Calculate the log base n with Shift left or Shift right

Question

I have a little problem. Who knows how we can calculate the log base n with Shift_L or Shift_R?

for example: for n=2 we had this solution:

int log(int n){
int res = 0;
while((n>>=1))
    res++;
return res;
}

This question appears to be off-topic because it is not about programming — Walter, Sep 18 '13 at 14:44
Sorry, I do not write well. I want to solve it using shift >> — Amirhossein Mahdinejad, Sep 18 '13 at 15:18

score 32 · Accepted Answer · answered Sep 18 '13 at 14:09

32

You don't seem to want the logarithm for a base b, but the largest integer n so that n <= log_b(x). If that's the case, the following function should serve your needs:

int intlog(double base, double x) {
    return (int)(log(x) / log(base));
}

answered Sep 18 '13 at 14:09

Carsten

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Thanks! And how about doubleLog? – Amirhossein Mahdinejad Sep 18 '13 at 14:30
1

@AmirhosseinMehdinejad What's a "doubleLog"? – Carsten Sep 18 '13 at 15:04
detailed log of x base n – Amirhossein Mahdinejad Sep 18 '13 at 15:14
1

@AmirhosseinMehdinejad What is a "detailed log"? If you mean the logarithm without truncating everything after the decimal point, just get rid of the `int` cast in the function. – Carsten Sep 18 '13 at 16:49
Due to floating point precision, `int` casting (or applying `floor`, which is equivalent) will mess up the result in cases like `intlog(10, 1000)`: the base 10 log of 1000 is 3, but it will be stored as `2.999999[...]`, and thus casting it to `int` will return `2`. – Anakhand Sep 15 '18 at 11:12

score 8 · Answer 2 · answered Sep 18 '13 at 14:07

8

well this is rather a math problem instead of an actuall programming problem, if i understand your problem correctly:

log_2 (x) = log_a (x) / log_a (2) where a can be any base.

Therefore you could use the math.h's function log(double)

double res = log(x)/log(2);

answered Sep 18 '13 at 14:07

Teh Suu

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I'm just 16 years old, and i don't know so much about logarithm ! – Amirhossein Mahdinejad Sep 18 '13 at 14:14
16

I have a Phd in engineering and I had to look this up too! :D – Dr. Andrew Burnett-Thompson Jun 25 '19 at 10:15

Calculate the log base n with Shift left or Shift right

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