Convert float to string without trailing 0s

[Mike]

Hi there,

I am trying to convert a float/double number to a string using the
sprintf().
For example, if I have a float number 123.456, and I want to convert
it to "123.456". But with the sprintf() function, I always get
"123.456001". My question is how can I get the expected string without
those trailing 0s?

Thanks in advance.

Mike

 

[Martin Ambuhl]

Hi there,

I am trying to convert a float/double number to a string using the
sprintf().
For example, if I have a float number 123.456, and I want to convert
it to "123.456". But with the sprintf() function, I always get
"123.456001". My question is how can I get the expected string without
those trailing 0s?

#include <stdio.h>

int main(void)
{
double dx = 123.456;
float fx = 123.456f;
long double ldx = 123.456L;

printf("This example uses 'printf'.\n"
"It is obviously trivial to change it to use sprintf.\n\n");
printf("Printing 123.456\n"
" using %%f %f, %%g %g, %%.6g %.6g, %%.3f %.3f\n\n",
dx, dx, dx, dx);
printf("Printing 123.456f\n"
" using %%f %f, %%g %g, %%.6g %.6g, %%.3f %.3f\n\n",
fx, fx, fx, fx);
printf("Printing 123.456L\n"
" using %%Lf %Lf, %%Lg %Lg, %%.6Lg %.6Lg, %%.3Lf %.3Lf\n\n",
ldx, ldx, ldx, ldx);
return 0;
}


This example uses 'printf'.
It is obviously trivial to change it to use sprintf.

Printing 123.456
using %f 123.456000, %g 123.456, %.6g 123.456, %.3f 123.456

Printing 123.456f
using %f 123.456001, %g 123.456, %.6g 123.456, %.3f 123.456

Printing 123.456L
using %Lf 123.456000, %Lg 123.456, %.6Lg 123.456, %.3Lf 123.456

 

[Mike.Hao]

That's what I want. Thank you so much, Martin.

Mike

 

[Chris Croughton]

I am trying to convert a float/double number to a string using the
sprintf().
For example, if I have a float number 123.456, and I want to convert
it to "123.456". But with the sprintf() function, I always get
"123.456001". My question is how can I get the expected string without
those trailing 0s?

Use %g, not %f (%g will automatically go to 'scientific' format when the
number is too big or too small, though). If you know that you only want
a certain number of decomal places, you can use %.3f (for example).

Chris C

 

[Keith Thompson]

I am trying to convert a float/double number to a string using the
sprintf().
For example, if I have a float number 123.456, and I want to convert
it to "123.456". But with the sprintf() function, I always get
"123.456001". My question is how can I get the expected string without
those trailing 0s?

No you don't have 123.456; that value isn't exactly representable in
binary floating-point. You probably have something like
123.45600128173828125, and sprintf() is doing exactly what you asked
it to.

You can limit the precision with a format specifier:

#include <stdio.h>
int main(void)
{
float f = 123.456;
char s[80];
sprintf(s, "%.20f", f);
printf("s = \"%s\"\n", s);
sprintf(s, "%.3f", f);
printf("s = \"%s\"\n", s);
return 0;
}

produces:

s = "123.45600128173828125000"
s = "123.456"

Determining how many digits to use is left as an excercise (it's not
necessarily obvious).

 

[Keith Thompson]

No you don't have 123.456; that value isn't exactly representable in
binary floating-point. You probably have something like
123.45600128173828125, and sprintf() is doing exactly what you asked
it to.

And I forgot to mention: Read section 14 of the C FAQ
<http://www.eskimo.com/~scs/C-faq/top.html>.

 

[Mike]

Hi All,

Thank you all for your replies. Now I got a new problem when I tried
to use the sprintf() with double arrays. I have two double arrays,
e.g.:

double values[] = {1234, 23456.89};
double values2[] = {34709.22, 3746.55, 23456};

After applied to sprinf(), I got:

{"1234", "23456.9"}
{"34709.2", "3746.55", "23456"}

Since what I want to do is a "pure" conversion from floating point to
string, and I don't know what the precision is in advance, so how
should I do it?

Thanks again.

Mike

 

[Walter Roberson]

Since what I want to do is a "pure" conversion from floating point to
string, and I don't know what the precision is in advance, so how
should I do it?

You cannot. Most numbers have no exact representation as floating
point / double . For example, no matter what the precision you use,
there is no exact representation for 1/3 or even 0.3 . When
your code has double values[] = {1234, 23456.89};
then the knowledge that the second number had exactly two decimal
places is gone and what gets stored internally is a number which
is -close to- 23456.89, but not exactly the same.

 

[Al Bowers]

Hi All,

Thank you all for your replies. Now I got a new problem when I tried
to use the sprintf() with double arrays. I have two double arrays,
e.g.:

double values[] = {1234, 23456.89};
double values2[] = {34709.22, 3746.55, 23456};

After applied to sprinf(), I got:

{"1234", "23456.9"}
{"34709.2", "3746.55", "23456"}

Since what I want to do is a "pure" conversion from floating point to
string, and I don't know what the precision is in advance, so how
should I do it?

It seems that this question has been answered.
I might add that you can write code to remove trailing '0'
as function DoubletoStr does below, but you will still
have problems addressed in the faq question 14.1 at
http://www.eskimo.com/~scs/C-faq/q14.1.html

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

char *DoubletoStr(char *s, double dd);

int main(void)
{
double values[] = {1234, 23456.89};
double values2[] = {34709.22, 3746.55, 23456};
char value[32];
int i;

for(i = 0; i < 2;i++)
printf("values[%d] = \"%s\"\n",i,
DoubletoStr(value,values));
putchar('\n');
for(i = 0; i < 3;i++)
printf("values2[%d] = \"%s\"\n",i,
DoubletoStr(value,values2));
return 0;
}

char *DoubletoStr(char *s, double dd)
{
char *endp;

sprintf(s,"%f",dd);
for(endp = s+strlen(s); endp!=s;endp--)
{
if(*(endp-1) != '0'&& *(endp-1) != '.')
{
*endp = '\0';
break;
}
}
return s;
}

 

[Martin Ambuhl]

Hi All,

Thank you all for your replies. Now I got a new problem when I tried
to use the sprintf() with double arrays. I have two double arrays,
e.g.:

double values[] = {1234, 23456.89};
double values2[] = {34709.22, 3746.55, 23456};

After applied to sprinf(), I got:

{"1234", "23456.9"}
{"34709.2", "3746.55", "23456"}

Since what I want to do is a "pure" conversion from floating point to
string, and I don't know what the precision is in advance, so how
should I do it?

#include <stdio.h>
#include <float.h>

int main(void)
{
double values[] = { 1234, 23456.89 };
double values2[] = { 34709.22, 3746.55, 23456 };
size_t i;

printf("double values[] -> \"");
for (i = 0; i < 2; i++)
printf("%.*g\"%s", DBL_DIG, values, (i) ? "\n" : ", ");

printf("double values2[] -> \"");
for (i = 0; i < 3; i++)
printf("%.*g\"%s", DBL_DIG, values2, (i == 2) ? "\n" : ", ");

return 0;
}

[output]
double values[] -> "1234", 23456.89"
double values2[] -> "34709.22", 3746.55", 23456"