Rounding a floating point number

[jacob navia]

Hi

"How can I round a number to x decimal places" ?

This question keeps appearing. I would propose the following
solution

#include <float.h>
#include <math.h>

double roundto(double x, int digits)
{
int sgn=1;

if (x == 0)
return 0;
if (digits DBL_DIG)
digits = DBL_DIG;
else if (digits < 1)
digits = 1;

if(x < 0.0) {
sgn = -sgn;
x = -x;
}
double p = floorl(log10l(x));
p--;
p = digits-p;
double pow10 = pow(10.0, p);
return sgn*floor(x*pow10+0.5)/pow10;
}

long double roundtol(long double x, int digits)
{
int sgn=1;

if (x == 0)
return 0;
if (digits LDBL_DIG)
digits = LDBL_DIG;
else if (digits < 1)
digits = 1;

if(x < 0.0) {
sgn = -sgn;
x = -x;
}
long double p = floorl(log10l(x));
p--;
p = digits-p;
long double pow10 = powl(10.0, p);
return sgn*floorl(x*pow10+0.5)/pow10;
}
#include <stdio.h>
int main(void)
{
double d = 1.7888889988996678;
long double ld = 1.7888889988996678998877L;

for (int i = 0; i<=DBL_DIG;i++) {
printf("i=%d: %.15g\n",i,roundto(d,i));
}
printf("\n 1.7888889988996678\n");
for (int i = 0; i<=LDBL_DIG;i++) {
printf("i=%d: %.18Lg\n",i,roundtol(ld,i));
}
printf("\n 1.7888889988996678998877L\n");
return 0;
}

--------------------------------------------------------------------
I would propose it to add it to the FAQ.
--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique
http://www.cs.virginia.edu/~lcc-win32

[Richard Heathfield]

jacob navia said:

Hi

"How can I round a number to x decimal places" ?

This question keeps appearing.

<snip>

I would propose it to add it to the FAQ.

Good idea. You could call it Question 14.6.

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Bartc]

"Richard Heathfield" <rj*@see.sig.invalidwrote in message
news:xc******************************@bt.com...

jacob navia said:

>Hi

"How can I round a number to x decimal places" ?

This question keeps appearing.

<snip>

>I would propose it to add it to the FAQ.

Good idea. You could call it Question 14.6.

14.6 is already used by How to Round a Number [to an integer]

--
Bart

[jacob navia]

Richard Heathfield wrote:

jacob navia said:

>Hi

"How can I round a number to x decimal places" ?

This question keeps appearing.

<snip>

>I would propose it to add it to the FAQ.

Good idea. You could call it Question 14.6.

Your kill file is not working apparently
:-)

That question addresses the rounding to nearest integer.
This answers the question of rounding to a number of decimal places

Not the same thing. But yes, we could add this to 14.6

--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique
http://www.cs.virginia.edu/~lcc-win32

[MisterE]

14.6 is already used by How to Round a Number [to an integer]

Then what need is there for this question? Its already answered in the faq.
Multiply, round then divide.

[Richard Heathfield]

Bartc said:

>
"Richard Heathfield" <rj*@see.sig.invalidwrote in message
news:xc******************************@bt.com...

>jacob navia said:

>>Hi

"How can I round a number to x decimal places" ?

This question keeps appearing.

<snip>

>>I would propose it to add it to the FAQ.

Good idea. You could call it Question 14.6.

14.6 is already used by How to Round a Number [to an integer]

From the FAQ, Q14.6: "You can round to a certain precision by scaling:

(int)(x / precision + 0.5) * precision"

(int)(3.141 / 0.01 + 0.5) * 0.01 gives us:

(int)(314.1 + 0.5) * 0.01 which is

(int)314.6 * 0.01 which is

314 * 0.01 which is

3.14

QED

(Of course, the usual caveats about floating point apply here.)

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Martien Verbruggen]

On Mon, 25 Feb 2008 12:19:35 +0100,
jacob navia <ja***@nospam.comwrote:

Richard Heathfield wrote:

>jacob navia said:

>>Hi

"How can I round a number to x decimal places" ?

This question keeps appearing.

<snip>

>>I would propose it to add it to the FAQ.

Good idea. You could call it Question 14.6.

