How to calculate HTML table columns (using javascript or jquery) with a user defined equation

Question

I need some help with dynamically calculating an HTML table column using data from other columns and using a user-defined equation.

For example, if the user inputs the equation C1 + C2 * 0.5 + C3 * 0.8 into a input box the table would need to calculate the last column based on the data from the columns defined in the equation (C1 = column 1, C2 = column 2...).

My table data looks like this:

Student ID | Homework 1 | Homework 2 | Exam points | Final Grade
1            8.75         7.60         55.50         -
2            9.00         4.50         63.00         -
3            7.75         7.40         45.50         -

If the user typed in the equation C1 + C2 * 0.5 + C3 * 0.8 in the input the table should perform the operations and fill the column Final Grade based on that equation.

The result should look something like this.

Student ID | Homework 1 | Homework 2 | Exam points | Final Grade
1            8.75         7.60         55.50         56.95
2            9.00         4.50         63.00         61.65
3            7.75         7.40         45.50         47.85

The first row in final grade would be calcualted like this (8.75 + 7.60 * 0.5 + 55.50 * 0.8).

This is my body in HTML:

<div>
    <input id="equation">
</div>
<table>
    <tr>
        <th>Student ID</th>
        <th>Homework 1</th>
        <th>Homework 2</th>
        <th>Exam points</th>
        <th>Final grade</th>
    </tr>
    <tr>
        <td>1</td>
        <td>8.75</td>
        <td>7.60</td>
        <td>55.50</td>
        <td class="final-grade">-</td>
    </tr>
    <tr>
        <td>2</td>
        <td>9.00</td>
        <td>4.50</td>
        <td>63.00</td>
        <td class="final-grade">-</td>
    </tr>
    <tr>
        <td>3</td>
        <td>8.75</td>
        <td>7.60</td>
        <td>55.50</td>
        <td class="final-grade">-</td>
    </tr>
</table>

Any help would be greatly appreciated!

Answer 1

Here's an example that uses the evil eval (!).

• Works with any number of cells, uppercase or lowercase C identifier.
• If the subject cell is invalid the result will be "NaN".
• If the provided equation is invalid the result will be "-"

Basically it creates an object with references like {"c1":8.75, "c2":7.60, ...} , than it evals the input string replacing the c* occurrences with the respective object value.

function calcGrades() {

  const val = this.value.toLowerCase().trim();
  
  $("#grades tbody tr").each((i, TR) => {
    let res;
    const refs = $("td",TR).get().reduce((ob, TD, i) =>
        [ob["c"+i] = parseFloat(TD.textContent), ob][1], {});
        
    try { res = eval(val.replace(/c\d+/g, $1 => refs[$1])).toFixed(2) } 
    catch (err) { res = "-" }
    
    $(".final-grade",TR).text( res );
  });
}

$("#equation").on("input", calcGrades).trigger("input");

<input id="equation" type="text" value="C1 + c2 * 0.5 + C3 * 0.8">
<table id="grades">
  <thead>
    <tr>
        <th>ID</th><th>Homework 1</th><th>Homework 2</th><th>Exam pts</th><th>Final</th>
    </tr>
  </thead>
  <tbody>
    <tr><td>1</td><td>8.75</td><td>7.60</td><td>55.50</td><td class="final-grade">-</td></tr>
    <tr><td>2</td><td>9.00</td><td>4.50</td><td>63.00</td><td class="final-grade">-</td></tr>
    <tr><td>3</td><td>7.75</td><td>7.40</td><td>45.50</td><td class="final-grade">-</td></tr>
    <tr><td>4</td><td>0</td><td>0.0</td><td>0</td><td class="final-grade">-</td></tr>
    <tr><td>4</td><td>foo</td><td>bar</td><td>baz</td><td class="final-grade">-</td></tr>
  </tbody>
</table>
<script src="//ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>