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subtract two fields on submit and alert if negative

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Sep 09.2008

How could I take two form fields and subtract them from each other on submit and then have an alert come up only if the amount is negative?

Example:

67 in field 1 is the users limit. 69 in field 2 is what they have ordered.

When user submits I want it to popup and say something like, “we are sorry your order amount exceeds your limit.”

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JavaScript

7 Comments(s) ↴

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@gil_davisSep 09.2008 — <i>
</i>&lt;script type="text/javascript"&gt;
function subtract(f1, f2) {
if (f1 - f2 &lt; 0)
{alert("we are sorry your order amount exceeds your limit.");
return false;}
return true;
}
&lt;/script&gt;
...
&lt;form onsubmit="return subtract(this.fld1.value, this.fld2.value)"&gt;
&lt;input type="text" name="fld1"&gt;
&lt;input type="text" name="fld2"&gt;
&lt;/form&gt;
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@designopauthorSep 09.2008 — Thank you very much Gil. I highly appreciate your help. Thumbs up my friend. ?
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@designopauthorSep 10.2008 — Is it possible to edit this so that in the alert it says how much you are over your limit?
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@cgishackSep 10.2008 — just store the difference between f1 - f2 into a variable.
<i>
</i>var difference = f1 - f2;
if (difference &lt; 0)
{
alert("we are sorry your order amount exceeds your limit. You entered "+ difference);
return false;
}


Drew
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@designopauthorSep 10.2008 — Thank you. Worked awesome!

One final question. How can I have a text field elsewhere on the page populated on the fly with the + difference? Or would this not be possible?
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@cgishackSep 10.2008 — if your textbox was like so..
[code=html]
 <input type="text" id="ERROR">
[/code]

you can populate it like so... (by its ID)

<i>
</i>var difference = f1 - f2;
if (difference &lt; 0)
{
alert("we are sorry your order amount exceeds your limit. You entered "+ difference);
[B]document.getElementById('[COLOR="Red"]ERROR[/COLOR]').value = difference;[/B]
return false;
}
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@designopauthorSep 10.2008 — Thanks. That did not work however. Not sure what I did wrong. What I mean by "on the fly", is that in real time on the page as the math is being done the text field changes.

ie. User enters item. math is done on the fly and this field is updated with leftover balance.
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