Cuberoot doesn't give correct answer

Question

4.00/5 (1 vote)

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Hello!

I used this:

C#

double secondnum = Math.Pow(num, 1.0 / 3);
            if (secondnum - (int)secondnum == 0)
            {
                return true;
            }
            else return false;

But, if I check it with this numb:66430125 then it gives 404.99999999999989, and in calculator it gives 405. So How could I solve this one?

Posted 23-Dec-10 12:24pm

velvet7

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Comments

Dr.Walt Fair, PE 23-Dec-10 21:10pm

Welcome to the wonderful world of floating point math on limited precision computers!

2 solutions

Solution 1

Ha-ha. This is a correct answer with expected accuracy.

Posted 23-Dec-10 12:30pm

Sergey Alexandrovich Kryukov

Comments

Dalek Dave 23-Dec-10 20:18pm

Close enough for Windows! :)

Espen Harlinn 26-Feb-11 11:10am

A 40 >> 3 :)

Sergey Alexandrovich Kryukov 26-Feb-11 19:17pm

Is it shift or "much more"? :-)
--SA

Espen Harlinn 27-Feb-11 4:56am

An attempt at humor yielding 5 :)

Sergey Alexandrovich Kryukov 1-Mar-11 3:01am

I maybe need to sleep. Could not even figure out about humor and the humorist...
--SA

Espen Harlinn 1-Mar-11 12:33pm

40 >> 3 gives you 5

Sergey Alexandrovich Kryukov 1-Mar-11 13:47pm

You have wonderful patience. And now, after I had enough sleep...
--SA

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Toli Cuturicu · Accepted Answer · 2010-12-23T13:47:00

Solution 2

Floating point Types (double and float) are not exact. You have to do some rounding, for example:

C#

double secondNum = Math.Round(Math.Pow(num, 1.0 / 3), 9);

Posted 23-Dec-10 13:47pm

Toli Cuturicu

Comments

Dalek Dave 23-Dec-10 20:18pm

Good answer.

Sergey Alexandrovich Kryukov 25-Dec-10 0:20am

Toli, I don't think rounding can improve accuracy. You just pick a special case when the accuracy looks better after rounding. In average situation, the rounding makes accuracy worse.
It's very bad to do rounding in intermediate result in calculation program, even if you count dollars and cents. Practically, one can create a calculation algorithm yielding any preset relative error.

Don't mix this issue with nice rounding used for textual presentation; this is more about formatting than about real accuracy.

One real purpose of rounding is preparation for conversion to integer; it also used in random number generations, etc.