How do I check the contents of this textbox in my php/mysql application

I have the attached checkbox section in my code.  Each checkbox has a value attached to it and calls the attached function which tallys everything and displays it in another textbox.  The client wanted me to add a Quantity box that will multiple anything selected by whatever the user puts in the quantity textbox.  I got that to work, but i want to check if the box is empty,and if empty, make the value 1,  or check if the value entered is a non-numeric value before it does the calculation.  I thought this would be simple, but I am too brain drained to get this working.  

I get this error in FF:
document.getElementById("Q_379") is null
[Break on this error] if (document.getElementById('Q_379').value == "")

Help please
function calc(theForm)
{
  //debugger;
  var elements = theForm.elements;
  var total = 0;
  var multiplier=1;
  var imgQty = 0;
  var imgTotal = 0;
  var photoQty = 0
  for(var i=0; i<elements.length; i++)
  {
      var element = elements[i];
  
        if(element.type == 'checkbox' && element.checked)
        {
            total += parseFloat(element.value);
        }
 
        else if(element.name=="Q_361" && element.checked)
        {
            multiplier = parseFloat(element.value);
        }
        if (document.getElementById('Q_379').value == "")
        {
        	projQtyBox = 1;
        }
        else
        {       
			projQtyBox = document.getElementById('Q_379').value;
        }
        
  document.getElementById('Q_330').value = total*multiplier*projQtyBox;
  
}
}

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checkbox.JPG
LVL 1
justmelatAsked:
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RobinSoftware EngineerCommented:
Probably because the new textbox you added does not have the id 'Q_379' but it's its name.

0
 
Michael701Commented:
I hope that quantity box is large enough. Ok, just kidding, it's been one of those days.

And could you be any less descriptive with your element names?

Here's a snippet when I mulitply quantity and price of an item.
    if (isNaN(quantity))
    {
      error=1
      notes.innerHTML="Invalid quantity!"
    }
    else if (isNaN(price))
    {
      error=1
      notes.innerHTML="Invalid price!"
    }
    else
    {
    answer=quantity * price
    ....
    }

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0
 
Michael701Commented:
oh guess I should have also shown the variable definitions

  for (x=1; x<=<?PHP echo MAX_ORDER_LINES ?>; x++)
  {
    quantity=document.getElementById("quantity_"+x).value
    price=document.getElementById("price_"+x).value
    extension=document.getElementById("extension_"+x)
    notes=document.getElementById("notes_"+x)

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0
 
justmelatAuthor Commented:
Ahhh, can't believe i didn't see that. thank you.
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