Re: fscanf performance > 10x slower in 2.1.69 compared to 2.1.50 on Windows

On 12-Apr-2005, Wlmsears Comcast <wlmsears@xxxxxxxxxxx> wrote:

| I did some investigating as to the source of this problem, and have 
| determined that the stream inserter for floating point numbers is quite 
| slow.  This accounts for about 2/3s of the time. About 10% of the time is 
| spent extracting and replacing characters to handle NA, Nan, and Inf 
| entries.
| 
| The following code is about 3x faster (slower than before, but perhaps 
| acceptable). It replaces the "octave_scan" template specialization for 
| double* in oct-stream.cc.
| 
| #define TMP_FP_BUFSIZE 128
| template <>
| std::istream&
| octave_scan (std::istream& is, const scanf_format_elt& fmt, double* valptr)
| {
|   double& ref = *valptr;
| 
|   char buf[TMP_FP_BUFSIZE];
|   switch (fmt.type)
|     {
|     case 'e':
|     case 'f':
|     case 'g':
|       {
|         is.width(TMP_FP_BUFSIZE-1);    // Protect image against strange file
|         buf[TMP_FP_BUFSIZE-2] = 0;     // Zero char sentinel
|         is >> buf;
|         if (is && buf[TMP_FP_BUFSIZE-2]==0) // Not eof, and field wasn't 
| gigantic
|           {
|             if (isdigit(buf[0])  || buf[0]=='+'  || buf[0] == '-' || 
| buf[0]=='.')
|               {
|                 ref =  atof(buf);
|               }
|             else if (strcmp(buf, "NA")==0)
|               {
|                 ref = octave_NA;
|               }
|             else if (strcmp(buf, "NaN")==0)
|               {
|                 ref = octave_NaN;
|               }
|             else if (strcmp(buf, "Inf") == 0)
|               {
|                 ref = octave_Inf;
|               }
|             else {
|                 is.setstate(std::ios_base::failbit);
|              }
|            }
|         else {  // Unparsable number (too long).  Set fail and quit
|             if (buf[TMP_FP_BUFSIZE-2]==0) 
| is.setstate(std::ios_base::failbit);
|         }
|         break;
|       }
|     default:
|       panic_impossible ();
|       break;
|     }
|   return is;
| } 

With this solution, what happens to the file position after reading
a file

  123 abc

?  I would expect to be able to read a file like this with

  number = fscanf (f, "%f");
  text = fscanf (f, "%s");

and have number = 123 and text = "abc".  It seems that the code above
would read the 123 set failbit and skip the part containing "abc".

If the slowdown is in the C++ class that implements the stream
extractor for double precision numbers, then perhaps the best place to
report the problem is to the libstdc++ maintainers?

jwe



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