using fscanf to read data for a class member ...

[blacksoil]

Hi,

I have a question regarding reading data from a file and put it to a
member of a class. I use fscanf.

The class looks like this

class myclass
{
double a;
...
}

Now in one of its member function, I want to do the following:

fscanf(fp,"%f\n",&myclass.a);

However, it turns out that myclass.a is not given the data.

I am not familiar with syntax in c++ and could anyone give me some
suggestions? Thanks a lot!

--Zhi

 

[mlimber]

Hi,

I have a question regarding reading data from a file and put it to a
member of a class. I use fscanf.

The class looks like this

class myclass
{
double a;
...
}

Now in one of its member function, I want to do the following:

fscanf(fp,"%f\n",&myclass.a);

Inside a member function, you would do this:

fscanf(fp,"%lf\n",&a); // Note the 'l'

However, it turns out that myclass.a is not given the data.

I am not familiar with syntax in c++ and could anyone give me some
suggestions? Thanks a lot!

In C++, iostreams are generally preferred to C-style I/O because they
are type safe (e.g., you wouldn't be able omit the 'l' without doing so
intentionally with a nasty cast) and because you can overload the <<
and >> operators for your own classes. Look up how to use
std::ifstream. See also these FAQs:

http://www.parashift.com/c++-faq-lite/input-output.html

Cheers! --M

 

[Victor Bazarov]

I have a question regarding reading data from a file and put it to a
member of a class. I use fscanf.

The class looks like this

class myclass
{
double a;
...
}

Now in one of its member function, I want to do the following:

fscanf(fp,"%f\n",&myclass.a);

However, it turns out that myclass.a is not given the data.

I am not familiar with syntax in c++ and could anyone give me some
suggestions? Thanks a lot!

RTFM. To convert a 'double' using 'scanf' (and its relatives), you
need to use '%lf' format.

V

 

[Bart]

fscanf(fp,"%f\n",&myclass.a);

However, it turns out that myclass.a is not given the data.

Why not use iostreams instead?

myfile >> myclass;

For this to work you'll have to overload the >> for your class and open
a fstream of some sort.

Regards,
Bart.

 

[blacksoil]

thanks a lot for the reply... However, I just mis-represented my
problem ... Here is the updated question:


class Aclass
{
double a;
...
}



class Bclass{

Aclass B[10];
double ...;
...

}

now in one of Bclass's member function, I want to do the following:

fscanf(fp, "%d\n",&B.a);

and it failed to give B.a the value from the data file.

I know the expression "&B.a" must be wrong, but I don't know what is
the correct form.

Thanks,

--Zhi

 

[blacksoil]

"%lf" just solved my problem.

Thanks a lot, Victor.


Regards,
Zhi

 

[blacksoil]

changing "%f" to "%lf" solved the problem. &B.a is not wrong, ....

thanks all

thanks a lot for the reply... However, I just mis-represented my
problem ... Here is the updated question:


class Aclass
{
double a;
...
}



class Bclass{

Aclass B[10];
double ...;
...

}

now in one of Bclass's member function, I want to do the following:

fscanf(fp, "%d\n",&B.a);

and it failed to give B.a the value from the data file.

I know the expression "&B.a" must be wrong, but I don't know what is
the correct form.

Thanks,

--Zhi

Inside a member function, you would do this:

fscanf(fp,"%lf\n",&a); // Note the 'l'


In C++, iostreams are generally preferred to C-style I/O because they
are type safe (e.g., you wouldn't be able omit the 'l' without doing so
intentionally with a nasty cast) and because you can overload the <<
and >> operators for your own classes. Look up how to use
std::ifstream. See also these FAQs:

http://www.parashift.com/c++-faq-lite/input-output.html

Cheers! --M

 

[Default User]

thanks a lot for the reply...

Please don't top-post. Your replies belong following or interspersed
with properly trimmed quotes. See the majority of other posts in the
newsgroup, or the group FAQ list:
<http://www.parashift.com/c++-faq-lite/how-to-post.html>





Brian