Pointer to convert fraction to binary

Hi,
I am using a function which takes a floating point fraction and converts it to binary upto 24 sig fig.I am passing a pointer pointing to an array (where the binary will be stored)to the function and then increasing the address by 1 (pointer arithmetic allows to point the variable to the next entry of the array).But when I run the code it seems the only one item gets copied into the array.
Here is my code:
/*this program converts a fractional decimal to binary e,g 0.625 should give .1010000000000000 upto 22 sig fig*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char myMantissa[25]; /* where the binary number is stored*/
char *fractiontoBinary(double a,char *b){
for(int i=0;i<22;i++)
{
if(a*2>=1)
{
*b='1';
b=b+1;
a=(a*2)-1;
}
else
{
*b='0';
b=b+1;
a=a*2;}
}

return b;
}

int main (void)
{double input;
scanf("%d",&input);
char *s= fractiontoBinary(input,myMantissa);

myMantissa[23]='\0';
printf("%d",myMantissa[1]);
return 0;
}

It looks like you are missing a closing curly brace. So...how did this even compile? BTW...this is a C-Sharp Programming forum. It looks like this is C code.

You are only displaying Mantissa[1], that is one character. You need to pass a pointer to the start of the array in the printf statement as follows...

printf("&#37;s", myMantissa);

I recoded this in C# for the benfit of others in this forum, but unless my palgarisum is bad, and I'm an expert at it so ;)... it seems the algorithm does not work?

Not the best C# code, but just tried to match it to the exact C code given for the benefit of the author.

Code:

---

public Foo()
{
    String b;
    b = FractionToBinary(0.625, out b);  // returns all '1's
}

private String FractionToBinary(Double a, out String b)
{
    b = String.Empty;

    for (Int32 i = 0; i < 22; i++)
    {
        if (a * 2 >= 1)    // always true...  and 'a' only increases in size
        {
            b += '1';
            a = (a * 2) + 1;  // should this be divide not multiply?
        }
        else
        {
            b += '0';
            a *= 2;  // does not work even with divide in both cases?
        }
    }

    return b;
}

---

Quote:

---

Originally Posted by rliq

You are only displaying Mantissa[1], that is one character. You need to pass a pointer to the start of the array in the printf statement as follows...

printf("%s", myMantissa);

I recoded this in C# for the benfit of others in this forum, but unless my palgarisum is bad, and I'm an expert at it so ;)... it seems the algorithm does not work?

Not the best C# code, but just tried to match it to the exact C code given for the benefit of the author.

---

Notice that rliq used "%s" as a format specifier in his printf statement which is very different from "%d". The "%s" tells the printf function to read bytes from an array until a null byte is encountered. The %d on the other hand tells the printf function to read a value as an integer. So the binary representation of your character at myMantissa[1] (e.g. '1') would be interpreted as the ASCII value not the character you intended. I also noticed that I was wrong about the missing curly brace. Sorry about that.

For those interested, here is a C# function that will take a base 10 Double N (0.0 < N < 1.0) and return the Binary equivelent to a maximum number of decimal places.

Code:

---

using System;

namespace BinFract
{
    class Program
    {
        static void Main(string[] args)
        {
            String binary1 = DoubleToBinary(0.625, 22);

            String binary2 = DoubleToBinary(Math.PI - 3.0, 100);

            // binary1 = 0.101  (as expected)

            // binary2 = 0.001001000011111101101010100010001000010110100011

            // so PI = 11.001001000011111101101010100010001000010110100011... ;)
        }

        //  Decimal    Binary
        //      4      100
        //      2        10
        //      1        1
        //      0.5      0.1      // dividing the decimal by 2, moves the binary '1', one place to the right,
        //      0.25      0.01      // same as dividing it by 10 in base10 (1000, 100, 10, 1, 0.1, 0.01, 0.001...)
        //      0.125    0.001
        // etc
        //
        // So in decimal 0.625 = 0.5 + 0.125.  Therefore in Binary = 0.1 + 0.001 = 0.101

        /// <summary>
        /// Return the binary of a Base 10 number to a maximum number of decimal places
        /// </summary>
        /// <param name="base10">The number to be converted (must be less that 1.0)</param>
        /// <param name="decimalPlaces">The maximum number of decimal places</param>
        /// <returns>A string containing the binary representation</returns>
        private static String DoubleToBinary(Double base10, Int32 decimalPlaces)
        {
            // divided by 2 (because binary)
            Double divisor = 1.0 / 2.0; 

            // this will hold the output
            String binary = "0.";

            for (Int32 i = 0; i < decimalPlaces; i++)
            {
                if (base10 >= divisor)
                {
                    binary += '1';
                    base10 -= divisor;
                }
                else
                    binary += '0';

                // divide by 2 (because binary)
                divisor /= 2.0;
            }

            // trim the insignificant zero's
            return binary.TrimEnd('0');
        }
    }
}

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