That question addresses the rounding to nearest integer.
This answers the question of rounding to a number of decimal places

It also answers the question of how to round to a certain number of
digits:

\begin{quote}
You can round to a certain precision by scaling:
\end{quote}

Martien
--
|
Martien Verbruggen | prepBut nI vrbLike adjHungarian! qWhat's
| artThe adjBig nProblem? -- Alec Flett
|

[Bartc]

"Richard Heathfield" <rj*@see.sig.invalidwrote in message
news:oZ*********************@bt.com...

Bartc said:

>>
"Richard Heathfield" <rj*@see.sig.invalidwrote in message
news:xc******************************@bt.com...

>>jacob navia said:

Hi

"How can I round a number to x decimal places" ?

This question keeps appearing.

<snip>

I would propose it to add it to the FAQ.

Good idea. You could call it Question 14.6.

14.6 is already used by How to Round a Number [to an integer]

From the FAQ, Q14.6: "You can round to a certain precision by scaling:

(int)(x / precision + 0.5) * precision"

(int)(3.141 / 0.01 + 0.5) * 0.01 gives us:

(int)(314.1 + 0.5) * 0.01 which is

(int)314.6 * 0.01 which is

314 * 0.01 which is

3.14

Yeah, you're right. Perhaps 14.6 should be written more clearly, with regard
to what exactly it means by Precision. And an example added, like yours.

>(int)(3.141 / 0.01 + 0.5) * 0.01

Somehow I would be happier if that was written as:

>(int)(3.141 *100 + 0.5) /100

Then any errors due to 0.01 not being exactly 1/100 are someone else's
fault.

--
Bart

[Mark McIntyre]

MisterE wrote:

>14.6 is already used by How to Round a Number [to an integer]

Then what need is there for this question?

<Fx: swoosh sound of clue going over head>

--
Mark McIntyre

CLC FAQ <http://c-faq.com/>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt>

[user923005]

On Feb 25, 2:18*am, jacob navia <[e-mail]:

Hi

"How can I round a number to x decimal places" ?

This question keeps appearing. I would propose the following
solution

#include <float.h>
#include <math.h>

double roundto(double x, int digits)
{
* * * * int sgn=1;

* * * * if (x == 0)
* * * * * * * * return 0;
* * * * if (digits DBL_DIG)
* * * * * * * * digits = DBL_DIG;
* * * * else if (digits < 1)
* * * * * * * * digits = 1;

* * * * if(x < 0.0) {
* * * * * * * * sgn = -sgn;
* * * * * * * * x = -x;
* * * * }
* * * * double p = floorl(log10l(x));
* * * * p--;
* * * * p = digits-p;
* * * * double pow10 = pow(10.0, p);
* * * * return sgn*floor(x*pow10+0.5)/pow10;

}

long double roundtol(long double x, int digits)
{
* * * * int sgn=1;

* * * * if (x == 0)
* * * * * * * * return 0;
* * * * if (digits LDBL_DIG)
* * * * * * * * digits = LDBL_DIG;
* * * * else if (digits < 1)
* * * * * * * * digits = 1;

* * * * if(x < 0.0) {
* * * * * * * * sgn = -sgn;
* * * * * * * * x = -x;
* * * * }
* * * * long double p = floorl(log10l(x));
* * * * p--;
* * * * p = digits-p;
* * * * long double pow10 = powl(10.0, p);
* * * * return sgn*floorl(x*pow10+0.5)/pow10;}

#include <stdio.h>
int main(void)
{
* * * * double d = 1.7888889988996678;
* * * * long double ld = 1.7888889988996678998877L;

* * * * for (int i = 0; i<=DBL_DIG;i++) {
* * * * * * * * printf("i=%d: %.15g\n",i,roundto(d,i));
* * * * }
* * * * printf("\n * * *1.7888889988996678\n");
* * * * for (int i = 0; i<=LDBL_DIG;i++) {
* * * * * * * * printf("i=%d: %.18Lg\n",i,roundtol(ld,i));
* * * * }
* * * * printf("\n * * *1.7888889988996678998877L\n");
* * * * return 0;

}

--------------------------------------------------------------------
I would propose it to add it to the FAQ.

This is the snippet's solution (which is very similar to yours):
/* round number n to d decimal points */
double fround(double n, unsigned d)
{
return floor(n * pow(10., d) + .5) / pow(10., d);
}

Of course, no solution is really going to be satisfying, as long as
the output format is floating point.
Another question is "What kind of rounding?" Do we want banker's
rounding or round to nearest or what?
Should we create a round function for each distinct floating point
type?

