read and store from unknown file name

[carmelo]

Hi! I need to read and store data from a file, but I don't the name of
it; Is this code correct and is there a better way to do this thing??
int main()
{
cout << " type file name:"
char * name;
cin >name;

ifstream indata;
int num1; // variables for input values
int num2;
int num3;
indata.open(nam e); // opens the file
if(!indata) { // file couldn't be opened
cerr << "Error: file could not be opened" << endl;
exit(1);
}
indata >num1;
indata >num2;
indata >num3;
}

[Jim Langston]

carmelo wrote:

Hi! I need to read and store data from a file, but I don't the name of
it; Is this code correct and is there a better way to do this thing??

Define "better".

It sounds like the filename is unknown at compile time, but known at run
time. There are a few ways you could get the name, a few include:
1. Accept filename as run time paramater. I.e.
myprog filename.ext
2. Accept the filename as user input
3. Accept the file through std::cin using OS piping.
4. Accept the filename through some other method (graphical input, stored in
a file, etc..)

It depends on what you mean by "better".

int main()
{
cout << " type file name:"
char * name;
cin >name;

This is an error. name is an uninitialied pointer to a character. Yet you
never allocated any memmory for it, so it points .. somewhere. Anywhere.
But not to anything valid. Most likely when the program hits the cin >>
name a run time error will occur as cin attempts to write to memory your
program doesn't "own". Better is:

std::string name;
std::getline( std::cin, name );

getline is better than >in this case because getline will allow a space,

>will stop when it hits the first space leaving the remainder in the

buffer.

ifstream indata;

Might as well just open it here.

std::ifstream indata( name );

int num1; // variables for input values
int num2;
int num3;
indata.open(nam e); // opens the file

So that line isn't needed anymore.

if(!indata) { // file couldn't be opened
cerr << "Error: file could not be opened" << endl;
exit(1);
}
indata >num1;
indata >num2;
indata >num3;
}

You are not checking if the data is valid. Better is:

if ( ! indata >num1 >num2 >num3 )
{
std::cerr << "File dataformat error." << std:;endl;
exit( 1 );
}

// Data is good, proceed
}

--
Jim Langston
ta*******@rocke tmail.com

[Juha Nieminen]

Jim Langston wrote:

> if(!indata) { // file couldn't be opened
cerr << "Error: file could not be opened" << endl;
exit(1);
}

That would be even better like this:

if(!indata)
{
std::cerr << "Could not open ";
std::perror(nam e.c_str());
exit(EXIT_FAILU RE);
}

[=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?=]

On 2008-03-01 11:36, Jim Langston wrote:

carmelo wrote:

>Hi! I need to read and store data from a file, but I don't the name of
it; Is this code correct and is there a better way to do this thing??

Define "better".

It sounds like the filename is unknown at compile time, but known at run
time. There are a few ways you could get the name, a few include:
1. Accept filename as run time paramater. I.e.
myprog filename.ext
2. Accept the filename as user input
3. Accept the file through std::cin using OS piping.
4. Accept the filename through some other method (graphical input, stored in
a file, etc..)

It depends on what you mean by "better".

>int main()
{
cout << " type file name:"
char * name;
cin >name;

This is an error. name is an uninitialied pointer to a character. Yet you
never allocated any memmory for it, so it points .. somewhere. Anywhere.
But not to anything valid. Most likely when the program hits the cin >>
name a run time error will occur as cin attempts to write to memory your
program doesn't "own". Better is:

std::string name;
std::getline( std::cin, name );

getline is better than >in this case because getline will allow a space,

>will stop when it hits the first space leaving the remainder in the

buffer.

> ifstream indata;

Might as well just open it here.

std::ifstream indata( name );

std::ifstream indata(name.c_s tr());

I can't believe that they missed this in the current standard, but at
least it is fixed in the next.

--
Erik Wikström

[Jim Langston]

Erik Wikström wrote:

On 2008-03-01 11:36, Jim Langston wrote:

>carmelo wrote:

>>Hi! I need to read and store data from a file, but I don't the name
of it; Is this code correct and is there a better way to do this
thing??

Define "better".

It sounds like the filename is unknown at compile time, but known at
run time. There are a few ways you could get the name, a few
include:
1. Accept filename as run time paramater. I.e.
myprog filename.ext
2. Accept the filename as user input
3. Accept the file through std::cin using OS piping.
4. Accept the filename through some other method (graphical input,
stored in a file, etc..)

It depends on what you mean by "better".

>>int main()
{
cout << " type file name:"
char * name;
cin >name;

This is an error. name is an uninitialied pointer to a character.
Yet you never allocated any memmory for it, so it points ..
somewhere. Anywhere. But not to anything valid. Most likely when
the program hits the cin >name a run time error will occur as cin
attempts to write to memory your program doesn't "own". Better is:

std::string name;
std::getline ( std::cin, name );

getline is better than >in this case because getline will allow a
space, >will stop when it hits the first space leaving the
remainder in the buffer.

>> ifstream indata;

Might as well just open it here.

std::ifstrea m indata( name );

std::ifstream indata(name.c_s tr());

I can't believe that they missed this in the current standard, but at
least it is fixed in the next.

Oh, yeah. I forgot that ifstream takes a char* and not a std::string. I
sure hope they fix it.

--
Jim Langston
ta*******@rocke tmail.com