Efficiently Convert Between Hex, Binary, and Decimal in C/C++

Efficiently convert between Hex, Binary, and Decimal in C/C++

As others have pointed out, I would start with sscanf(), printf() and/or strtoul(). They are fast enough for most applications, and they are less likely to have bugs. I will say, however, that these functions are more generic than you might expect, as they have to deal with non-ASCII character sets, with numbers represented in any base and so forth. For some domains it is possible to beat the library functions.

So, measure first, and if the performance of these conversion is really an issue, then:

1) In some applications / domains certain numbers appear very often, for example zero, 100, 200, 19.95, may be so common that it makes sense to optimize your functions to convert such numbers with a bunch of if() statements, and then fall back to the generic library functions.
2) Use a table lookup if the most common 100 numbers, and then fall back on a library function. Remember that large tables may not fit in your cache and may require multiple indirections for shared libraries, so measure these things carefully to make sure you are not decreasing performance.

You may also want to look at boost lexical_cast functions, though in my experience the latter are relatively compared to the good old C functions.

Tough many have said it, it is worth repeating over and over: do not optimize these conversions until you have evidence that they are a problem. If you do optimize, measure your new implementation to make sure it is faster and make sure you have a ton of unit tests for your own version, because you will introduce bugs :-(

Convert a hexadecimal string to an integer efficiently in C?

You want strtol or strtoul. See also the Unix man page

Efficient Conversion of a Binary Number to Hexadecimal String

Here's a solution with nothing but shifts, and/or, and add/subtract. No loops either.

uint64_t x, m;
x = 0xF05C1E3A;
x = ((x & 0x00000000ffff0000LL) << 16) | (x & 0x000000000000ffffLL);
x = ((x & 0x0000ff000000ff00LL) << 8)  | (x & 0x000000ff000000ffLL);
x = ((x & 0x00f000f000f000f0LL) << 4)  | (x & 0x000f000f000f000fLL);
x += 0x0606060606060606LL;
m = ((x & 0x1010101010101010LL) >> 4) + 0x7f7f7f7f7f7f7f7fLL;
x += (m & 0x2a2a2a2a2a2a2a2aLL) | (~m & 0x3131313131313131LL);

Above is the simplified version I came up with after a little time to reflect. Below is the original answer.

uint64_t x, m;
x = 0xF05C1E3A;
x = ((x & 0x00000000ffff0000LL) << 16) | (x & 0x000000000000ffffLL);
x = ((x & 0x0000ff000000ff00LL) << 8) | (x & 0x000000ff000000ffLL);
x = ((x & 0x00f000f000f000f0LL) << 4) | (x & 0x000f000f000f000fLL);
x += 0x3636363636363636LL;
m = (x & 0x4040404040404040LL) >> 6;
x += m;
m = m ^ 0x0101010101010101LL;
x -= (m << 2) | (m << 1);

See it in action: http://ideone.com/nMhJ2q

Convert strings between hex format and binary format

You can use a combination of std::stringstream, std::hex and std::bitset to convert between hex and binary in C++03.

Here's an example:

#include <iostream>
#include <sstream>
#include <bitset>
#include <string>

using namespace std;

int main()
{
    string s = "0xA";
    stringstream ss;
    ss << hex << s;
    unsigned n;
    ss >> n;
    bitset<32> b(n);
    // outputs "00000000000000000000000000001010"
    cout << b.to_string() << endl;
}

EDIT:

About the refined question, here's a code example about converting between hex strings and binary strings (you can refactor with a helper function for the hex char<>bits part, and use a map or a switch instead, etc).

const char* hex_char_to_bin(char c)
{
    // TODO handle default / error
    switch(toupper(c))
    {
        case '0': return "0000";
        case '1': return "0001";
        case '2': return "0010";
        case '3': return "0011";
        case '4': return "0100";
        case '5': return "0101";
        case '6': return "0110";
        case '7': return "0111";
        case '8': return "1000";
        case '9': return "1001";
        case 'A': return "1010";
        case 'B': return "1011";
        case 'C': return "1100";
        case 'D': return "1101";
        case 'E': return "1110";
        case 'F': return "1111";
    }
}

std::string hex_str_to_bin_str(const std::string& hex)
{
    // TODO use a loop from <algorithm> or smth
    std::string bin;
    for(unsigned i = 0; i != hex.length(); ++i)
       bin += hex_char_to_bin(hex[i]);
    return bin;
}

How would I convert decimal into binary?

A simple function could be defined such as this:

void binary(int decimal) {
   int remainder;

   if(decimal <= 1) {
       std::cout << decimal;
       return;
   }
   remainder = decimal % 2;
   binary(decimal >> 1);    
   std::cout << remainder;
}

Although there are many other resources on the web on how to do this..

A noteworthy question for efficiency of it here, as you may want more than just that:
Efficiently convert between Hex, Binary, and Decimal in C/C++

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