Decimal to Irrational fraction approximation

Question

I have implemented an algorithm for floating point decimal to rational fraction approximation (example: 0.333 -> 1/3) and now I wonder, is there a way to find an irrational number which satisfies the condition. For example, given the input 0.282842712474 I want the result to be sqrt(2)/5 and not 431827/1526739 which my algorithm produces. The only condition is that the first digits of the result (converted back to floating point) should be the digits of the input, the rest doesn't matter. Thanks in advance!

Answer 1

I came up with solution, that from given set of possible denominators and nominators finds best approximation of given number.

For example this set can contain all numbers that can be created by:
1 <= radicand <= 100000
1 <= root_index <= 20

If set has N elements, than this solution finds best approximation in O(N log N).

In this solution X represents denominator and Y nominator.

1. sort numbers from set
2. for each number X from set:
   using binary find smallest Y such that Y/X >= input_number
   compare Y/X with currently best approximation of input_number

I couldn't resist and I implemented it:

#include <cstdio>
#include <vector>
#include <algorithm>
#include <cmath>
using namespace std;

struct Number {
  // number value
  double value;

  // number representation
  int root_index;
  int radicand;

  Number(){}
  Number(double value, int root_index, int radicand)
    : value(value), root_index(root_index), radicand(radicand) {}

  bool operator < (const Number& rhs) const {
    // in case of equal numbers, i want smaller radicand first
    if (fabs(value - rhs.value) < 1e-12) return radicand < rhs.radicand;
    return value < rhs.value;
  }

  void print() const {
    if (value - (int)value < 1e-12) printf("%.0f", value);
    else printf("sqrt_%d(%d)",root_index, radicand); 
  }
};

std::vector<Number> numbers;
double best_result = 1e100;
Number best_numerator;
Number best_denominator;

double input;

void compare_approximpation(const Number& numerator, const Number& denominator) {
   double value = numerator.value / denominator.value;

   if (fabs(value - input) < fabs(best_result - input)) {
      best_result = value;
      best_numerator = numerator;
      best_denominator = denominator;
   }
}

int main() {

  const int NUMBER_LIMIT = 100000;
  const int ROOT_LIMIT = 20;

  // only numbers created by this loops will be used
  // as numerator and denominator
  for(int i=1; i<=ROOT_LIMIT; i++) {
     for(int j=1; j<=NUMBER_LIMIT; j++) {
        double value = pow(j, 1.0 /i);
        numbers.push_back(Number(value, i, j));
     }
  }

  sort(numbers.begin(), numbers.end());

  scanf("%lf",&input); 

  int numerator_index = 0;

  for(int denominator_index=0; denominator_index<numbers.size(); denominator_index++) {
    // you were interested only in integral denominators
    if (numbers[denominator_index].root_index == 1) {
      // i use simple sweeping technique instead of binary search (its faster)
      while(numerator_index < numbers.size() && numbers[numerator_index].root_index &&
    numbers[numerator_index].value / numbers[denominator_index].value <= input) {
      numerator_index++;
      }

      // comparing approximations
      compare_approximpation(numbers[numerator_index], numbers[denominator_index]);
      if (numerator_index > 0) {
    compare_approximpation(numbers[numerator_index - 1], numbers[denominator_index]);
      }
    }
  }

  printf("Best approximation %.12lf = ", best_numerator.value / best_denominator.value);
  best_numerator.print();
  printf(" / ");
  best_denominator.print();
  printf("\n");
}