C Read file doubles with leading spaces

Question

I have a file with a header of two lines, and the rest contains two columns of double data like

    0.0030556304       -0.0078125

The first column is 16 characters long, and the second column is 17 characters long, after there is a space and the '\n'.

My code to read this file is

nscan = fscanf(sound_file, "%lf %lf %c", &value1,
                                          &value2,
                                          &termch);

I use termch to test further for newline character.

I've tried with

nscan = fscanf(sound_file, "%16lf %16lf %c", &value1,
                                          &value2,
                                          &termch);

too.

But when I printf the with

printf("%f %f\n", value1, value2);

the result is

-0.000000 -0.000000

Am I missing something?

@CoolGuy: that is not useful: `printf("%f")` is exactly the same as `printf("%lf")`. The documentation says so. — John Zwinck, Oct 04 '14 at 14:21
@JohnZwinck ,So the difference comes when using it in `scanf`? — Spikatrix, Oct 04 '14 at 14:25
Thanks to all guys, I finally discovered that I already has read the file until the end. So I made **rewind** to put pointer in beginning of the file. Now it's works. — Caco, Oct 04 '14 at 14:26
@CoolGuy: http://stackoverflow.com/questions/210590/why-does-scanf-need-lf-for-doubles-when-printf-is-okay-with-just-f — mafso, Oct 04 '14 at 15:14
fscanf is fine, however, %lf format specifier for double variable and %f for float, be consistent in that — Dr. Debasish Jana, Oct 04 '14 at 17:37

chux - Reinstate Monica · Answer 1 · 2014-10-04T16:30:13.083

fscanf(sound_file, "%lf %lf %c"... is not doing what you think.

The space before "%c" consumes all white-space including ' ' and '\n'. The following "%c" will consume the next char (which must be non-white-space at this point), if any.

Better to read all files lines using fgets() and then scan the buffer with sscanf()

char buf[100];


// Read 2 header lines and toss
if (fgets(buf, sizeof buf, sound_file) == NULL) Handle_EOForIOError();
if (fgets(buf, sizeof buf, sound_file) == NULL) Handle_EOForIOError();

// Insure code is using a type the match format specifiers
// Could be a problem in OP's unposted code.
double value1, value2;

while (fgets(buf, sizeof buf, sound_file) != NULL) [
  int cnt = sscanf(buf, "%lf%lf", &value1, &value2);
  if (cnt != 2) Handle_MissingData();
  else Use(value1, value2);
}

fscanf("%lf",... scans for optional leading white-space and then for a double. A leading " " before the "%lf" is not needed.

fscanf("%16lf",... will limit the total number of non-white-space char scanned to 16.

C Read file doubles with leading spaces

1 Answers1