Question:

How to solve the issue of producing the wrong output value in PHP?

Problem

I wrote the PHP code that calculates the school grade (including weight and targeting rate), however, it produces the wrong output value.

Here's the PHP code so far

<?php

$numassignments = isset($_GET['numassignments']) ? $_GET['numassignments'] : 0;

<!doctype html>

<head>

<title>Lab 01 - Final Grade Calculator</title>

</head>

<body>

<h1 class="mt-5">Final Grade Calculator</h1>

<p>Welcome to the Final Grade Calculator!</p>

<h2 class="form-text fs-4 mb-4">To get started, please enter the number of assignments that you've completed so far.</h2>

<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="get">

<label for="numassignments" class="form-label">Number of Assignments:</label>

<input type="number" class="form-control" name="numassignments" id="numassignments" min="1" max="10" value="<?php echo $numassignments ?>" required>

</div>

<p class="form-text">How many assignments have you already completed in this course?</p>

</fieldset>

</div>

</form>

</div>

Enter the input value and hit 'Generate Row' to produce the input rows in the second form field. For me, I put 3.

<?php if (isset($_GET['numassignments']) && $_GET['numassignments'] > 0) : ?>

<h2 class="form-text fs-4 mb-4">Next, for each assignment, add the grade that you received and the assignment's overall weight.</h2>

<form action="<?php echo $_SERVER['REQUEST_URI']; ?>" method="post">

<?php

$desired = isset($_POST["desired"]) ? $_POST["desired"] : 0;

for ($i = 1; $i <= $numassignments; $i++) {

echo "<div class=\"row\">";

echo "\n<div class=\"col-md-6\">"; //grade input

echo "\n<div class=\"mb-3\">";

echo "\n<label for=\"keep-numassignments{$i}\" class=\"form-label\">Assignment " . $i . " Grade:</label>";

echo "\n<input type=\"number\" class=\"form-control\" name=\"keep-numassignments[$i]\" id=\"keep-numassignments{$i}\" min=\"0\" max=\"100\" value=\"$keep_numassignments\" required>";

echo "\n</div>";

echo "\n<div class=\"col-md-6\">"; //weight input

echo "\n<div class=\"mb-3\">";

echo "\n<label for=\"weightnum{$i}\" class=\"form-label\">Assignment " . $i . " Weight:</label>";

echo "\n<input type=\"number\" class=\"form-control\" name=\"weightnum[$i]\" id=\"weightnum{$i}\" min=\"1\" max=\"100\" value=\"$weightnum\" required>";

echo "\n</div>";

}

<p class="form-text">Finally, if you would like to calculate what you will need to get on the remainder of your coursework in order to receive a certain grade, enter your desired grade below.</p>

<label for="desired" class="form-label">Desired Final Grade (Optional):</label>

</div>

</fieldset>

</div>

</form>

</div>

<?php endif; ?>

<?php

if (isset($_POST['gradesubmit'])) {

// Retrieve the values from the form

$grades = isset($_POST["keep-numassignments"]) ? $_POST["keep-numassignments"] : [];

$weights = isset($_POST["weightnum"]) ? $_POST["weightnum"] : [];

$desired = isset($_POST["desired"]) ? $_POST["desired"] : 0;

// Calculate the weighted average of completed assignments

$weightedSum = 0;

$totalWeight = 0;

for ($i = 0; $i < count($grades); $i++) {

$weightedSum += ($weights[$i] * $grades[$i]);

$totalWeight += $weights[$i];

}

$currentAverage = ($totalWeight > 0) ? ($weightedSum / $totalWeight) : 0;

// Calculate the required grade on remaining assessments

if ($totalWeight < 100 && $desired > 0) {

$remainingWeight = 100 - $totalWeight;

$requiredGrade = ($desired - (1 - ($remainingWeight / 100)) * $currentAverage) / $remainingWeight;

} else {

$requiredGrade = 0;

}

// Display the results

echo "<div class=\"alert alert-success mb-5\">";

echo "\n<p>Current Class Average: " . number_format($currentAverage, 2) . "%</p>";

if ($desired > 0) {

echo "\n<p>Required Grade on Remaining Assessments: " . number_format($requiredGrade, 2) . "%</p>";

}

echo "\n</div>";

}

</div>

</body>

</html>

enter image description here

I put the input value like in the picture, but the wrong value was output. If it had been calculated correctly, the output would be produced 61.67% for the Current Class Average and 135% for the Required Grade on Remaining Assessments.

Solution

I think you mixed up the list indices for numassignments when you added items and when you read items.

When you're adding items to the list you start at 1

for ($i = 1; $i <= $numassignments; $i++)

When you're reading items from the list you start at 0

$grades = isset($_POST["keep-numassignments"]) ? $_POST["keep-numassignments"] : [];

for ($i = 0; $i < count($grades); $i++) {

$weightedSum += ($weights[$i] * $grades[$i]);

$totalWeight += $weights[$i];

}

If you check the average of your first two items (50, 65) you get the average of 57.5% that you're seeing in your output.

To fix this, I would start reading your grades and weights starting from index 1 just like when you added them.