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Converting string of 1s and 0s into binary value

Lets say that I have string which contains 1024 characters (represents 0 and 1). I want to present it as an number in decimal base (also as a string). The tricky part is that i have to do it in C/C++ without 3rd part libraries. Any clues?

Answer 1

There are probably more efficient ways, but I’d have an array of decimal digits, and implement a ‘left-shift’ function on it which runs from the least significant digit, doubling them and carrying over into the next.

It’s then just a job of reading your binary data in one bit at a time and ‘left-shifting’ the decimal array and ‘OR’ing in the binary digit as required.

Just iterate through the decimal digits to print out the answer.

void outputAsDecimal(char *binary)
{
   char digits[1000]; // arbitrary size for now

   for (int i=0; i< 1000; ++i)
     digits[i] = 0;

   while (*binary != 0)
   {

   // shift the digits, with carry
     int carry = 0;

     for (int i = 0; i< 1000; ++i)
     {
       int d = digits[i] *2 + carry;
       carry = d > 9;
       digits[i] = d % 10;
     }

   // or in the new bit
     if (*binary++ == '1') 
       digits[0] |= 1;
   }

    // output with leading zeroes!
    for (int i = 999; i >=0; --i)
    {
      putchar(digits[i] + '0'); // convert to ascii
    }
}

See it running here: http://ideone.com/CibAfw