This is my form: I want to calculate Units Billed when user enter present and previous reading before submission.

<form action="EIMS_North_Inserted.php" method="post">

<table class="mytable" align="center">
<tr>
<td><label>Site ID
<span class="small">Example N-WAH-8620</span>
</label></td>
<td>
<input type="text" name="a" id="course"  value=""  class="input"/> </td>

<td>
<label>Previous Reading
<span class="small">Regular bills</span>
</label></td>
<td>
<input type="text" name="e" id="e" value="<?php echo $e; ?>"  class="input"/> </td>

<td><label>Present Reading
<span class="small">Regular bills</span>
</label></td>
<td>
<input type="text" name="f" id="f" value=""  class="input"/> </td></tr>
<tr>
<td><label>Units Billed
<span class="small">Regular bills</span>
</label></td>
<td>
<input type="text" name="g" id="g" value=""  class="input"/> </td>

<tr>
<td colspan="2">
<input type="submit" name="submit" style="width: 200px;
	height: 20px;
	cursor: pointer;
	border: none;
	margin-left:100px;
	background: transparent url(/images/sims_update_button.jpg) no-repeat 0 0;" value=""/>
</td>
</tr>
 </form>
</table>

Seems to me that you want the values calculated on the fly. You can do this on a simple Javascript, IMO this cannot be done by php without parsing the form values first.

Add following javascript code before html body tag.

<script lang='javascript'>
function calculate()
{
 //e is previous
 //f is current
 //g is new is f-e
 	previous=parseFloat(document.getElementById("e").value);
 	curr=parseFloat(document.getElementById("f").value);
	if(!isNaN(previous) && !isNaN(curr) )
	{
	 	if(curr>=previous)
			document.getElementById("g").value=curr-previous;
		else
			document.getElementById("g").value="";
	}
	else
	{
		document.getElementById("g").value="";
	}
	var total = 0;      
	for (i = 1 ;i <= 15; i++)      
	{        
		if (i<10)
			labelname='amount_0'+i;
		else
			labelname='amount_'+i;
	
	
		if (!isNaN(parseFloat(document.getElementById(labelname).innerHTML)))
			total += parseFloat(document.getElementById(labelname).innerHTML);      
	}      
	
	document.getElementById(label2).innerHTML = total;
}
<script>

in your html part add onblur events as shown below in your text box elements

<input type="text" name="e" id="e" value="<?php echo $e; ?>"  class="input" onblur='javascritp:calculate();' /> 
<input type="text" name="f" id="f" value=""  class="input"  onblur='javascritp:calculate();' />

I see what you did there.

some code was not required given in my previous post. following is the final one.
Add following javascript code before html body tag.

<script lang='javascript'>
function calculate()
{
 //e is previous
 //f is current
 //g is new is f-e
 	previous=parseFloat(document.getElementById("e").value);
 	curr=parseFloat(document.getElementById("f").value);
	if(!isNaN(previous) && !isNaN(curr) )
	{
	 	if(curr>=previous)
			document.getElementById("g").value=curr-previous;
		else
			document.getElementById("g").value="";
	}
	else
	{
		document.getElementById("g").value="";
	}
}
<script>

in your html part add onblur events as shown below in your text box elements

<input type="text" name="e" id="e" value="<?php echo $e; ?>"  class="input" onblur='javascritp:calculate();' /> 
<input type="text" name="f" id="f" value=""  class="input"  onblur='javascritp:calculate();' />

Utrivedi, the below code is also working for me without the validation then why we should check for that? browser compatability?

You mistakently ommited the g value textbox and added here

<!doctype html public "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title> New Document </title>
<meta name="Generator" content="EditPlus">
<meta name="Author" content="">
<meta name="Keywords" content="">
<meta name="Description" content="">
<script lang='javascript'>
function calculate()
{
 	previous=parseFloat(document.getElementById("e").value);
 	curr=parseFloat(document.getElementById("f").value);
	 	if(curr>=previous)
			document.getElementById("g").value=curr-previous;
		else
			document.getElementById("g").value="";
}
</script>
</head>
<body>
<input type="text" name="e" id="e" value="10"  onblur='javascritp:calculate();' /> 
<input type="text" name="f" id="f" value="20"  class="input"  onblur='javascritp:calculate();' />
<input type="text" name="f" id="g" value=""  class="input"  onblur='javascritp:calculate();' />
</body>
</html>

g field is calculated field, user may not change it so onblur='javascritp:calculate();' is not required for g field. User is not going to type there anything.

