string to double conversion - atof() vs istringstream

[sharmaharish]

I need a conversion function that converts values from string to a
particular type. For this I have a template function that looks like
this ...

template<class T>
T value(const string& s)
{
istringstream(s );
T val;
is >val;
return val;
}

Now for double type the lowest positive value is 2.22507-308 and I get
different answers when I use this function vs atof() function. I am not
getting the reason here. Interestingly, if I reduce the exponent value
from -308 to -307, I get similar results.

Regards,
Harry.

[sharmaharish]

Forgot to mention this before. atof() behaviour is expected for -308
exp and that of istringstream is wrong.

Regards,
Harry.

sh**********@gm ail.com wrote:

I need a conversion function that converts values from string to a
particular type. For this I have a template function that looks like
this ...

template<class T>
T value(const string& s)
{
istringstream(s );
T val;
is >val;
return val;
}

Now for double type the lowest positive value is 2.22507-308 and I get
different answers when I use this function vs atof() function. I am not
getting the reason here. Interestingly, if I reduce the exponent value
from -308 to -307, I get similar results.

Regards,
Harry.

[mlimber]

sh**********@gm ail.com wrote:

I need a conversion function that converts values from string to a
particular type. For this I have a template function that looks like
this ...

template<class T>
T value(const string& s)
{
istringstream(s );
T val;
is >val;
return val;
}

Now for double type the lowest positive value is 2.22507-308 and I get
different answers when I use this function vs atof() function. I am not
getting the reason here. Interestingly, if I reduce the exponent value
from -308 to -307, I get similar results.

How different are the results? Floating point numbers are notoriously
tricky. See, e.g., these FAQs:

http://www.parashift.com/c++-faq-lit...html#faq-29.16
http://www.parashift.com/c++-faq-lit...html#faq-29.18

Cheers! --M

[Roland Pibinger]

On 24 Jul 2006 20:12:21 -0700, sh**********@gm ail.com wrote:

>I need a conversion function that converts values from string to a
particular type. For this I have a template function that looks like
this ...

template<cla ss T>
T value(const string& s)
{
istringstream(s );
T val;
is >val;
return val;
}

You don't check any return or error value in that function. Moreover,
I would prefer an explicit stringToDoulble () function to a template.

>Now for double type the lowest positive value is 2.22507-308 and I get
different answers when I use this function vs atof() function. I am not
getting the reason here. Interestingly, if I reduce the exponent value
from -308 to -307, I get similar results.

Use strtod
(http://www.opengroup.org/onlinepubs/...ns/strtod.html)
and check endptr, errno and return value.

Best wishes,
Roland Pibinger

[sharmaharish]

Roland,
stringToDouble( ) is definitely something that will resolve the issue,
but that is not I am looking for. I was interested in minimizing the
no. of specialized functions that I write. Moreover, I am using this
only for the intrinsic types. As for the error checking, I don't know
how I can add that with the template types without specializing it for
a certain type.

Regards,
Harish Sharma
Roland Pibinger wrote:

On 24 Jul 2006 20:12:21 -0700, sh**********@gm ail.com wrote:

I need a conversion function that converts values from string to a
particular type. For this I have a template function that looks like
this ...

template<class T>
T value(const string& s)
{
istringstream(s );
T val;
is >val;
return val;
}

You don't check any return or error value in that function. Moreover,
I would prefer an explicit stringToDoulble () function to a template.

Now for double type the lowest positive value is 2.22507-308 and I get
different answers when I use this function vs atof() function. I am not
getting the reason here. Interestingly, if I reduce the exponent value
from -308 to -307, I get similar results.

Use strtod
(http://www.opengroup.org/onlinepubs/...ns/strtod.html)
and check endptr, errno and return value.

Best wishes,
Roland Pibinger

[sharmaharish]

mlimber,
The difference in the values is quite large to be neglected for a
rounding error as the faq says. And anyways I am not interested in
comparing any double type no.s here. The result that I get while using
this func is 2.64167e-308 while atof() returns 2.22507e-308. I
understand that atof() has minimal functionality and that too is type
specific, whereas the istringstream (or istream) has be generic.
However, I think the precision of default 6 digits is lost somewhere
while extracting the value from the string.

