how to read data?

[kathy]

to read float type data, we can use:

....
float f;
fscanf(outfile, "%f", &f);
....

but for double type, how to do that?
....
double d;
fscanf(outfile, "%f", &d);
....

Does that mean I lost data precision?

[Ian Collins]

kathy wrote:

to read float type data, we can use:

....
float f;
fscanf(outfile, "%f", &f);
....

but for double type, how to do that?
....
double d;
fscanf(outfile, "%f", &d);
....

Does that mean I lost data precision?

Do it in C++:

double d;

std::cin >> d;

--
Ian Collins.

[Mike Wahler]

"kathy" <yq*****@yahoo. com> wrote in message
news:11******** **************@ o13g2000cwo.goo glegroups.com.. .

to read float type data, we can use:

...
float f;
fscanf(outfile, "%f", &f);
...

but for double type, how to do that?
...
double d;
fscanf(outfile, "%f", &d);
fscanf(outfile, "%lf", &d); ...

Does that mean I lost data precision?

Using "%f" with 'fscanf()' for type 'double'
means you get undefined behavior.

BTW have you considered using std::cin
instead of a FILE* stream? It has overloaded
operators defined for it; the one for the
right type gets called automatically.

float f;
double d;

std::cin >> f; // reads a float
std::cin >> d; // reads a double

-Mike

[upashu2]

In C, you can do it like:
//read as a string
char d[40];
fscanf(file,"%s ",d);
///convert it into double using strtod()
call strtod()

[Ben Pope]

upashu2 wrote:

In C, you can do it like:
//read as a string
char d[40];
fscanf(file,"%s ",d);
///convert it into double using strtod()
call strtod()

That's also a classic example of buffer overflow.

At least do:

char d[40];
fscanf(file,"%3 9s",d);
Ben Pope
--
I'm not just a number. To many, I'm known as a string...

[kathy]

could you give me example how to use std::cin to read data from file?

[Ben Pope]

kathy wrote:

could you give me example how to use std::cin to read data from file?

He probably means std::ifstream. std::cin and std::ifstream inherit
from std::istream.

Ben Pope
--
I'm not just a number. To many, I'm known as a string...

[kathy]

could you point it to me where is the article compare std::fstream with
FILE* file = fopen(...). What is the benefit to use the std::fstream ?

[Ben Pope]

kathy wrote:

could you point it to me where is the article compare std::fstream with
FILE* file = fopen(...). What is the benefit to use the std::fstream ?

fstream manages the resources for you, you do not have to explicitly
close the file, fstream will close the file when its destructor is
called, whether explicitly, because it goes out of scope, or because an
exception was thrown.

Additionally it provides the stream operators, buffering etc. for
convenience.

Probably best to see your favourite C++ book for how to use fstream.

Ben Pope
--
I'm not just a number. To many, I'm known as a string...

[osmium]

"kathy" writes:

could you give me example how to use std::cin to read data from file?

You can't, cin is an object which is by deault, tied to the keyboard. At
least you can't without an extraordinary amount of effort. He meant to use
the overloaded >> operator which is part of <iostream>.

#include <iostream>

int main()
{
using std::ifstream;
using std::cout;
using std::endl;

ifstream in_file("junk07 .txt");
if(!in_file)
{
cout << "Can't open file\n";
//system("pause") ; // needed by my compiler, not really a necessary
part of the program
return 1; // error exit
}
int n;
float x;
double y;
in_file >> n >> x >> y;
cout << n << ' ' << x << ' ' <<y << endl;
//system("pause") ;
}
----------------
Junk27 looked like this:
1024 2.71828 3.1416

All on one line. It could have been on three lines.

It must be in the same directory as the executable produced by your
compiler. You can probably create it with the editor that comes with your
compiler.