Find Recurring Sequence in a Fraction

Given a fraction, find a recurring sequence of digits if it exists, otherwise, print “No recurring sequence”.

Examples:

Input  : Numerator = 8, Denominator = 3
Output : Recurring sequence is 6 
Explanation : 8/3 = 2.66666666.......  

Input : Numerator = 50, Denominator = 22
Output : Recurring sequence is 27
Explanation : 50/22 = 2.272727272..... 

Input : Numerator = 11, Denominator = 2
Output : No recurring sequence
Explanation : 11/2 = 5.5

We strongly recommend that you click here and practice it, before moving on to the solution.

When does the fractional part repeat?
Let us simulate the process of converting fractions to decimals. Let us look at the part where we have already figured out the integer part, which is floor(numerator/denominator). Now we are left with ( remainder = numerator%denominator ) / denominator. 

If you remember the process of converting to decimal, at each step we do the following : 

1. Multiply the remainder by 10.
2. Append the remainder/denominator to the result.
3. Remainder = remainder % denominator.

At any moment, if the remainder becomes 0, we are done.
However, when there is a recurring sequence, the remainder never becomes 0. For example, if you look at 1/3, the remainder never becomes 0.

Below is one important observation : 
If we start with the remainder ‘rem’ and if the remainder repeats at any point in time, the digits between the two occurrences of ‘rem’ keep repeating.
So the idea is to store seen remainders in a map. Whenever a remainder repeats, we return the sequence before the next occurrence. 

Below is the implementation of the above idea. 

C++

// C++ program to find repeating
// sequence in a fraction
#include <bits/stdc++.h>
using namespace std;

// This function returns repeating sequence of
// a fraction.  If repeating sequence doesn't
// exist, then returns empty string
string fractionToDecimal(int numr, int denr)
{
    string res; // Initialize result

    // Create a map to store already
    // seen remainders, remainder is used
    // as key and its position in
    // result is stored as value.
    // Note that we need
    // position for cases like 1/6.
    // In this case,the recurring sequence
    // doesn't start from first
    // remainder.
    map<int, int> mp;
    mp.clear();

    // Find first remainder
    int rem = numr % denr;

    // Keep finding remainder until either remainder
    // becomes 0 or repeats
    while ((rem != 0)
           && (mp.find(rem) == mp.end()))
    {
        // Store this remainder
        mp[rem] = res.length();

        // Multiply remainder with 10
        rem = rem * 10;

        // Append rem / denr to result
        int res_part = rem / denr;
        res += to_string(res_part);

        // Update remainder
        rem = rem % denr;
    }

    return (rem == 0) ? "" : res.substr(mp[rem]);
}

// Driver code
int main()
{
    int numr = 50, denr = 22;
    string res = fractionToDecimal(numr, denr);
    if (res == "")
        cout << "No recurring sequence";
    else
        cout << "Recurring sequence is " << res;
    return 0;
}

Java

// Java program to find
// repeating sequence
// in a fraction
import java.util.*;
class GFG {

    // This function returns repeating
    // sequence of a fraction. If
    // repeating sequence doesn't
    // exist, then returns empty String
    static String fractionToDecimal(int numr, int denr)
    {
        // Initialize result
        String res = "";

        // Create a map to store already
        // seen remainders. Remainder is
        // used as key and its position in
        // result is stored as value.
        // Note that we need position for
        // cases like 1/6.  In this case,
        // the recurring sequence doesn't
        // start from first remainder.
        HashMap<Integer, Integer> mp = new HashMap<>();
        mp.clear();

        // Find first remainder
        int rem = numr % denr;

        // Keep finding remainder until
        //  either remainder becomes 0 or repeats
        while ((rem != 0) && (!mp.containsKey(rem)))
        {
            // Store this remainder
            mp.put(rem, res.length());

            // Multiply remainder with 10
            rem = rem * 10;

            // Append rem / denr to result
            int res_part = rem / denr;
            res += String.valueOf(res_part);

            // Update remainder
            rem = rem % denr;
        }

        if (rem == 0)
            return "";
        else if (mp.containsKey(rem))
            return res.substring(mp.get(rem));

        return "";
    }

    // Driver code
    public static void main(String[] args)
    {
        int numr = 50, denr = 22;
        String res = fractionToDecimal(numr, denr);
        if (res == "")
            System.out.print("No recurring sequence");
        else
            System.out.print("Recurring sequence is "
                             + res);
    }
}

