fputs vs. fprintf and double

Question

I need to put out a double value into an file. So for all my other string i have to put out i use fputs because i think and timed it that fputs is faster then fprintf.

But if i want to put out the double value i can't do this with fputs so i tried fprintf. This is very slow. I need more time if i replace the fprintf and puts there an fputs("TEST",file);

How can i put out the double as fast as the string. Need i to convert it first to string?

I found some conversion. But if i use them i am as slow as if i use the fprintf.

So how can i put out the double to a file so fast as if i put out a string to the file?

  fputs("TEST",fileout); //FAST
  fprintf(fileout, "%f",p); //SLOW

EDIT:

Times with fprintf double:

END.PROCESSING.DATA: 2013-04-26 08:10:33.000
END.CALC.IN.FILE: 2013-04-26 08:11:13.000

Times with fputs strings:

END.PROCESSING.DATA: 2013-04-26 08:14:10.000
END.CALC.IN.FILE: 2013-04-26 08:14:37.000

The code for that times:

now = time (0);
strftime (buff, 100, "%Y-%m-%d %H:%M:%S.000", localtime (&now));
printf ("END.PROCESSING.DATA: %s\n", buff);

//output file
FILE *fileout;
fileout = fopen("out.txt","w");
double p;
for(i=0; i<17000;i++){
  for(j=0; j<i;j++){
    for(z=0;z<400;z++){
      //DO SOME WORK
    }
    p = 1.0;
    fputs(allsubsts[i],fileout);
    fputs(";",fileout);
    fputs(allsubsts[j],fileout);
    fputs(";",fileout);
    //fprintf(fileout, "%.21g", p);
    fputs("TEST",fileout);
    fputs(";\n",fileout);
  }
}
fclose(fileout);

now = time(0);
strftime(buff,100,"%Y-%m-%d %H:%M:%S.000", localtime(&now));
printf("END.CALC.IN.FILE: %s\n",buff);

Changes between the time are only the change that i put the double instead the TEST string in the file.

1. Is it text or binary file? 2. `%f` is for `float`, `%lf` is for `double`. — Crozin, Apr 25 '13 at 15:22
@Crozin: No, both `%f` and `%lf` are for double. There is no way to pass a `float` to `fprintf`. — R.. GitHub STOP HELPING ICE, Apr 25 '13 at 15:23
@R..: [Of cource, you're right](http://www.cplusplus.com/reference/cstdio/fprintf/) - my mistake. — Crozin, Apr 25 '13 at 15:28
Measuring speed with date prints before and after writing to the file...very simple — bladepit, Apr 25 '13 at 18:37
have edited my question with a example and how i measuring speed — bladepit, Apr 26 '13 at 06:19

score 4 · Answer 1 · answered Apr 25 '13 at 15:22

Converting a floating point number to a decimal representation is a fundamentally slow operation. You might try using the %a specifier instead of %f to store it as a hex float; this should be much faster and would safely preserve the original value rather than a poor approximation. If you need decimal, you should be using %.21g or similar, not %f, as the latter will lose most of the precision.

If none of this is fast enough for you, then you probably need to save the binary representation of the value directly.

the binary representation is a very good idea but the file needs to be loaded by an other program so it needs so be in this format...i will try your formats — bladepit, Apr 25 '13 at 18:21

fputs vs. fprintf and double

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