Regards,
Harish Sharma

mlimber wrote:

sh**********@gm ail.com wrote:

I need a conversion function that converts values from string to a
particular type. For this I have a template function that looks like
this ...

template<class T>
T value(const string& s)
{
istringstream(s );
T val;
is >val;
return val;
}

Now for double type the lowest positive value is 2.22507-308 and I get
different answers when I use this function vs atof() function. I am not
getting the reason here. Interestingly, if I reduce the exponent value
from -308 to -307, I get similar results.

How different are the results? Floating point numbers are notoriously
tricky. See, e.g., these FAQs:

http://www.parashift.com/c++-faq-lit...html#faq-29.16
http://www.parashift.com/c++-faq-lit...html#faq-29.18

Cheers! --M

[Default User]

sh**********@gm ail.com wrote:

Roland,
stringToDouble( ) is definitely something that will resolve the issue,

Please don't top-post. Your reply belongs following or interspersed
with properly trimmed quotes. See the newsgroup FAQ list:
<http://www.parashift.c om/c++-faq-lite/how-to-post.html#faq-5.4>


Brian

[mlimber]

Roland Pibinger wrote:

On 24 Jul 2006 20:12:21 -0700, sh**********@gm ail.com wrote:

I need a conversion function that converts values from string to a
particular type. For this I have a template function that looks like
this ...

template<class T>
T value(const string& s)
{
istringstream(s );
T val;
is >val;
return val;
}

You don't check any return or error value in that function.

That is a good point. Are you sure the conversion didn't abort
prematurely? Please post a *complete* but *minimal* program (see FAQ
5.8) that demonstrates your problem so we can reproduce it.

Cheers! --M

[sharmaharish]

#include <iostream>
#include <sstream>
#include <cstdlib>

using namespace std;

template<class T>
T value(string s)
{
istringstream(s );
T val; is >val;
return val;
}

int
main()
{
cout << "\natof(2.22507 e-308) ->" << atof("2.22507e-308");
cout << "\nvalue<double >(2.22507e-308) ->" <<
value<double>(" 2.22507e-308");
return 0;
}

Above is the complete demo program illustrating the problem. Actually
the value displayed by the template function is not consistent across
compilation and is sorta random (atleast I didn't see any pattern or
consistent values with every compilation). I checked the output with
MSVC as well as with GCC compiler. They give different values but
neither gives the right output.

Regards,
Harish Sharma

[mlimber]

sh**********@gm ail.com wrote:

#include <iostream>
#include <sstream>
#include <cstdlib>

using namespace std;

template<class T>
T value(string s)
{
istringstream(s );
T val; is >val;
return val;
}

Try this instead:

class MyException {};

template<class T>
T value(const string& s)
{
istringstream is(s);
T val;
if( !(is >val) || !(is >ws).eof() )
{
throw MyException();
}
return val;
}

Or better, use boost::lexical_ cast, which does basically the same thing
'cept better. I get the same answer for both with the above code:

atof(2.22507e-308) ->2.22507e-308
value<double>(2 .22507e-308) ->2.22507e-308

>
int
main()
{
cout << "\natof(2.22507 e-308) ->" << atof("2.22507e-308");
cout << "\nvalue<double >(2.22507e-308) ->" <<
value<double>(" 2.22507e-308");
return 0;
}

Above is the complete demo program illustrating the problem. Actually
the value displayed by the template function is not consistent across
compilation and is sorta random (atleast I didn't see any pattern or
consistent values with every compilation). I checked the output with
MSVC as well as with GCC compiler. They give different values but
neither gives the right output.

That's probably because you forgot the variable name for the string
stream (it's using 's', which means you can't access the string that
was passed in and you have a global 'is' somewhere). GIGO.

Cheers! --M