Simple fprintf question

[GGarramuno]

I have a program that expects its input in a specific format.
Mainly, it expects floating-point values to be formatted in the form:

1.32
1.
3.
3.2345

In case you missed it, all floating point values have a period, even when they
are round numbers. This is a somewhat similar behavior to the %g flag of
printf, but %g always strips the period from round numbers.
Using %#f or %#g is also no good as it just fills in trailing zeros.
I seem to be unable to force fprintf() to print out in such a way, but I'm sure
I am missing something obvious. Any ideas?

 

[Barry Schwarz]

I have a program that expects its input in a specific format.
Mainly, it expects floating-point values to be formatted in the form:

1.32
1.
3.
3.2345

In case you missed it, all floating point values have a period, even when they
are round numbers. This is a somewhat similar behavior to the %g flag of
printf, but %g always strips the period from round numbers.
Using %#f or %#g is also no good as it just fills in trailing zeros.
I seem to be unable to force fprintf() to print out in such a way, but I'm sure
I am missing something obvious. Any ideas?

You could use sprintf and then adjust the string to suit your needs.


<<Remove the del for email>>

 

[Emmanuel Delahaye]

GGarramuno wrote on 02/08/04 :

I have a program that expects its input in a specific format.
Mainly, it expects floating-point values to be formatted in the form:

1.32
1.
3.
3.2345

This is a special formatting. I'm not sure you can avoid to code it
yourself.

In case you missed it, all floating point values have a period, even when
they are round numbers.

Floating points are rarely 'round numbers'. There are mainly
approximations.

This is a somewhat similar behavior to the %g flag of
printf, but %g always strips the period from round numbers.
Using %#f or %#g is also no good as it just fills in trailing zeros.
I seem to be unable to force fprintf() to print out in such a way, but I'm
sure I am missing something obvious. Any ideas?

Yes, what's wrong with "%.4f"

1.3200
1.0000
3.0000
3.2345

A space problem?

If so, you have to use it with sscanf() to prepare the string, and to
'manually' trim the trailing zeros.

 

[Martin Ambuhl]

I have a program that expects its input in a specific format.
Mainly, it expects floating-point values to be formatted in the form:

1.32
1.
3.
3.2345

In case you missed it, all floating point values have a period, even when they
are round numbers. This is a somewhat similar behavior to the %g flag of
printf, but %g always strips the period from round numbers.
Using %#f or %#g is also no good as it just fills in trailing zeros.
I seem to be unable to force fprintf() to print out in such a way, but I'm sure
I am missing something obvious. Any ideas?

#include <stdio.h>
#include <string.h>

void remove_trailing_zero(char *s);

int main(void)
{
double tvals[] = { 1.32, 1., 3., 3.2345 };
size_t nvals = sizeof tvals / sizeof *tvals, i;
char buf[BUFSIZ];
for (i = 0; i < nvals; i++) {
sprintf(buf, "%f", tvals);
printf("%%f -> %s\n", buf);
remove_trailing_zero(buf);
printf(" remove_trailing_zero -> %s\n", buf);
}
return 0;
}


void remove_trailing_zero(char *s)
{
char *t;
if (s && *s && strchr(s, '.'))
for (t = s + strlen(s) - 1; *t == '0'; t--)
*t = 0;
}



%f -> 1.320000
remove_trailing_zero -> 1.32
%f -> 1.000000
remove_trailing_zero -> 1.
%f -> 3.000000
remove_trailing_zero -> 3.
%f -> 3.234500
remove_trailing_zero -> 3.2345

 

[CBFalconer]

I have a program that expects its input in a specific format.
Mainly, it expects floating-point values to be formatted in the
form:

1.32
1.
3.
3.2345

In case you missed it, all floating point values have a period,
even when they are round numbers. This is a somewhat similar
behavior to the %g flag of printf, but %g always strips the
period from round numbers. Using %#f or %#g is also no good as
it just fills in trailing zeros.

