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November 1st, 2006, 04:35 PM
 blkskullwork Authorized User Join Date: Nov 2006 Posts: 15 Thanks: 0 Thanked 0 Times in 0 Posts
Strange calculation problem

I have a very strange problem. I am multiplying quantity by amount to get a sub total and for most quantities it works ok but for some reason when I enter 5 in the quantity I get a sub total with around 8 decimal places eg 2.99*5 gives me (\$14.950000000000001), multiplied by4 gives me \$11.96. This also happens on quantity 10,20,30 and so on.

Here is the function:

function totalord()
{
var totitems
var itm1 = document.form1.hambqty.value
var itm2 = document.form1.cheeseqty.value
var itm3 = document.form1.chickqty.value
var itm4 = document.form1.friesqty.value
var itm5 = document.form1.gravyq.value
var itm6 = document.form1.chilliq.value

totitems = eval(itm1+itm2+itm3+itm4+itm5+itm6)
alert("Item 1 is " + itm1)
if (totitems == "0")
{
alert("you have not orderd any items, please enter at least one item(s)")
}

document.form1.hambsub.value = eval(document.form1.hambprice.value.substring(1,6) *itm1)

document.form1.hambsub.value =("\$"+document.form1.hambsub.value)
alert("Subtotal 1 is " + document.form1.hambsub.value)

}

November 3rd, 2006, 11:33 AM
 joefawcett Wrox Author Join Date: Jun 2003 Posts: 3,074 Thanks: 1 Thanked 38 Times in 37 Posts

Just as in decimal system some numbers cannot be expressed perfectly, e.g. 1/3. The same happens in binary so you have to round numbers if it's important.
As a side issue, why are you using eval? It's completely unnecessary and a severe waste of resources.

--

Joe (Microsoft MVP - XML)

November 3rd, 2006, 01:13 PM
 blkskullwork Authorized User Join Date: Nov 2006 Posts: 15 Thanks: 0 Thanked 0 Times in 0 Posts

I'm new to javascript so you'll have to excuse me for not doing things the right way sometimes. I have removed the eval and used )itm1+"+"+itm2+"+"+itm3...etc, I was using eval because when adding the vars I was getting 122345 for an answer instead of the sum 17. Thanks for your input though, every little bit helps.

November 3rd, 2006, 01:43 PM
 joefawcett Wrox Author Join Date: Jun 2003 Posts: 3,074 Thanks: 1 Thanked 38 Times in 37 Posts

There are a number of ways to do arithmetic summation, rather than string concatenation. You can use Math.floor:
Code:
var itm1 = document.form1.hambqty.value
var itm2 = document.form1.cheeseqty.value
or the unary plus:
Code:
You will often see parseInt used as well but it is best avoided.

--

Joe (Microsoft MVP - XML)

November 3rd, 2006, 03:16 PM
 blkskullwork Authorized User Join Date: Nov 2006 Posts: 15 Thanks: 0 Thanked 0 Times in 0 Posts

I've seen Math.floor used before but not with the alert, does that actually add them up or just display the sum. Thanks for the ideas, I'll try them out.

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