Modulo for doubles

Question

I need to work out if a massive integer (I mean 20 digits...) is prime.

I'm trying to use the brute-force method, but (of course) I need to use doubles to contain the original number. However, the modulo operator (%) is an integer operator - thus it is useless to me!

score 18 · Accepted Answer · edited May 23 '17 at 12:26

18

That's not possible, a double only has 15 significant digits. Look for an implementation of a BigInt class. C specific is discussed here.

edited May 23 '17 at 12:26

Community

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answered Dec 16 '09 at 13:53

Hans Passant

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Interesting link on first glance, I'll make sure and read it! Thanks for taking the time. – silentcontributor Dec 16 '09 at 14:07

score 3 · Answer 2 · answered Dec 16 '09 at 14:05

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Since the double data type is stored as a fractional value scaled to some power of two, and since it only has a precision of 15 decimal digits, a 20-digit number stored as a double will always be divisible by two, and therefore, is not prime.

answered Dec 16 '09 at 14:05

Jeffrey L Whitledge

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That explains one of the bugs I was getting - thanks for the input! – silentcontributor Dec 16 '09 at 14:06

score 1 · Answer 3 · answered Dec 16 '09 at 13:54

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Are you looking for fmod in the C standard library? Or possibly fmodlfor long double?

answered Dec 16 '09 at 13:54

It's fmodl for long double. Thanks! – silentcontributor Dec 16 '09 at 14:04

Modulo for doubles

3 Answers3