Convert double to BigInteger

Question

How do you convert a double to a BigInteger, as accurately as possible? BigInteger doesn't have a valueOf(double) method. It does have a valueOf(long) method, but converting via long, would destroy any value outside the 2^64 range, and in particular I'm trying to get large numbers like 1e100 to convert to their corresponding integer values to the sixteen or so significant figures allowed by floating point precision.

i hope you know that `Double.MAX_VALUE` is somewhere around 1.7*10^308... — Martin Frank, Feb 14 '18 at 10:50

score 5 · Accepted Answer · answered Feb 14 '18 at 10:48

5

You can convert the double into a BigDecimal and then into a BigInteger:

BigInteger k = BigDecimal.valueOf(doublevalue).toBigInteger();

answered Feb 14 '18 at 10:48

Mathias G.

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score 1 · Answer 2 · answered Feb 14 '18 at 10:47

1

You could use BigDecimal.valueOf(double) and then call toBigIntegerExact() on that. Like,

BigInteger bi = BigDecimal.valueOf(Math.PI).toBigIntegerExact();

answered Feb 14 '18 at 10:47

Elliott Frisch

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That would throw an error, because is *π* not exactly 3 anyway. If you want to avoid errors, use `toBigInteger()`. – Rudy Velthuis Feb 14 '18 at 19:54

Convert double to BigInteger

2 Answers2