Reverse binary representation of int (only significant bits)

Question

I'm trying to write a program for reversing numbers in binary. For instance, the binary representation of 13 is 1101, and reversing it gives 1011, which corresponds to number 11 right?

Here's my code:

static void Main(string[] args)
{
 Console.WriteLine("Enter a Number");
 int numb = int.Parse(Console.ReadLine());
 int reverse = 0;
 while (numb > 0)
 {
 int rem = numb % 10;
 reverse = (reverse * 10) + rem;
 numb = numb / 10;
 }
 Console.WriteLine("Reverse number={0}", reverse);
 Console.ReadLine();
}

By this code I only get the numbers to reverse (13 -> 31)...

The input should contain a single line with an integer N, 1≤N≤1000000000 and I want my output in one line with one integer, the number I want to get by reversing the binary representation of N.

• Jon Skeet: "By this code I only get the numbers to reverse..." - Yes, because you're using 10 everywhere, which means you're getting the decimal digits, not the binary ones...

Answer 1

Something like that

// 13 = 1101b
int value = 13;
// 11 = 1011b
int result = Convert.ToInt32(new String(
 Convert.ToString(value, 2)
 .Reverse()
 .ToArray()), 2);

Explanation:

• Convert.ToString(value, 2) returns value in binary representation ("1101")
• Reverse().ToArray() - reverse the string ('1','0','1','1') as sequence of characters and converts to array char[].
• new String(...) constructs string "1011" from array of char
• finally, Convert.ToInt32(..., 2) convert binary representation back to int

Answer 2

You can use Convert.ToString and Convert.ToInt32 methods, where 2 means binary:

int numb = int.Parse(Console.ReadLine());
var reversedString = Convert.ToString(numb, 2).ReverseString();
var result = Convert.ToInt32(reversedString, 2);
...
public static string ReverseString(this string s)
{
 char[] arr = s.ToCharArray();
 Array.Reverse(arr);
 return new string(arr);
}

Answer 3

A fun excercise would be doing this without using the string conversion.

I have very little experience with bit twiddling so there is probably a faster and better way of doing this, but this seems to work:

 public static IEnumerable<bool> ToBinary(this int n)
 {
 for (int i = 0; i < 32; i++)
 {
 yield return (n & (1 << i)) != 0;
 }
 }
 public static int ToInt(this IEnumerable<bool> b)
 {
 var n = 0;
 var counter = 0;
 foreach (var i in b.Trim().Take(32))
 {
 n = n | (i ? 1 : 0) << counter;
 counter++
 }
 return n;
 }
 private static IEnumerable<bool> Trim(this IEnumerable<bool> list)
 {
 bool trim = true;
 foreach (var i in list)
 {
 if (i)
 {
 trim = false;
 }
 if (!trim)
 {
 yield return i;
 }
 }
 }

And now you'd call it like this:

var reversed = n.ToBinary().Reverse().ToInt();