Fast way to convert a binary number to a decimal number

Question

I have to convert a binary number like for example unsigned int bin_number = 10101010 into its decimal representation (i.e. 170) as quickly as possible

Answer 1

If you know the number of binary digits that you're dealing with and it's always fixed and the binary number comes in a string (as it would if read from a file or stdin) at runtime (i.e. compile time conversion not possible) then you could adopt this approach:

int to_binary( const char* c )
{
    return ( ( c[0] & 1 ) ? 0x80 : 0x00 ) |
           ( ( c[1] & 1 ) ? 0x40 : 0x00 ) |
           ( ( c[2] & 1 ) ? 0x20 : 0x00 ) |
           ( ( c[3] & 1 ) ? 0x10 : 0x00 ) |
           ( ( c[4] & 1 ) ? 0x08 : 0x00 ) |
           ( ( c[5] & 1 ) ? 0x04 : 0x00 ) |
           ( ( c[6] & 1 ) ? 0x02 : 0x00 ) |
           ( ( c[7] & 1 ) ? 0x01 : 0x00 );
}

This assumes an fixed eight digit binary number. called like this:

std::cout << to_binary("10101010") << std::endl;

If you had a sixteen bit number you could still use it:

const char* bin_number = "1010101010101010";

// Deal with 16 bits
std::cout << ( to_binary( bin_number ) << 8 | to_binary( bin_number + 8 ) ) << std::endl;

Note that there is clearly no bounds checking here and I'm relying on the fact that the LSB of '1' is always 1 and '0' is always 0 (so not validating that it's actually a binary input.)

Naturally, it's pretty specific and not very flexible, but it does the job and I'm not sure that you'd get much faster.