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# What is the correct result of this computation?

Bartender
Posts: 1679
17
Using BigDecimal for the simple calculation below, I get one result in Java (and Python). But using an online calculator and another program, I get another result.

Java Code:

Result:

0.000000000000045454545454545453631211255875255243966038341187705575727952898226001045920632102272727272727272727272727272727272727272727272727272727273

----

But, at this website: https://apfloat.appspot.com/

I get a different result:

.000000000001 / 22
0.00000000000004545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545

--

Which is correct and why?

Thanks,

-- mike

Ranch Foreman
Posts: 3298
22
• 1
The second is correct - at least, it's what most of us would want.  To see what's wrong with the first, add this line:

I get

which is not what we expect for something that should be 0.000000000001.  Unfortunately, that's the sort of thing that can happen when we use the double primitive type.  The solution is to not use the constructor BigDecimal(double) - see the [url=https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/math/BigDecimal.html#%3Cinit%3E(double)]API for BigDecimal(double) for an explanation why.  Instead, try using BigDecimal(String):

That should give you what you want...

Mike London
Bartender
Posts: 1679
17

Mike Simmons wrote:The second is correct - at least, it's what most of us would want.  To see what's wrong with the first, add this line:

I get

which is not what we expect for something that should be 0.000000000001.  Unfortunately, that's the sort of thing that can happen when we use the double primitive type.  The solution is to not use the constructor BigDecimal(double) - see the [url=https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/math/BigDecimal.html#%3Cinit%3E(double)]API for BigDecimal(double) for an explanation why.  Instead, try using BigDecimal(String):

That should give you what you want...

Thanks very much!

- mike

Marshal
Posts: 65037
247
Also look up the BigDecimal#valueOf(double) method and see whether that would help you.

Ranch Hand
Posts: 573
1
You could also do 1/22, the only thing that changes is the decimal point.  This shows the second result is indeed correct (adjusting the decimal point, of course).

As others have said, you've just discovered the joys of big numbers and floating point math on computers.

Campbell Ritchie
Marshal
Posts: 65037
247

Jim Venolia wrote:You could also do 1/22 . . .

You mean 1.0 / 22; 1 / 22 evaluates to 0

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