CODEWITHSUNDEEP
c++
Dmitriano
Dmitriano

Reputation: 2060

How to find the position of the first 1 in a double value?

How to by a given value like 0.00...001 find the position of 1?

I have a sample code like this:

constexpr int PrecisionFromDouble(double val)
{
    int d = 1.0 / val;
    int log = 0;

    while (true)
    {
        d = d / 10;
        if (d == 0)
            break;
        ++log;
    }
    return log;
}

It works with the following values:

static_assert(PrecisionFromDouble(0.0001) == 4);
static_assert(PrecisionFromDouble(0.001) == 3);
static_assert(PrecisionFromDouble(0.01) == 2);
static_assert(PrecisionFromDouble(0.1) == 1);
static_assert(PrecisionFromDouble(1.0) == 0);

but does not work with 0.00001, because d becomes 9999.

EDIT1:

It works a bit better with std::round in the first line of the function:

    int d = static_cast<int>(std::round(1.0 / val));

this should work with the most double literals like 0.00..001

Upvotes: 1

Views: 191

Answers (3)

Vlad Feinstein
Vlad Feinstein

Reputation: 11311

This one works (corrected after phuclv's comment):

constexpr int PrecisionFromDouble(double val)
{
  int log = 0;
  unsigned long long m(10);

  while (1. / m >= val)
    log++, m*= 10;

  return log;
}

Upvotes: 0

AbdelAziz AbdelLatef
AbdelAziz AbdelLatef

Reputation: 3744

I think you can use log10

#include <math.h>
constexpr int PrecisionFromDouble(double val)
{
    return -(int)log10(val);
}

Upvotes: 1

user14289352
user14289352

Reputation: 

main(){ 
    int error;
    float fvalue=0.0001;
    char *cstr = ftoa(dvalue,&error);
    string str(cstr);
    size_t position = str.find_first_of("1",0);
    cout << position <<endl;
}

Upvotes: 1

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