Truncating decimal places in a float (possibly OT)

[Todd]

I have a function that takes a double and truncates it to a specified
number of decimal places. I've found that I have to add a small
number to the input value otherwise I get errors due to the way
floating points work.

double TruncateToDigit s(double dValue, int iDigitsToRightO fDecimal)
{
double dEpsilon = 1e-8;
// high limit, about 100,000,000 for this epsilon (1e-8)
// low limit, about 7 decimal places for this epsilon (1e-8)
// modifying it will increase one limit while reducing the other
ASSERT(fabs(dVa lue) < 100000000.0);
ASSERT(iDigitsT oRightOfDecimal <= 7);

int iSignFactor = (dValue >= 0.0 ? 1 : -1);
dValue *= iSignFactor; // make positive
dValue += dEpsilon; // make it error towards the positive
double dMulti = pow(10.0, iDigitsToRightO fDecimal);
dValue *= dMulti; // move valid digits left of decimal
dValue = floor(dValue); // truncate to whole value
dValue /= dMulti; // move digits back
dValue *= iSignFactor; // put sign back
return dValue;
}

This appears to work, however it only reliably works within a certain
range, as listed in the comments.

Is there a better way to do this? One that would work for any
floating point range?

[Victor Bazarov]

Todd wrote:

I have a function that takes a double and truncates it to a specified
number of decimal places. [..]

What does that mean? How would it "truncate" 0.111 to 1 decimal point
when neither 0.111 nor 0.1 can be represented _exactly_ in the binary
system of FP numbers (employed by most computers nowadays)?

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Todd]

On Jul 18, 4:11 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:

What does that mean? How would it "truncate" 0.111 to 1 decimal point
when neither 0.111 nor 0.1 can be represented _exactly_ in the binary
system of FP numbers (employed by most computers nowadays)?

To be more specific, the input is a fixed decimal place number
(converted to a double) and the output needs to be the floating point
closest to the truncated decimal number. If that makes sense.

[Victor Bazarov]

Todd wrote:

On Jul 18, 4:11 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:

>What does that mean? How would it "truncate" 0.111 to 1 decimal
point when neither 0.111 nor 0.1 can be represented _exactly_ in the
binary system of FP numbers (employed by most computers nowadays)?

To be more specific, the input is a fixed decimal place number
(converted to a double) and the output needs to be the floating point
closest to the truncated decimal number. If that makes sense.

The closest you can get in this case is by stopping reading after
the designated digit in the character input. No manipulation with
FP representation required _at_all_. For example, if your input
contains "321.123456 " and you need to truncate to 2 digits after
the decimal point, input all digits before the point, then the
point itself, then only 2 digits after the point, you get "321.12"
and then convert that to your internal representation. That's
the best you can do, really.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[sonison.james]

On Jul 18, 2:25*pm, Todd <toddbitr...@gm ail.comwrote:

On Jul 18, 4:11 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:

What does that mean? *How would it "truncate" 0.111 to 1 decimal point
when neither 0.111 nor 0.1 can be represented _exactly_ in the binary
system of FP numbers (employed by most computers nowadays)?

To be more specific, the input is a fixed decimal place number
(converted to a double) and the output needs to be the floating point
closest to the truncated decimal number. *If that makes sense.

How about doing it using strings.

#include <sstream>
#include <string>
#include <iostream>
using namespace std;

double foo( double d, size_t n )
{
stringstream ss;
ss.setf( ios::fixed, ios::floatfield );
ss << d;
n += ss.str().find( '.' ) + 1;
string str = ss.str().substr ( 0, n );
stringstream ss2;
ss2 << str;
double d2;
ss2 >d2;

return d2;
}

int main()
{
double d = 1.1234567;
cout << "1 decimal place:" << foo( d, 1 ) << endl;
cout << "2 decimal place:" << foo( d, 2 ) << endl;
cout << "3 decimal place:" << foo( d, 3 ) << endl;
cout << "4 decimal place:" << foo( d, 4 ) << endl;
cout << "5 decimal place:" << foo( d, 5 ) << endl;

return 0;
}