reverse equivalent of strtod, etc, other than sprintf

And for strtod's inverse :

    double value = ...;
    sprintf(buf + strlen(buf), "%lf", value);

of course :)

There's itoa, ltoa, etc.

>> There's itoa, ltoa, etc.

Those are not ANSI C, nor are they POSIX ... if that matters to drunnels.

infinity08, your solution does what he explicitly said not to do, ie scan multiple times through the data using sprintf. You first find the length of the buffer (one scan), then parse out the format specifier. Next time, you would have to do it again.

I looked, and though I can see libc.a has itoa etc. for djgpp, it does not compile under linux, and the routines are listed as not being posix. This is rather strange. Can anyone comment?

#include <stdlib.h>
#include <stdio.h>

int main(int argc, char* argv[]) {
  int x = 5;
  char buffer[256];
  printf("%d\n", itoa(x, buffer, 10));
}

And in addition to these integer conversion routines, there's 'gcvt()', 'fcvt()' a´nd 'ecvt()' for floating point data (http://www.icewalkers.com/Linux/ManPages/fcvt-3.html).

>> Can anyone comment?

As I said : they are not standard functions (not ANSI C and not POSIX). They are non-standard extensions that are supported by some compilers.


>> infinity08, your solution does what he explicitly said not to do, ie scan multiple times through the data using sprintf.

No, his biggest problem with sprintf() was that it didn't keep track of the position ... that is solved by what I proposed.
His second-biggest problem with sprintf was that the format string needs to be parsed ... that's still the case with my solution obviously, and that's why I asked if that was acceptable.

The alternative is to write the function yourself, so I'd argue that my solution is the simplest way of doing what he wants to do, and on top of that it's portable.

drunnels, what do you think ?

A C++ solution is to use stringstream:

#include <sstream>
#include <iomanip>
using namespace std;

   i = 5;
   ostringstream oss;
   oss << i;
   cout << oss.str() << endl;

oss.str() is a std::string. If you need a char buffer you could get it by

   char buf[32];
   strcpy(buf, oss.str().c_str());

You also could make formatting like

     // right justified 5 digits padded up with zeros
    oss << setw(5) << right << setfill('0') << i;

The reverse is safe and easier than strtod, strtol.

   istringstream iss("12345.12ABC");
   double d;
   iss >> d;
   if (iss.fail())
        cout << "12345.12ABC" << " is not a number. " << endl;

Regards, Alex

check out %n format specifier: http://www.cplusplus.com/reference/clibrary/cstdio/printf.html
you can get rid of strlen() proposed by inifinity08.

other than that, you can use c++ code already mentioned.

Linux (i.e.glibc) sprintf returns the number of bytes output (not including trailing NUL). (BSD doesn't - it returns buffer address). If you need to constrain the output to avoid buffer overflow, use snprintf, which also guarantees to fit in a trailing NUL (which simpler functions like strncpy() don't). Not all implementations of snprintf() are that smart - so far I have only encountered  Windows libraries that aren't.

the ANSI standard says that sprintf returns the number of characters written in the array, not
counting the terminating null character, or a negative value if an encoding error occurred.