Efficiently convert an unsigned short to a char*

Question

What would be an efficient, portable way to convert a unsigned short to a char* (i.e. convert 25 to '25').

I'd like to avoid things such as getting (std::string) strings involved. Performance is important in this case since this conversion will need to happen quickly and often.

I was looking into things such as using sprintf but would like to explorer any and all ideas.

Answer 1

First off, do it right, then do it fast--only optimize if you can see for certain that a piece of code is not performant.

snprintf() into a buffer will do what you want. Is it the fastest possible solution? Not at all. But it is among the simplest, and it will suffice to get your code into a working state. From there, if you see that those calls to snprintf() are so laborious that they need to be optimized, then and only then seek out a faster solution.

Answer 2

An array of strings such that

array[25] = "25";
array[26] = "26";

array[255] = "255";

maybe? You could write a small program that generates the table source code for you quite easily, and then use this file in your project.

Edit: I don't get what you mean by you don't want to ge strings involved.

Answer 3

I would say at least try sprintf and since you have this tagged as C++, try StringStream, and actually profile them. In many cases the compiler is smart enough to build something that works pretty well. Only when you know it's going to be a bottleneck do you need to actually find a faster way.

Answer 4

I hacked together a test of various functions here, and this is what I came up with:

write_ushort: 7.81 s
uShortToStr: 8.16 s
convert: 6.71 s
use_sprintf: 49.66 s

(Write_ushort is my version, which I tried to write as clearly as possible, rather than micro-optimize, to format into a given character buffer; use_sprintf is the obvious sprintf(buf, "%d", x) and nothing else; the other two are taken from other answers here.)

This is a pretty amazing difference between them, isn't it? Who would ever think to use sprintf faced with almost an order of magnitude difference? Oh, yeah, how many times did I iterate each tested function?

// Taken directly from my hacked up test, but should be clear.
// Compiled with gcc 4.4.3 and -O2.  This test is interesting, but not authoritative.
int main() {
  using namespace std;
  char buf[100];

#define G2(NAME,STMT) \
  { \
    clock_t begin = clock(); \
    for (int count = 0; count < 3000; ++count) { \
      for (unsigned x = 0; x <= USHRT_MAX; ++x) { \
        NAME(x, buf, sizeof buf); \
      } \
    } \
    clock_t end = clock(); \
    STMT \
  }
#define G(NAME) G2(NAME,) G2(NAME,cout << #NAME ": " << double(end - begin) / CLOCKS_PER_SEC << " s\n";)
  G(write_ushort)
  G(uShortToStr)
  G(convert)
  G(use_sprintf)
#undef G
#undef G2

  return 0;
}

Sprintf converted the entire possible range of unsigned shorts, then did the whole range again 2,999 more times at about 0.25 µs per conversion, on average, on my ~5 year old laptop.

Sprintf is portable; is it also efficient enough for your requirements?

---

My version:

// Returns number of non-null bytes written, or would be written.
// If ret is null, does not write anything; otherwise retlen is the length of
// ret, and must include space for the number plus a terminating null.
int write_ushort(unsigned short x, char *ret, int retlen) {
  assert(!ret || retlen >= 1);

  char s[uint_width_10<USHRT_MAX>::value];  // easy implementation agnosticism
  char *n = s;
  if (x == 0) {
    *n++ = '0';
  }
  else while (x != 0) {
    *n++ = '0' + x % 10;
    x /= 10;
  }

  int const digits = n - s;
  if (ret) {
    // not needed by checking retlen and only writing to available space
    //assert(retlen >= digits + 1);

    while (--retlen && n != s) {
      *ret++ = *--n;
    }
    *ret = '\0';
  }
  return digits;
}

Compile-time log TMP functions are nothing new, but including this complete example because it's what I used:

template<unsigned N>
struct uint_width_10_nonzero {
  enum { value = uint_width_10_nonzero<N/10>::value + 1 };
};
template<>
struct uint_width_10_nonzero<0> {
  enum { value = 0 };
};
template<unsigned N>
struct uint_width_10 {
  enum { value = uint_width_10_nonzero<N>::value };
};
template<>
struct uint_width_10<0> {
  enum { value = 1 };
};

Answer 5

try this:

int convert(unsigned short val, char* dest)
{
  int i = 0;
  if (val > 10000)
  {
    dest[i++] = (val / 10000) | 0x30;
    val %= 10000;
  }
  if (val > 1000)
  {
    dest[i++] = (val / 1000) | 0x30;
    val %= 1000;
  }
  if (val > 100)
  {
    dest[i++] = (val / 100) | 0x30;
    val %= 100;
  }
  if (val > 10)
  {
    dest[i++] = (val / 10) | 0x30;
    val %= 10;
  }
  dest[i++] = (val) | 0x30;
  dest[i] = 0;
  return i;
}