// This code is contributed by gauravrajput1

Python3

# Python3 program to find repeating
# sequence in a fraction

# This function returns repeating sequence
# of a fraction.If repeating sequence doesn't
# exist, then returns empty string


def fractionToDecimal(numr, denr):

    # Initialize result
    res = ""

    # Create a map to store already seen
    # remainders. Remainder is used as key
    # and its position in result is stored
    # as value. Note that we need position
    # for cases like 1/6.  In this case,
    # the recurring sequence doesn't start
    # from first remainder.
    mp = {}

    # Find first remainder
    rem = numr % denr

    # Keep finding remainder until either
    # remainder becomes 0 or repeats
    while ((rem != 0) and (rem not in mp)):

        # Store this remainder
        mp[rem] = len(res)

        # Multiply remainder with 10
        rem = rem * 10

        # Append rem / denr to result
        res_part = rem // denr
        res += str(res_part)

        # Update remainder
        rem = rem % denr

    if (rem == 0):
        return ""
    else:
        return res[mp[rem]:]


# Driver code
numr, denr = 50, 22
res = fractionToDecimal(numr, denr)

if (res == ""):
    print("No recurring sequence")
else:
    print("Recurring sequence is", res)

# This code is contributed by divyeshrabadiya07

C#

// C# program to find repeating sequence
// in a fraction
using System;
using System.Collections.Generic;

class GFG {

    // This function returns repeating
    // sequence of a fraction. If
    // repeating sequence doesn't
    // exist, then returns empty String
    static string fractionToDecimal(int numr, int denr)
    {
        // Initialize result
        string res = "";

        // Create a map to store already
        // seen remainders. Remainder is
        // used as key and its position in
        // result is stored as value.
        // Note that we need position for
        // cases like 1/6.  In this case,
        // the recurring sequence doesn't
        // start from first remainder.
        Dictionary<int, int> mp
            = new Dictionary<int, int>();

        // Find first remainder
        int rem = numr % denr;

        // Keep finding remainder until
        // either remainder becomes 0
        // or repeats
        while ((rem != 0) && (!mp.ContainsKey(rem)))
        {

            // Store this remainder
            mp[rem] = res.Length;

            // Multiply remainder with 10
            rem = rem * 10;

            // Append rem / denr to result
            int res_part = rem / denr;
            res += res_part.ToString();

            // Update remainder
            rem = rem % denr;
        }

        if (rem == 0)
            return "";
        else if (mp.ContainsKey(rem))
            return res.Substring(mp[rem]);

        return "";
    }

    // Driver code
    public static void Main(string[] args)
    {
        int numr = 50, denr = 22;
        string res = fractionToDecimal(numr, denr);

        if (res == "")
            Console.Write("No recurring sequence");
        else
            Console.Write("Recurring sequence is " + res);
    }
}

// This code is contributed by rutvik_56

Javascript

<script>

// Javascript program to find
// repeating sequence
// in a fraction

    // This function returns repeating
    // sequence of a fraction. If
    // repeating sequence doesn't
    // exist, then returns empty String
    function fractionToDecimal(numr, denr)
    {
        // Initialize result
        let res = "";

        // Create a map to store already
        // seen remainders. Remainder is
        // used as key and its position in
        // result is stored as value.
        // Note that we need position for
        // cases like 1/6.  In this case,
        // the recurring sequence doesn't
        // start from first remainder.
        let mp = new Map();
        mp.clear();

        // Find first remainder
        let rem = numr % denr;

        // Keep finding remainder until
        //  either remainder becomes 0 or repeats
        while ((rem != 0) && (!mp.has(rem)))
        {
            // Store this remainder
            mp.set(rem, res.length);

            // Multiply remainder with 10
            rem = rem * 10;

            // Append rem / denr to result
            let res_part = Math.floor(rem / denr);
            res += res_part.toString();

            // Update remainder
            rem = rem % denr;
        }

        if (rem == 0)
            return "";
        else if (mp.has(rem))
            return res.substr(mp.get(rem));

        return "";
    }

// Driver program

      let numr = 50, denr = 22;
      let res = fractionToDecimal(numr, denr);
      if (res == "")
          document.write("No recurring sequence");
      else
          document.write("Recurring sequence is "
                             + res);

</script>

Output

Recurring sequence is 27

Time Complexity : O(N) 

Auxiliary Space : O(N) , as we use map as extra space.

This article is contributed by Dhruv Mahajan. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.