This may be of use. Compile it with:

gcc -o putflt -D TESTING putflt.c

/* ---------- FILE putflt.h ---------- */
/* Print out floating point numbers
Public Domain, by C.B. Falconer.
Attribution appreciated */

#ifndef H_putflt_h
# define H_putflt_h 1
# ifdef __cplusplus
extern "C" {
# endif

/* (putchar) is a suitable value */
typedef int (*putchr)(int ch);

/* ------------ */

/* Returns the count of columns used, or -ve for error
Calling with a dummy dest allows evaluating fields */
int putflt(double val, int sigdigs, putchr dest);

/* ------------ */

/* write the value right justified in field, do not truncate
* Negative value for field left justifies.
* return value signifies field actually used */
int putfltfld(double val, int sigdigs, int field, putchr dest);

/* ------------ */

# ifdef __cplusplus
}
# endif
#endif
/* ---------- EOF FILE putflt.h ---------- */

/* Print out floating point numbers demo, file putflt.c */
/* Public Domain, by C.B. Falconer. Attribution appreciated */

#include "putflt.h"

/* ------------ */

/* Returns the count of columns used, or -ve for error
Calling with a dummy dest allows evaluating fields */
int putflt(double val, int sigdigs, putchr dest)
{
int dp;
int ch;
int used;

used = 0;
if (sigdigs <= 0) sigdigs = 1;

if (val < 0) {
dest('-'); used++;
val = - val;
}
/* Normalize */
dp = 0;
if (val != 0.0) {
while (val >= 10.0) {
val = val / 10.0; dp++;
}
while (val < 1.0) {
val = 10.0 * val; dp--;
}
}
/* Handle leading zeroes */
if (dp < 0) {
dest('0'); dest('.'); used += 2;
while (++dp) {
dest('0'); used++;
}
dp--;
}
/* The real work */
do {
ch = val; val = val - ch;
dest(ch + '0'); used++;
val = 10.0 * val;
if (0 == dp--) {
dest('.'); used++;
}
} while (--sigdigs > 0);

/* handle trailing zeroes */
while (dp >= 0) {
dest('0'); used++;
if (0 == dp--) {
dest('.'); used++;
}
}
/* Handle isolated '.' at end */
if (-1 == dp) {
dest('0'); used++;
}
return used; /* no errors handled yet */
} /* putflt */

/* ------------ */

/* do nothing, to allow evaluating field needed */
static int dummy(int ch)
{
return ch;
} /* dummy */

/* ------------ */

/* write the value right justified in field, do not truncate
* Negative value for field left justifies.
* return value signifies field actually used */
int putfltfld(double val, int sigdigs, int field, putchr dest)
{
int needed;

needed = putflt(val, sigdigs, dummy);
if (field > 0)
while (needed < field) {
dest(' '); needed++;
}
putflt(val, sigdigs, dest);
if (field < 0)
while ((needed + field) < 0) {
dest(' '); needed++;
}
return needed;
} /* putfltfld */

/* ------------ */

#ifdef TESTING

#include <stdio.h>

#define show(fld) printf(" Field = %d\n", fld)

int main(void)
{
double val;
int i, fld;

val = 10.0 / 7.0; /* A normalized value, 6 dig rep */

fld = putflt(-val / 100, 10, (putchar)); show(fld);
fld = putflt(-val, 10, (putchar)); show(fld);
fld = putflt(-val * 100, 10, (putchar)); show(fld);
fld = putflt(val / 100, 10, (putchar)); show(fld);
fld = putflt(val, 10, (putchar)); show(fld);
for (i = 12; i > 0; i--) {
val = 10 * val;
fld = putflt(val, 10, (putchar)); show(fld);
}
fld = putflt(0.0, 10, (putchar)); show(fld);
fld = putflt(1e-30, 10, (putchar)); show(fld);
fld = putflt(1e30, 10, (putchar)); show(fld);
fld = putflt(0.1e-30, 10, (putchar)); show(fld);
fld = putflt(0.2e-30, 10, (putchar)); show(fld);
fld = putflt(0.3e-30, 10, (putchar)); show(fld);
fld = putflt(0.01e-30, 10, (putchar)); show(fld);
fld = putflt(0.001e-30, 10, (putchar)); show(fld);

val = 30.0 / 13.0; /* Another normalized value */
fld = putfltfld(-val, 10, 20, (putchar)); show(fld);
fld = putfltfld(-val * 100, 10, -20, (putchar)); show(fld);
fld = putfltfld(val / 100, 10, -20, (putchar)); show(fld);
fld = putfltfld(val, 10, 20, (putchar)); show(fld);
fld = putfltfld(val, 25, 35, (putchar)); show(fld);

val = 5419351.0 / 1725033.0; /* Another normalized value */
fld = putfltfld(val, 25, 35, (putchar)); show(fld);

return 0;
} /* main putflt demo */
#endif
/* ---------- EOF FILE putflt.c ---------- */