How do you randomly zero a bit in an integer?

Question

Updated with newer answer and better test

Let\'s say I have the number 382 which is 101111110.

How could I randomly turn a bit which is not 0 to

Answer 1

You can generalize this by using BitArray.

public static BitArray FlipRandomTrueBit(BitArray bits)
{
    List trueBits = new List();

    for (int i = 0; i < bits.Count; i++)
        if (bits[i])
            trueBits.Add(i);

    if (trueBits.Count > 0)
    {
        int index = rnd.Next(0, trueBits.Count);
        bits[trueBits[index]] = false;
    }

    return bits;
}

However then you will have to write helper functions for simple data types.

public static int FlipRandomTrueBit(int input)
{
    BitArray bits = new BitArray(new int[] { input });
    BitArray flipedBits = FlipRandomTrueBit(bits);

    byte[] bytes = new byte[4];
    flipedBits.CopyTo(bytes, 0);

    int result = BitConverter.ToInt32(bytes, 0);
    return result;
}

If your using a large bit array you could save memory by iterating twice.

public static void FlipRandomTrueBitLowMem(ref BitArray bits)
{
    int trueBits = 0;

    for (int i = 0; i < bits.Count; i++)
        if (bits[i])
            trueBits++;

    if (trueBits > 0)
    {
        int flip = rnd.Next(0, trueBits);

        for (int i = 0; i < bits.Count; i++)
        {
            if (bits[i])
            {
                if (flip == 0)
                {
                    bits[i] = false;
                    break;
                }

                flip--;
            }
        }
    }
}

Test Program.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace bitarray
{
    class Program
    {
        private static Random rnd = new Random((int)DateTime.Now.Ticks);

        public static BitArray FlipRandomTrueBit(BitArray bits)
        {
            List trueBits = new List();

            for (int i = 0; i < bits.Count; i++)
                if (bits[i])
                    trueBits.Add(i);

            if (trueBits.Count > 0)
            {
                int index = rnd.Next(0, trueBits.Count);
                bits[trueBits[index]] = false;
            }

            return bits;
        }

        public static int FlipRandomTrueBit(int input)
        {
            BitArray bits = new BitArray(new int[] { input });
            BitArray flipedBits = FlipRandomTrueBit(bits);

            byte[] bytes = new byte[4];
            flipedBits.CopyTo(bytes, 0);

            int result = BitConverter.ToInt32(bytes, 0);
            return result;
        }

        static void Main(string[] args)
        {
            int test = 382;
            for (int n = 0; n < 200; n++)
            {
                int result = FlipRandomTrueBit(test);
                Console.WriteLine(result);
            }

            Console.ReadLine();
        }
    }
}