Solved

# Javascript subtraction help needed

Posted on 2007-03-31
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I can't see to figure this one out... yet it seems so simple

when I calculate the following equation I am getting a very out results  5.21-5 = .2099999999999999998.

I need it to compute the actual value which is .2100000

function ComputeChange5(Start, Change, End) {
document.MainForm[Change].value = formatNum(eval(5.21-5),"^#,###.5^");
}

I am looking for a very simple solution... I apss into the function the start and end values, and then look for the change value (which is the subtraction). I tried to simplify the code above after I narrowed down that its not my passed in variables, its simply the calucation. thx.
0
Question by:mrferrari
• 5
• 4

LVL 36

Accepted Solution

Zyloch earned 500 total points
The problem is caused by round off error in the Javascript. You might want to try something like:

var number = 5.21 - 5;
0

LVL 63

Expert Comment

The fractal number is the nature of digital computer number representation.
That effect is stumping already others, but what do you want to ahieve?
Do you need help for that formatNum() function?
Do you know the method toFixed(2)
Like this:

0

LVL 36

Expert Comment

The toFixed() function will round your number to the number of decimal places that you want. I am not too sure why you need to use eval() either.
0

Author Comment

this is my full function

function ComputeChange5(Start, Change, End) {
var vartotal = 0;
var varname = "";
var FieldName = End;

if (document.MainForm[FieldName].value.replace(/\,/g,"") == ''|| isNaN(document.MainForm[FieldName].value.replace(/\,/g,""))){
document.MainForm[FieldName].value = formatNum(eval(document.MainForm.oldvarstorage.value.replace(/\,/g,"")),"^#,###.5^");
document.MainForm[FieldName].focus();}
else
{
if (document.MainForm[End].value == document.MainForm.oldvarstorage.value)
{}
else
{
vartotal = numbertofixed(eval(document.MainForm[End].value.replace(/\,/g,"")),5) - eval(document.MainForm[Start].value.replace(/\,/g,""));
document.MainForm[End].value = formatNum(eval(document.MainForm[End].value),"^#,###.5^");
document.MainForm[Change].value = formatNum(eval(vartotal),"^#,###.5^");
}
}
}
0

Author Comment

thanks...

I just change the last statement formatNum(eval(vartotal),"^#,###.5^");

to vartotal.toFixed(5)

any other suggestion on the function would be helpful, I am very new to java and frankly don;t use it much except these calculation functions... here and there.

0

LVL 36

Expert Comment

There are parts of your function that seem repetitive or useless. For instance, what is the purpose of the FieldName variable?

if (document.MainForm[FieldName].value.replace(/\,/g,"") == ''|| isNaN(document.MainForm[FieldName].value.replace(/\,/g,""))){

if may be easier to use

var num = parseFloat(document.MainForm[End].value.replace(/,/g, ''));
if (isNaN(num)) {

Also,

if (document.MainForm[End].value == document.MainForm.oldvarstorage.value)
{}

does not seem necessary. It does not hurt to format the number again if it is the old one.
0

Author Comment

cool. super thanks. look at it now...

note, I have another function which is compute change. It is virtually the same code, but I just changed field name, that is why I left it in there at the top

function ComputeChange5(Start, Change, End) {
var vartotal = 0;
var FieldName = End;
var Num = parseFloat(document.MainForm[FieldName].value.replace(/\,/g,""))

if (isNaN(Num))
{
document.MainForm[FieldName].value = formatNum(eval(document.MainForm.oldvarstorage.value.replace(/\,/g,"")),"^#,###.5^");
document.MainForm[FieldName].focus();}
else
{
vartotal = eval(document.MainForm[End].value.replace(/\,/g,"")) - eval(document.MainForm[Start].value.replace(/\,/g,""));
document.MainForm[End].value = formatNum(eval(document.MainForm[End].value),"^#,###.5^");
document.MainForm[Change].value = vartotal.toFixed(5);
}
}
0

Author Comment

sad is I also have a function

function ComputeChange2(Start, Change, End)

which just replaces all the 5's with 2's and return 2 deciaml place results...

it was easier for me to copy the code that incorporate the logic into the function and pass in another parameter... looking back it seems lazy to me... im guilty and will fix it.

Marc
0

Author Comment

should I even be using eval and the format num?

how do I make the formatnum parameter, ,"^#,###.5^" dynamic, so I can pass in either a 2 or 5 to specify how many decimals I want...

I would like to award you some more points....

Marc
0

LVL 36

Expert Comment

It depends on how formatnum is coded. In fact, it may be easier to take out the .5 part of the format number and just use toFixed() explicitly.

If you want, you can always use something such as:

"^#,###." + VariableHoldingDecimalPlaces + "^"

Finally, you do need the majority of your eval() functions. You only need that if you have complex Javascript as a string and need to run it.
0

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