Calculate number of decimal digits in BigInteger

Question

I'm trying to use BigInteger values along side Math.Log10 method.

final BigInteger answerNo = fact;
final int digits = 1 + (int)Math.floor(Math.log10(answerNo));

Unfortunately the compiler says incompatible types.

If I change the ints to BigIntegers it still doesn't like it.

I've changed the title of this question to make it comply with the information in the question and accepted answer. Obviously counting the digits is not as precise as performing an actual 10 based logarithm. — Maarten Bodewes, Sep 26 '15 at 14:19

score 7 · Accepted Answer · answered Sep 30 '12 at 17:01

7

Instead of doing a log10 you can find the number of digits by simply doing:

int digits = answerNo.toString().length();

answered Sep 30 '12 at 17:01

epsalon

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If that's the goal, then it would indeed be a lot simpler! – assylias Sep 30 '12 at 17:03
Might want to subtract 1 from that :) – Ray Toal Sep 30 '12 at 17:04
That's irrelevant. You need neither subtract nor add one if you want to get the number of digits. @RayToal Only if you wanted to compute the floored log_10 – phant0m Sep 30 '12 at 17:10
This is great. So much simpler. Thanks a lot. Perfect answer. – binary101 Sep 30 '12 at 17:25
@epsalon OH! Of course, the OP wanted one plus the log. My miss. – Ray Toal Sep 30 '12 at 21:57

score 1 · Answer 2 · answered Sep 30 '12 at 16:59

1

You can't assign an int to a BigInteger - you could write: final BigInteger answerNo = BigInteger.valueOf(fact);
Mat.log10 expects a double, not a BigInteger

If you don't mind a loss of precision, you could simply use doubles:

final int digits = 1 + (int)Math.floor(Math.log10(fact));

If you care about precision, you will need to calculate the log manually.

answered Sep 30 '12 at 16:59

assylias

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Good idea, but it breaks when digits > 314, for example. – WebComer Dec 27 '18 at 20:15

score 0 · Answer 3 · answered Jul 16 '20 at 09:54

0

BigInteger.toString() can be inefficient in terms of memory if you only care for count of digits.

You can instead try converting BigInteger into a BigDecimal and use the BigDecimal.precision() to achieve the same. See this answer for details.

answered Jul 16 '20 at 09:54

ajay

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Calculate number of decimal digits in BigInteger

3 Answers3