[user923005]

On Feb 25, 2:18*am, jacob navia <[e-mail]:

Hi

"How can I round a number to x decimal places" ?

This question keeps appearing. I would propose the following
solution

#include <float.h>
#include <math.h>

double roundto(double x, int digits)
{
* * * * int sgn=1;

* * * * if (x == 0)
* * * * * * * * return 0;
* * * * if (digits DBL_DIG)
* * * * * * * * digits = DBL_DIG;
* * * * else if (digits < 1)
* * * * * * * * digits = 1;

* * * * if(x < 0.0) {
* * * * * * * * sgn = -sgn;
* * * * * * * * x = -x;
* * * * }
* * * * double p = floorl(log10l(x));
* * * * p--;
* * * * p = digits-p;
* * * * double pow10 = pow(10.0, p);
* * * * return sgn*floor(x*pow10+0.5)/pow10;

}

long double roundtol(long double x, int digits)
{
* * * * int sgn=1;

* * * * if (x == 0)
* * * * * * * * return 0;
* * * * if (digits LDBL_DIG)
* * * * * * * * digits = LDBL_DIG;
* * * * else if (digits < 1)
* * * * * * * * digits = 1;

* * * * if(x < 0.0) {
* * * * * * * * sgn = -sgn;
* * * * * * * * x = -x;
* * * * }
* * * * long double p = floorl(log10l(x));
* * * * p--;
* * * * p = digits-p;
* * * * long double pow10 = powl(10.0, p);
* * * * return sgn*floorl(x*pow10+0.5)/pow10;}

#include <stdio.h>
int main(void)
{
* * * * double d = 1.7888889988996678;
* * * * long double ld = 1.7888889988996678998877L;

* * * * for (int i = 0; i<=DBL_DIG;i++) {
* * * * * * * * printf("i=%d: %.15g\n",i,roundto(d,i));
* * * * }
* * * * printf("\n * * *1.7888889988996678\n");
* * * * for (int i = 0; i<=LDBL_DIG;i++) {
* * * * * * * * printf("i=%d: %.18Lg\n",i,roundtol(ld,i));
* * * * }
* * * * printf("\n * * *1.7888889988996678998877L\n");
* * * * return 0;

}

--------------------------------------------------------------------
I would propose it to add it to the FAQ.

The snippets solution, tweaked a little bit:

#include <math.h>
/* round number n to d decimal points */
long double roundl(const long double n, const unsigned d)
{
long double p = powl(10., d);
return floorl(n * p + .5) / p;
}

/* round number n to d decimal points */
double roundd(const double n, const unsigned d)
{
return (double) roundl((long double) n, d);
}

/* round number n to d decimal points */
double roundf(const float n, const unsigned d)
{
return (float) roundl((long double) n, d);
}
#ifdef UNIT_TEST
#include <stdio.h>
#include <stdlib.h>
#include <float.h>
int main(void)
{
long double pi = 3.1415926535897932384626433832795;
long double npi = -pi;
unsigned digits;
for (digits = 0; digits <= LDBL_DIG; digits++) {
printf("Rounding by printf gives: %20.*f\n",
digits, pi);
printf("Rounding approximation by function gives: %20.*f\n",
LDBL_DIG, roundl(pi, digits));
printf("Rounding approximation by function gives: %20.*f\n",
DBL_DIG, roundd(pi, digits));
printf("Rounding approximation by function gives: %20.*f\n\n",
FLT_DIG, roundf(pi, digits));
}
for (digits = 0; digits <= LDBL_DIG; digits++) {
printf("Rounding by printf gives: %20.*f\n",
digits, npi);
printf("Rounding approximation by function gives: %20.*f\n",
LDBL_DIG, roundl(npi, digits));
printf("Rounding approximation by function gives: %20.*f\n",
DBL_DIG, roundd(npi, digits));
printf("Rounding approximation by function gives: %20.*f\n\n",
FLT_DIG, roundf(npi, digits));
}
return 0;
}
#endif
/*
Rounding by printf gives: 3
Rounding approximation by function gives: 3.000000000000000
Rounding approximation by function gives: 3.000000000000000
Rounding approximation by function gives: 3.000000

Rounding by printf gives: 3.1
Rounding approximation by function gives: 3.100000000000000
Rounding approximation by function gives: 3.100000000000000
Rounding approximation by function gives: 3.100000

