fputs vs. fprintf and double

问题

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.

回答1:

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.

来源：https://stackoverflow.com/questions/16218614/fputs-vs-fprintf-and-double

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