I am not checking for browser compatibility anywhere, why do you feel so?

Okay, but without using the tag you retrieved in the javascript. I want to know in which scenario we must need isNAN to check because without that itself working for me?

But I require the result in input feild where id is g

Dear Raja
Some time if you parse string to float it my give Nan (NOT a number) error, to avoid such error I have used isNaN function.

Dear Ayesha
Dont worry, this code will give you result in g field

if(!isNaN(previous) && !isNaN(curr) )

Why you write this?

Ayeshya,

Please add the one more input code

<input type="text" name="g" id="g" value=""  class="input"/>

Then you can see the result.

Dont be scared for that isNAN code, that is fine as utrivedi said. Sometimes the given input show an error as NAN (Not a number) means then given input is not a number. To identified and avoid that utrivedit used that. When u give strings then you can't get display in inputbox.

Here is the complete code

<!doctype html public "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title> New Document </title>
<meta name="Generator" content="EditPlus">
<meta name="Author" content="">
<meta name="Keywords" content="">
<meta name="Description" content="">
<script lang='javascript'>
function calculate()
{
 	previous=parseFloat(document.getElementById("e").value);
 	curr=parseFloat(document.getElementById("f").value);
	if(!isNaN(previous) && !isNaN(curr) )
	{
	 	if(curr>=previous)
			document.getElementById("g").value=curr-previous;
		else
			document.getElementById("g").value="Not curr>=previous";
	}
	else
	{
		document.getElementById("g").value="Not a Number";
	}
}
</script>
</head>
<body>
<input type="text" name="e" id="e" value="10"  onblur='javascritp:calculate();' /> 
<input type="text" name="f" id="f" value="20"  class="input"  onblur='javascritp:calculate();' />
<input type="text" name="g" id="g" value=""  class="input" />
</body>
</html>

Test it in two ways:

Method I: Give your name in first text box. (and check the third box output)
Method II: Give a number greater than second text box (and check the third box output)

I hope u understand now!

I have g feild in my code thats why utrivedi did not write.
Now one more qustion I want to ask...
Can I?

Okay, I did not noticed. Ayesha, did you gone thru the above code?

Yea its working perfectly for me.
can I ask next question related to this?

e feild is Previous reading
f feild is Present Reading
g is Caclulated by f-e
now I have another feild name a which is code of bill
and in DB I store code and there MF(Multiplying Factor)
Now I want my result g=(f-e)*MF
if Site id 8620 and mf 2 in DB Table
when user write 8620 in feild a it gets value of MF from DB and calculate g=(f-e)*MF

you need to change as code given below

<?php
// find multiplier here 
$mfvalue=9;
?>
<script lang='javascript'>
function calcualte()
{
.
.
.
.
if(curr>=previous)
	document.getElementById("g").value=(curr-previous)*(<?php echo $mfvalue;?>);

.
.
.
}

for Example
When I write value in field a can it search value from DB table and store in hidden feild for calculation use?
Table name: MFCode

Code MF
8620 2
8621 1
8645 5

i) Create a new field with an id="a"
ii) Connect database retrieve the value of mf for relevant site.
iii) Do the calculation as before.

I suggested only the logic. for the above work you should use both the php and javascript elements within the document.

but I want mf on the basis of input of feild a.

no need to keep it in hidden field.
You may query database before script tag and find value in variable $mfvalue.
then echo value of $mfvalue in your javascript as i have shown in abouve post.

Then come out of playing with html and javascript.

i) create a form with your input fields and a button.
<form name="frm" method="post" action="process.php">
ii) On submit it will the process the seperate php file, pass your a field value
$siteno=$_POST;
iii) Then connect your database and retrieve the specific value
iv) Then do the calculation.

But my requirement is to show units calculated on client side before submit so they can verify. I can share image of scenario in next post

But my requirement is to show units calculated on client side before submit so they can verify. I can share image of scenario in next post

Please check image . I want g=(f-e)*mf

Its critical to pass javascript value to php script without using the post or get.

Please check image . I want g=(f-e)*mf

Please check my image