Binary Search for Rational Numbers without using floating point arithmetic


A rational is represented as p/qb, for example 2/3. Given a sorted array of rational numbers, how to search an element using Binary Search. Use of floating point arithmetic is not allowed.


Input:  arr[] = {1/5, 2/3, 3/2, 13/2}
        x = 3/2
Output: Found at index 2

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To compare two rational numbers p/q and r/s, we can compare p*s with q*r.

// C program for Binary Search for Rationalnal Numbers
// without using floating point arithmetic
#include <stdio.h>

struct Rational
    int p;
    int q;

// Utility function to compare two Rationalnal numbers
// 'a' and 'b'. It returns
// 0 --> When 'a' and 'b' are same
// 1 --> When 'a' is greater
//-1 --> When 'b' is greate
int compare(struct Rational a, struct Rational b)
    // If a/b == c/d  then  a*d = b*c:
    // method to ignore division
    if (a.p * b.q == a.q * b.p)
        return 0;
    if (a.p * b.q > a.q * b.p)
        return 1;
    return -1;

// Returns index of x in arr[l..r] if it is present, else
// returns -1. It mainly uses Binary Search.
int binarySearch(struct Rational arr[], int l, int r,
                 struct Rational x)
   if (r >= l)
        int mid = l + (r - l)/2;

        // If the element is present at the middle itself
        if (compare(arr[mid], x) == 0)  return mid;

        // If element is smaller than mid, then it can
        // only be present in left subarray
        if (compare(arr[mid], x) > 0)
            return binarySearch(arr, l, mid-1, x);

        // Else the element can only be present in right
        // subarray
        return binarySearch(arr, mid+1, r, x);

   return -1;

// Driver method
int main()
    Rational arr[] = {{1, 5}, {2, 3}, {3, 2}, {13, 2}};
    Rational x = {3, 2};
    int n = sizeof(arr)/sizeof(arr[0]);
    printf("Element found at index %d",
            binarySearch(arr, 0, n-1, x));


Element found at index 2

Thanks to Utkarsh Trivedi for suggesting above solution.

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