac)/(2a) - in the latter case division would have priority over addition without parenthesis.

And second one is the way you output your result, you should use concatenation operator .

print "x = " . $ans1 . "," . $ans2;

(though I would use echo instead of print)

answered Mar 09 '13 at 10:59

Marko D

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what was wrong with the maths bit? Also, thank you, because it works! – imulsion Mar 09 '13 at 11:00
but why use `echo` rather than `print`? – imulsion Mar 09 '13 at 11:03
i edited my comment and added the math bit. about `echo` vs `print`, i prefer `echo` for simple output (and most people do) – Marko D Mar 09 '13 at 11:05

PHP quadratic equation calculator strange output

1 Answers1