Javascript, weird floating point number endless decimal? - Stack Overflow

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I need some help with floating point numbers...please!

Here's the thing, I have code like below:

<script>

    function add()
    {
        document.getElementById("answer").value = parseFloat(document.getElementById("num1").value) + parseFloat(document.getElementById("num2").value);
    }

    function subtract()
    {
        document.getElementById("answer").value = parseFloat(document.getElementById("num1").value) - parseFloat(document.getElementById("num2").value);
    }

    function multiply()
    {
        document.getElementById("answer").value = parseFloat(document.getElementById("num1").value) * parseFloat(document.getElementById("num2").value);
    }

    function divide()
    {
        document.getElementById("answer").value = parseFloat(document.getElementById("num1").value) / parseFloat(document.getElementById("num2").value);
    }

</script>

Sorry it's a bit long! And then the html is pretty simple:

<h1>Calculator</h1>

<h3>Enter a value in each box below, then click an operation</h3>

<form id="calculatorForm">
    <input id="num1" value="0" type="text">
    <input id="num2" value="0" type="text">
    <br>
    <input value="+" onclick="add();" type="button">
    <input value="-" onclick="subtract();" type="button">
    <input value="*" onclick="multiply();" type="button">
    <input value="/" onclick="divide();" type="button">
    <br>
    <input id="answer" value="0" type="text">
</form>

You can pretty much guess what my question is gonna be: when I multiply, divide, or subtract two floating point numbers, I end up with an infinite decimal.

I need a quick solution that will round those numbers to two decimal points, and I need it to work later on, because I then need to implement Fahrenheit-to-Celsisus operations afterwards.

I don't care how this is done, but it must be Javascript. Sorry if this has been answered before, but I really need an answer soon! Thanks!

EDIT: A BIG Thankyou to the helpful people who answered my questions. Thank you!

I need some help with floating point numbers...please!

Here's the thing, I have code like below:

<script>

    function add()
    {
        document.getElementById("answer").value = parseFloat(document.getElementById("num1").value) + parseFloat(document.getElementById("num2").value);
    }

    function subtract()
    {
        document.getElementById("answer").value = parseFloat(document.getElementById("num1").value) - parseFloat(document.getElementById("num2").value);
    }

    function multiply()
    {
        document.getElementById("answer").value = parseFloat(document.getElementById("num1").value) * parseFloat(document.getElementById("num2").value);
    }

    function divide()
    {
        document.getElementById("answer").value = parseFloat(document.getElementById("num1").value) / parseFloat(document.getElementById("num2").value);
    }

</script>

Sorry it's a bit long! And then the html is pretty simple:

<h1>Calculator</h1>

<h3>Enter a value in each box below, then click an operation</h3>

<form id="calculatorForm">
    <input id="num1" value="0" type="text">
    <input id="num2" value="0" type="text">
    <br>
    <input value="+" onclick="add();" type="button">
    <input value="-" onclick="subtract();" type="button">
    <input value="*" onclick="multiply();" type="button">
    <input value="/" onclick="divide();" type="button">
    <br>
    <input id="answer" value="0" type="text">
</form>

You can pretty much guess what my question is gonna be: when I multiply, divide, or subtract two floating point numbers, I end up with an infinite decimal.

I need a quick solution that will round those numbers to two decimal points, and I need it to work later on, because I then need to implement Fahrenheit-to-Celsisus operations afterwards.

I don't care how this is done, but it must be Javascript. Sorry if this has been answered before, but I really need an answer soon! Thanks!

EDIT: A BIG Thankyou to the helpful people who answered my questions. Thank you!

• javascript
• floating-point

Share Improve this question Follow edited Sep 7, 2011 at 0:15 Singular1ty asked Sep 7, 2011 at 0:04 Singular1tySingular1ty 2,61511 gold badge2525 silver badges3939 bronze badges 16

• 8 Might not want to start your question off by saying you couldn't be bothered to read other questions that may answer yours. – James Montagne Commented Sep 7, 2011 at 0:06
• A failure to plan on your part does not constitute an emergency on my part. Read the damned answers. – Chris Eberle Commented Sep 7, 2011 at 0:08
• I'll have you know @Chris, I was given this code. I have to work with it. Thanks for your support. – Singular1ty Commented Sep 7, 2011 at 0:09
• 3 This isn't a troll. This question has already been asked, and as per the FAQ this question shouldn't even be here. – Chris Eberle Commented Sep 7, 2011 at 0:13
• 2 user-Unknown: Did you consider that people may share his feelings? When new to a site, perhaps you should try a bit harder to adhere to the rules/attitude of the site. The spirit of this site is not "here's my homework! Finish it! Now!!!" He was simply trying to point that out. (Note to anyone who reads this: The original post, before editing had a much different tone than the one now. It originally had something along the lines of "I can't be miffed to read any other posts, so do this for me") – Corbin Commented Sep 7, 2011 at 0:17

 |  Show 11 more ments

4 Answers 4

Sorted by: Reset to default 4

Use .toFixed():

var num = 45.34343434343;

num = num.toFixed(2); // "45.34"

Use the .toFixed() function in Javascript.

document.getElementById("answer").value = (parseFloat(document.getElementById("num1").value) - parseFloat(document.getElementById("num2").value)).toFixed(2);

Or an easier to see version:

var x = 3.14159265358979323;
alert(x.toFixed(2));   // 3.14

A very good description of how it works on MDN.

See here there is a function Number.toFixed(num) that do just that.

See more info and other options at W3Schools JS Numbers.

That problem occurs because in Javascript every Number is 64bit floating-point, there no such thing as an Integer.

The .toFixed() is useful, but be ware that correctly rounding number in Javascript (and many other languages) require more work.

There are different rounding rules for "Tie-Breaking" and you need to know what is the one you need because they are used in different scenarios. Look here.

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