Rounding by printf gives: 3.14
Rounding approximation by function gives: 3.140000000000000
Rounding approximation by function gives: 3.140000000000000
Rounding approximation by function gives: 3.140000

Rounding by printf gives: 3.142
Rounding approximation by function gives: 3.142000000000000
Rounding approximation by function gives: 3.142000000000000
Rounding approximation by function gives: 3.142000

Rounding by printf gives: 3.1416
Rounding approximation by function gives: 3.141600000000000
Rounding approximation by function gives: 3.141600000000000
Rounding approximation by function gives: 3.141600

Rounding by printf gives: 3.14159
Rounding approximation by function gives: 3.141590000000000
Rounding approximation by function gives: 3.141590000000000
Rounding approximation by function gives: 3.141590

Rounding by printf gives: 3.141593
Rounding approximation by function gives: 3.141593000000000
Rounding approximation by function gives: 3.141593000000000
Rounding approximation by function gives: 3.141593

Rounding by printf gives: 3.1415927
Rounding approximation by function gives: 3.141592700000000
Rounding approximation by function gives: 3.141592700000000
Rounding approximation by function gives: 3.141593

Rounding by printf gives: 3.14159265
Rounding approximation by function gives: 3.141592650000000
Rounding approximation by function gives: 3.141592650000000
Rounding approximation by function gives: 3.141593

Rounding by printf gives: 3.141592654
Rounding approximation by function gives: 3.141592654000000
Rounding approximation by function gives: 3.141592654000000
Rounding approximation by function gives: 3.141593

Rounding by printf gives: 3.1415926536
Rounding approximation by function gives: 3.141592653600000
Rounding approximation by function gives: 3.141592653600000
Rounding approximation by function gives: 3.141593

Rounding by printf gives: 3.14159265359
Rounding approximation by function gives: 3.141592653590000
Rounding approximation by function gives: 3.141592653590000
Rounding approximation by function gives: 3.141593

Rounding by printf gives: 3.141592653590
Rounding approximation by function gives: 3.141592653590000
Rounding approximation by function gives: 3.141592653590000
Rounding approximation by function gives: 3.141593

Rounding by printf gives: 3.1415926535898
Rounding approximation by function gives: 3.141592653589800
Rounding approximation by function gives: 3.141592653589800
Rounding approximation by function gives: 3.141593

Rounding by printf gives: 3.14159265358979
Rounding approximation by function gives: 3.141592653589790
Rounding approximation by function gives: 3.141592653589790
Rounding approximation by function gives: 3.141593

Rounding by printf gives: 3.141592653589793
Rounding approximation by function gives: 3.141592653589793
Rounding approximation by function gives: 3.141592653589793
Rounding approximation by function gives: 3.141593

Rounding by printf gives: -3
Rounding approximation by function gives: -3.000000000000000
Rounding approximation by function gives: -3.000000000000000
Rounding approximation by function gives: -3.000000

Rounding by printf gives: -3.1
Rounding approximation by function gives: -3.100000000000000
Rounding approximation by function gives: -3.100000000000000
Rounding approximation by function gives: -3.100000

Rounding by printf gives: -3.14
Rounding approximation by function gives: -3.140000000000000
Rounding approximation by function gives: -3.140000000000000
Rounding approximation by function gives: -3.140000

Rounding by printf gives: -3.142
Rounding approximation by function gives: -3.142000000000000
Rounding approximation by function gives: -3.142000000000000
Rounding approximation by function gives: -3.142000

Rounding by printf gives: -3.1416
Rounding approximation by function gives: -3.141600000000000
Rounding approximation by function gives: -3.141600000000000
Rounding approximation by function gives: -3.141600

Rounding by printf gives: -3.14159
Rounding approximation by function gives: -3.141590000000000
Rounding approximation by function gives: -3.141590000000000
Rounding approximation by function gives: -3.141590

Rounding by printf gives: -3.141593
Rounding approximation by function gives: -3.141593000000000
Rounding approximation by function gives: -3.141593000000000
Rounding approximation by function gives: -3.141593

Rounding by printf gives: -3.1415927
Rounding approximation by function gives: -3.141592700000000
Rounding approximation by function gives: -3.141592700000000
Rounding approximation by function gives: -3.141593

Rounding by printf gives: -3.14159265
Rounding approximation by function gives: -3.141592650000000
Rounding approximation by function gives: -3.141592650000000
Rounding approximation by function gives: -3.141593

