while(!FP.eof()) is reading the last data point of the file twice

[ramana]

I'm wondering if someone could point me to the flaw in the following
code that uses the while(!FP.eof()) condition to read the input data.
This condition is reading the last data point of the file twice.

#include <iostream>
#include <fstream>
using namespace std;
int main(int argc, char **argv)
{
double x;
ifstream FP("test.d");

//while(!FP.eof()){FP >> x; cout << x << endl;} // This reads the
last data point of test.d twice
while(FP >> x){cout<< x << endl;} // This doesn't.

return 0;
}
/* Using either gcc 3.4.6 or gcc 4.1.3
File "test.d" has the following 2 data points:
1.1
2.2
*/

Thanks...ramana

 

[Daniel T.]

I'm wondering if someone could point me to the flaw in the following
code that uses the while(!FP.eof()) condition to read the input data.
This condition is reading the last data point of the file twice.

#include <iostream>
#include <fstream>
using namespace std;
int main(int argc, char **argv)
{
double x;
ifstream FP("test.d");

//while(!FP.eof()){FP >> x; cout << x << endl;} // This reads the
last data point of test.d twice

The reason the last data point is "read" twice is this... When the last
data point is read the first time, eof() still false because you haven't
reached the end of the file yet, then it attempts to read another data
point, but none exists, so eof is set to true and x is left unchanged,
so the last data point outputs a second time.

while(FP >> x){cout<< x << endl;} // This doesn't.

This one works as follows... The last data point is read, then FP is
true so the data point is printed out, then when it goes to read more,
there isn't any os FP returns false and the loop isn't executed anymore.

 

[LR]

I'm wondering if someone could point me to the flaw in the following
code that uses the while(!FP.eof()) condition to read the input data.

This condition is reading the last data point of the file twice.

I don't think so.

#include <iostream>
#include <fstream>
using namespace std;
int main(int argc, char **argv)
{
double x;
ifstream FP("test.d");

//while(!FP.eof()){FP >> x; cout << x << endl;}
> // This reads the last data point of test.d twice

Are you sure about that?

while(FP >> x){cout<< x << endl;} // This doesn't.

I suspect, for many, but not all, apps this would be preferred over the
commented code above.

return 0;
}
/* Using either gcc 3.4.6 or gcc 4.1.3
File "test.d" has the following 2 data points:
1.1
2.2
*/

Thanks...ramana

May I suggest that you try this code,

#include <iostream>
#include <fstream>

int main() {
std::ifstream in("junk.txt");
while(!in.eof()) {
double d = 999.;
in >> d;
std::cout << d << std::endl;
}
}

And this,
#include <iostream>
#include <fstream>

int main() {
std::ifstream in("junk.txt");
while(!in.eof()) {
double d = 999.;
if(in >> d) {
std::cout << d << std::endl;
}
}
}

 

[ramana]

The reason the last data point is "read" twice is this... When the last
data point is read the first time, eof() still false because you haven't
reached the end of the file yet, then it attempts to read another data
point, but none exists, so eof is set to true and x is left unchanged,
so the last data point outputs a second time.


This one works as follows... The last data point is read, then FP is
true so the data point is printed out, then when it goes to read more,
there isn't any os FP returns false and the loop isn't executed anymore.

Thank you Daniel... It's very enlightening...ramana

 

[ramana]

Thank you LR...

May I suggest that you try this code,

#include <iostream>
#include <fstream>

int main() {
std::ifstream in("junk.txt");
while(!in.eof()) {
double d = 999.;
in >> d;
std::cout << d << std::endl;
}

}

This code still printed 999 after the contents of junk.txt. It's quite
revealing though...

And this,
#include <iostream>
#include <fstream>

int main() {
std::ifstream in("junk.txt");
while(!in.eof()) {
double d = 999.;
if(in >> d) {
std::cout << d << std::endl;
}
}

}

This code worked perfect.
And as Daniel's reply would mean, failing if-condition prevented its
stmnts from executing... Thanks. ramana

 

[red floyd]

I'm wondering if someone could point me to the flaw in the following
code that uses the while(!FP.eof()) condition to read the input data.
This condition is reading the last data point of the file twice.

#include <iostream>
#include <fstream>
using namespace std;
int main(int argc, char **argv)
{
double x;
ifstream FP("test.d");

//while(!FP.eof()){FP >> x; cout << x << endl;} // This reads the
last data point of test.d twice
while(FP >> x){cout<< x << endl;} // This doesn't.

return 0;
}

This is a FAQ, 15.5
http://parashift.com/c++-faq-lite/input-output.html#faq-15.5

Essentially, C++ doesn't return true until *after* you've read EOF. So
you hit EOF. The subsequent read doesn't change your data, so you get
it twice. Then testing EOF indicates eof.

The FAQ is more eloquent than I am.

 

[Sachin]

This is a FAQ, 15.5http://parashift.com/c++-faq-lite/input-output.html#faq-15.5

Essentially, C++ doesn't return true until *after* you've read EOF.  So
you hit EOF.  The subsequent read doesn't change your data, so you get
it twice.  Then testing EOF indicates eof.

The FAQ is more eloquent than I am.- Hide quoted text -

- Show quoted text -

I think main reason for the above mentioned behaviour is you might be
having SPACE or END OF LINE character at the end of the file.

 

[red floyd]

I think main reason for the above mentioned behaviour is you might be
having SPACE or END OF LINE character at the end of the file.

No, it's because the stream doesn't know it's at EOF until it's actually
*hit* the end of file. READ THE FAQ!!!!!

http://parashift.com/c++-faq-lite/input-output.html#faq-15.5