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

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

blacksoil@gmail.com wrote:

Quote:

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'

Quote:

>
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

blacksoil@gmail.com wrote:

Quote:

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

blacksoil@gmail.com wrote:
<snip>

Quote:

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.

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[i].a);

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

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

Thanks,

--Zhi

mlimber wrote:

Quote:

blacksoil@gmail.com wrote:

Quote:

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'
>

Quote:

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

"%lf" just solved my problem.

Thanks a lot, Victor.

Regards,
Zhi

Victor Bazarov wrote:

Quote:

blacksoil@gmail.com wrote:

Quote:

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

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

thanks all

blacksoil@gmail.com wrote:

Quote:

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[i].a);
>
and it failed to give B.a the value from the data file.
>
I know the expression "&B[i].a" must be wrong, but I don't know what is
the correct form.
>
Thanks,
>
--Zhi
>
>
>
>
>
>
>
mlimber wrote:

Quote:

blacksoil@gmail.com wrote:

Quote:

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'

Quote:

>
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

blacksoil@gmail.com wrote:

Quote:

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

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