Re: ifstream problem?
* kazik.lakomy@gmail.com:
Yes, this is exactly what is in tutorial page whose address I've
enclosed.
Oh dear.
And any ideas what was wrong in giving a stream as an argument to a
function [I mean previous, not working version]?
No problem with *that*.
And it seems to me that Victor did a good job of dissecting that code.
However, my take... :-)
<dissection>
#include <iostream>
#include <fstream>
using namespace std;
void ReadInitialConditions(double* TabX, double* TabY, int TableSize,
ifstream & InitCondFile){
Too many arguments, plus arguments of raw types; use std::vector.
int XorYCounter = 0;
int XYPairsCounter = 0;
if(InitCondFile.is_open()){
while(! InitCondFile.eof() ){
double num;
InitCondFile>>num;
Forgetting to check for failure.
if((XorYCounter % 2) == 0)
TabX[XYPairsCounter] = num;
Forgetting that raw array may not have enough room (buffer overflow).
if((XorYCounter % 2) != 0){
Repeating condition instead of using 'else'.
TabY[XYPairsCounter] = num;
XYPairsCounter++;
Non-idiomatic. Either hoist that postfix '++' into the indexing, or use prefix
'++'. I.e. ask only for what you actually want done, even here where it doesn't
matter regarding generated code.
}
XorYCounter++;
}
InitCondFile.close();
Confused resource ownership. The code that opened the file should close it. Ideally.
}
else
cout<<"Unable to open file";
In a non-interactive program, which this is, send error messages to the standard
error steam, not the standard output stream.
Also, confusion of responsibility, whose job it is to report errors to the user.
For example, this routine couldn't be used in a GUI program, because of this.
}
int main(){
ifstream InitCondFile("groundsol.dat");
const int TableSize = 128;
double TabX[TableSize];
double TabY[TableSize];
ReadInitialConditions(TabX, TabY, TableSize, InitCondFile);
for(int j=0; j<TableSize; j++)
cout<<"x"<<j<<": "<<TabX[j]<<" y"<<j<<": "<<TabY[j]<<endl;
Preferentially use braces around loop bodies, otherwise they'll bite you.
return 0;
This explicitly says the program succeeded, when it may very well have failed.
}
</dissection>
The following alternative code is just typed in, never handled by compiler:
<alternative>
#include <iostream>
#include <fstream>
#include <vector>
#include <stdlib.h>
using namespace std;
struct Condition
{
int x;
int y;
};
istream& operator>>( istream& stream, Condition& cond )
{
return (stream >> cond.x >> cond.y);
}
typedef vector<Condition> ConditionVec;
bool readFrom( istream& stream, ConditionVec& conditions )
{
ConditionVec result;
Condition cond;
while( stream >> cond ) { result.push_back( cond ); }
bool const ok = stream.eof();
if( ok ) { result.swap( conditions ); }
return ok;
}
int main()
{
ifstream condFile( "groundsol.dat" );
if( !condFile )
{
cerr << "Unable to open file." << endl;
return EXIT_FAILURE;
}
ConditionVec conditions;
if( !readFrom( condFile, conditions ) )
{
cerr << "File contained invalid data." << endl;
return EXIT_FAILURE;
}
for( size_t i = 0; i < conditions.size(); ++i )
{
Condition const& c = conditions.at( i );
cout << "x" << j << ": " << c.x << " y" << i << ": " << x.y << endl;
}
return EXIT_SUCCESS;
}
</alternative>
Of course this code will probably not even compile. But it should give you some
ideas about how to structure things. I've abstained from using exceptions in
order to keep the code simple for someone who possibly isn't used to them.
Cheers & hth.,
- Alf