Rounding by printf gives: -3.141592654
Rounding approximation by function gives: -3.141592654000000
Rounding approximation by function gives: -3.141592654000000
Rounding approximation by function gives: -3.141593

Rounding by printf gives: -3.1415926536
Rounding approximation by function gives: -3.141592653600000
Rounding approximation by function gives: -3.141592653600000
Rounding approximation by function gives: -3.141593

Rounding by printf gives: -3.14159265359
Rounding approximation by function gives: -3.141592653590000
Rounding approximation by function gives: -3.141592653590000
Rounding approximation by function gives: -3.141593

Rounding by printf gives: -3.141592653590
Rounding approximation by function gives: -3.141592653590000
Rounding approximation by function gives: -3.141592653590000
Rounding approximation by function gives: -3.141593

Rounding by printf gives: -3.1415926535898
Rounding approximation by function gives: -3.141592653589800
Rounding approximation by function gives: -3.141592653589800
Rounding approximation by function gives: -3.141593

Rounding by printf gives: -3.14159265358979
Rounding approximation by function gives: -3.141592653589790
Rounding approximation by function gives: -3.141592653589790
Rounding approximation by function gives: -3.141593

Rounding by printf gives: -3.141592653589793
Rounding approximation by function gives: -3.141592653589793
Rounding approximation by function gives: -3.141592653589793
Rounding approximation by function gives: -3.141593
*/

[CBFalconer]

jacob navia wrote:

>
"How can I round a number to x decimal places" ?

This question keeps appearing. I would propose the following

.... snip 75 or so lines ...

Why all this gyration? I found the following in the c standard:

7.12.9.6 The round functions
Synopsis
[#1]
#include <math.h>
double round(double x);
float roundf(float x);
long double roundl(long double x);

Description

[#2] The round functions round their argument to the nearest
integer value in floating-point format, rounding halfway
cases away from zero, regardless of the current rounding
direction.

Returns

[#3] The round functions return the rounded integer value.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
Try the download section.

--
Posted via a free Usenet account from http://www.teranews.com

[Ben Pfaff]

CBFalconer <cb********@yahoo.comwrites:

jacob navia wrote:

>>
"How can I round a number to x decimal places" ?

This question keeps appearing. I would propose the following

... snip 75 or so lines ...

Why all this gyration? I found the following in the c standard:

7.12.9.6 The round functions

Those functions are new in C99.
--
char a[]="\n .CJacehknorstu";int putchar(int);int main(void){unsigned long b[]
={0x67dffdff,0x9aa9aa6a,0xa77ffda9,0x7da6aa6a,0xa6 7f6aaa,0xaa9aa9f6,0x11f6},*p
=b,i=24;for(;p+=!*p;*p/=4)switch(0[p]&3)case 0:{return 0;for(p--;i--;i--)case+
2:{i++;if(i)break;else default:continue;if(0)case 1:putchar(a[i&15]);break;}}}

[user923005]

On Feb 25, 4:29*pm, Ben Pfaff <[e-mail]:

CBFalconer <[e-mail]:

jacob navia wrote:

"How can I round a number to x decimal places" ?

This question keeps appearing. I would propose the following

... snip 75 or so lines ...

Why all this gyration? *I found the following in the c standard:

* 7.12.9.6 *The round functions

Those functions are new in C99.

And they don't round to x decimal places, unless x happens to be
zero. (Admittedly, you could manually scale it.)

[Keith Thompson]

Ben Pfaff <bl*@cs.stanford.eduwrites:

CBFalconer <cb********@yahoo.comwrites:

>jacob navia wrote:

>>"How can I round a number to x decimal places" ?

This question keeps appearing. I would propose the following

... snip 75 or so lines ...

Why all this gyration? I found the following in the c standard:

7.12.9.6 The round functions

Those functions are new in C99.

So are floorl and log10l, which jacob's code uses; it also mixes
declarations and statements within a block. An implementation that
can handle jacob's code should provide the round functions.

--
Keith Thompson (The_Other_Keith) <ks***@mib.org>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[user923005]

On Feb 25, 10:15*pm, Keith Thompson <[e-mail]:

Ben Pfaff <[e-mail]:

CBFalconer <[e-mail]:

jacob navia wrote:
"How can I round a number to x decimal places" ?

>This question keeps appearing. I would propose the following

... snip 75 or so lines ...

Why all this gyration? *I found the following in the c standard:

* 7.12.9.6 *The round functions

Those functions are new in C99.

