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HTMLHelp Forums _ Client-side Scripting _ Get the number of edited inputs

Posted by: RainLover Mar 5 2019, 08:19 AM

Scenario

Every semester my students need to take at least one test. The following form gives the right average grade of a student:

CODE
<!DOCTYPE html>
<html lang="en">
<body>
<form>
Math: <input type="number" id="test1">
<input type="number" id="test2">
<input type="number" id="test3">
<output id="average"></output>
<br>
<input type="button" value="Calculate" id="calcBtn">
</form>
<script>
var test1 = document.getElementById('test1').value;
var test2 = document.getElementById('test2').value;
var test3 = document.getElementById('test3').value;
var average = document.getElementById('average');
average.value = (Number(test1)+Number(test2)+Number(test3)) / 3;
});
</script>
</body>
</html>

The problem is it works right only if all the fields are edited. If the student doesn't take some tests, the average grade won't show the right value. I know it's because of dividing by the fixed number 3 when it calculates the average grade:
CODE
average.value = (Number(test1)+Number(test2)+Number(test3)) / 3;

Question

What is a simple approach to get the number of changed input fields?

Posted by: Christian J Mar 5 2019, 03:44 PM

If the student doesn't take some tests, the average grade won't show the right value. I know it's because of dividing by the fixed number 3 when it calculates the average grade:

You mean the number of entered test fields are allowed to vary? Then you might use say getElementsByTagName to count the number of form fields with non-empty values, and divide them with the the length property.

Posted by: RainLover Mar 5 2019, 10:34 PM

If the student doesn't take some tests, the average grade won't show the right value. I know it's because of dividing by the fixed number 3 when it calculates the average grade:

You mean the number of entered test fields are allowed to vary? Then you might use say getElementsByTagName to count the number of form fields with non-empty values, and divide them with the the length property.

Dear Christian,
I'm afraid I don't know how to do it. Would you mind helping me with this:
QUOTE
count the number of form fields with non-empty values

Posted by: Christian J Mar 6 2019, 11:55 AM

Here's an idea:

CODE
Math:
<input type="number" class="test">
<input type="number" class="test">
<input type="number" class="test">

<output id="average"></output>
<br>
<input type="button" value="Calculate" id="calcBtn">

<script>
{
// any number of elements
var test_input=document.getElementsByClassName('test');

for(var i=0; i<test_input.length; i++)
{
var val=test_input[i].value;
//alert('Value: '+val+'\nType: '+typeof val);

if(val.length>0) // at least one character must be entered
{
val=val*1; // try to convert string type of form field  to number

if(!isNaN(val) && typeof val=='number') // try to check if type conversion was successful
{
grades_arr.push(val); // add valid number values to array
//alert('Row '+i+' is a number');
}
else
{
//alert('Row '+i+' is not a  number');
}
}
else
{
//alert('Row '+i+' is empty');
}
}

// calculate average
if(grades_arr.length>0) // at least one form field must be filled in correctly by user
{
for(var i=0; i<grades_arr.length; i++)
{
}
}
else
{
//alert('No valid data was entered');
}
}
);
</script>

You can use any number of form fields with the CLASS "test", and the user can fill in as many of them as he wants. I haven't added any rounding of the resulting average.

There are a few gothas about the type conversion though. Currently it seems only IE11 and Chrome-based browsers prevent the user from entering non-digit characters in number INPUTs. Edge and Firefox still seem to allow it, but will consider such user data as an empty string. Older browsers that don't support HTML5 number INPUTs will regard the form fields as text INPUTs, and pass along the whole string to the javascript.

Despite the above, javascript still considers INPUT data to be of "string" type, even when numbers have been entered by the user into a number INPUT. I tried to consider all the scenarios in the script, but don't trust me on this.

Posted by: pandy Mar 6 2019, 03:40 PM

Is checking for NaN really needed? Provided browser support is there I think input type="number" does its own validation and sends nothing if the input isn't a number. This sounds like a controlled environment so... Posted by: Christian J Mar 6 2019, 04:52 PM

Is checking for NaN really needed? Provided browser support is there I think input type="number" does its own validation and sends nothing if the input isn't a number. This sounds like a controlled environment so... I agree, but thought it didn't hurt to include it.

Posted by: pandy Mar 7 2019, 05:52 AM

No, it doesn't. I asked mainly because I wasn't sure I had understood this right.

Posted by: Christian J Mar 7 2019, 06:10 AM

Then again NaN is a tricky property that I don't understand very well, for example it can sometimes be of the type "number".

isNaN() or Number.isNaN() is supposedly better: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN

Posted by: Christian J Mar 7 2019, 10:56 AM

Seems IE11 does need checking with isNaN(), since the browser lets through user values like "1a" (but not "a1"). Also IE11 doesn't support the OUTPUT element.

Firefox does warn the user of invalid form field input (by creating a red selection rectangle), but not until the user clicks outside the field. Edge never gives any warning at all.

Posted by: pandy Mar 7 2019, 05:30 PM

Always IE, eh? Still.

I don't understand the discussion, because when he for instance wants to demonstrate why isNaN isn't reliable, the example he gives returns the wanted result. But, the solution at the bottom of the page is interesting!

Posted by: Christian J Mar 8 2019, 06:16 AM

I didn't understand that example either.

As an alternative to the NaN confusion one might use a regular expression to look for non-numerical characters, and then flag an error. (One shouldn't just strip non-numbers out though, since then something like "1foo2" might be changed to "12".)

Posted by: Christian J Mar 8 2019, 02:12 PM

Here's one with a regex instead of isNaN:

CODE
Math:
<input type="number" class="test">
<input type="number" class="test">
<input type="number" class="test">

<span id="average"></span>
<br>
<input type="button" value="Calculate" id="calcBtn">

<script>
{
// any number of elements
var test_input=document.getElementsByClassName('test');

for(var i=0; i<test_input.length; i++)
{
var val=test_input[i].value;

// remove numerical characters from sample, to see if any invalid characters remain
var invalid=val.replace(/[0-9]/g,'');

if(val.length>0 && invalid.length==0) // at least one character must be entered, but no invalid characters
{
val=1*val; // convert to number type
grades_arr.push(val); // add valid number values to array
}
else
{
//alert('Field '+i+' is empty or invalid');
}
}

// calculate average
if(grades_arr.length>0) // at least one form field must be filled in correctly by user
{
for(var i=0; i<grades_arr.length; i++)
{
}
}
else
{
//alert('No valid data was entered');
}
}
);
</script>

Posted by: RainLover Mar 9 2019, 10:41 AM

Here's one with a regex instead of isNaN:

CODE
Math:
<input type="number" class="test">
<input type="number" class="test">
<input type="number" class="test">

<span id="average"></span>
<br>
<input type="button" value="Calculate" id="calcBtn">

<script>
{
// any number of elements
var test_input=document.getElementsByClassName('test');

for(var i=0; i<test_input.length; i++)
{
var val=test_input[i].value;

// remove numerical characters from sample, to see if any invalid characters remain
var invalid=val.replace(/[0-9]/g,'');

if(val.length>0 && invalid.length==0) // at least one character must be entered, but no invalid characters
{
val=1*val; // convert to number type
grades_arr.push(val); // add valid number values to array
}
else
{
//alert('Field '+i+' is empty or invalid');
}
}

// calculate average
if(grades_arr.length>0) // at least one form field must be filled in correctly by user
{
for(var i=0; i<grades_arr.length; i++)
{
}
}
else
{
//alert('No valid data was entered');
}
}
);
</script>

Many thanks for the answers! 🌹

You're welcome!