So are floorl and log10l, which jacob's code uses; it also mixes
declarations and statements within a block. *An implementation that
can handle jacob's code should provide the round functions.

You will still have to multiply and divide by powers of 10 and use
floor() to achieve the same thing because the C99 round() functions
round to nearest integer. They do not round to nearest k decimal
places.

[CBFalconer]

user923005 wrote:

>

.... snip float rounding discussion ...

>
You will still have to multiply and divide by powers of 10 and use
floor() to achieve the same thing because the C99 round() functions
round to nearest integer. They do not round to nearest k decimal
places.

Are you claiming that such multiplication and division by 10 is too
complex for the average reader of c.l.c? :-)

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
Try the download section.

--
Posted via a free Usenet account from http://www.teranews.com

[user923005]

On Feb 26, 2:24*pm, CBFalconer <[e-mail]:

user923005 wrote:

... snip float rounding discussion ...

You will still have to multiply and divide by powers of 10 and use
floor() to achieve the same thing because the C99 round() functions
round to nearest integer. *They do not round to nearest k decimal
places.

Are you claiming that such multiplication and division by 10 is too
complex for the average reader of c.l.c? *:-)

No, but I am claiming that using the C99 functions to round to N
digits will take twice as much work as a function that does it
directly, since the multiplication and division by a power of ten and
the floor function are all that is necessary in either case. And so
while you can round to N digits using the C99 rounding functions, it
really does not make a lot of sense to do it that way.

[CBFalconer]

user923005 wrote:

CBFalconer <[e-mail]:

>user923005 wrote:

... snip float rounding discussion ...

>>You will still have to multiply and divide by powers of 10 and
use floor() to achieve the same thing because the C99 round()
functions round to nearest integer. They do not round to
nearest k decimal places.

Are you claiming that such multiplication and division by 10 is
too complex for the average reader of c.l.c? :-)

No, but I am claiming that using the C99 functions to round to N
digits will take twice as much work as a function that does it
directly, since the multiplication and division by a power of
ten and the floor function are all that is necessary in either
case. And so while you can round to N digits using the C99
rounding functions, it really does not make a lot of sense to do
it that way.

Let me also point out that such rounding is rarely needed. Most of
the time maintaining the original alleged precision and rounding
only the output, via printf, is needed.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
Try the download section.

--
Posted via a free Usenet account from http://www.teranews.com

[user923005]

On Feb 27, 8:10*am, CBFalconer <[e-mail]:

user923005 wrote:

CBFalconer <[e-mail]:

user923005 wrote:

... snip float rounding discussion ...

>You will still have to multiply and divide by powers of 10 and
use floor() to achieve the same thing because the C99 round()
functions round to nearest integer. *They do not round to
nearest k decimal places.

Are you claiming that such multiplication and division by 10 is
too complex for the average reader of c.l.c? *:-)

No, but I am claiming that using the C99 functions to round to N
digits will take twice as much work as a function that does it
directly, since the multiplication and division by a power of
ten and the floor function are all that is necessary in either
case. *And so while you can round to N digits using the C99
rounding functions, it really does not make a lot of sense to do
it that way.

Let me also point out that such rounding is rarely needed. *Most of
the time maintaining the original alleged precision and rounding
only the output, via printf, is needed.

I would go one step further:
It's better than rounding the original data.
But sometimes people want to do it anyway.

[Bartc]

"CBFalconer" <cb********@yahoo.comwrote in message
news:47**************@yahoo.com...

user923005 wrote:

>CBFalconer <[e-mail]:

>>user923005 wrote:

... snip float rounding discussion ...

You will still have to multiply and divide by powers of 10 and
use floor() to achieve the same thing because the C99 round()
functions round to nearest integer. They do not round to
nearest k decimal places.

Are you claiming that such multiplication and division by 10 is
too complex for the average reader of c.l.c? :-)

No, but I am claiming that using the C99 functions to round to N
digits will take twice as much work as a function that does it
directly, since the multiplication and division by a power of
ten and the floor function are all that is necessary in either
case. And so while you can round to N digits using the C99
rounding functions, it really does not make a lot of sense to do
it that way.

Let me also point out that such rounding is rarely needed. Most of
the time maintaining the original alleged precision and rounding
only the output, via printf, is needed.

There was a long discussion about this stuff last November in c.l.c with
various examples of why such rounding can be useful. One example was
cleaning noisy data.

--
Bart