Text file reading problem

[pkiskool]

I've got a problem regarding reading a text file.

I have a text file which is this format:

3 2 3 4
-3 1 1
1 -3 0
1 0 -4
-5 -5 0

Basically, first line has 4 numbers (each representing a specific variable values - the first nubmer which is 3, represents the dimension of the matrix), then second line through 4th line is a matrix (3 by 3 matrix), then the last line is the right-hand-side of that matrix. Thus I should be able to solve this linear equations eventually.

The problem comes from reading this file. At the moment I've got it so that:

int *indx, s, d, ns;
int num, job=0, job2=0;
double p, **aa ;
double *x;
double sum;
ifstream infile;
string line2;
char * inname = "matrix.txt";
infile.open(inname);

if (!infile) {
cout << "Cannot open file " << inname << " for reading." << endl;
exit(EXIT_FAILURE);
}

infile >> dim >> ns >> s >> d; //Reads the first line (4 variables)
for (i = 1; i <= dim; i++) {
for (j = 1; j <= dim; j++) {
if (!(infile >> a[i][j])) {
cout << "Couldn't read a[" << i << "][" << j << "]" << endl;
exit(EXIT_FAILURE);
}
aa[i][j] = a[i][j];
}
}
.
.
.
other stuff
.
.
.


This results in the exit always (end up with the "couldn't read a[1][1]" error message).
I think the problem is that it doesn't go to the next line after reading the first 4 variables, and i'm sure there's an easy way of fixing this problem but my head is blank at the moment.

Someone please help?

[Zaita]

Where is A declared? Have you allocated memory for it?

[pkiskool]

It is declared else where as:

double **a;
a = dmatrix(1, dim, 1, dim);

[Mitsakos]

But you read from the file to dim value here. You can't use the dim before it...
Also you didn't allocate any memory for **aa

[pkiskool]

Oh it is read again in "matrix.txt" file...
The thing is that dim value updates as it is looped.
Thus I need to read in new updated-dim value everytime.

It is read first in a another file called "contour.txt", then it uses that dim value to decalre "a".
Then this function I put up above comes in later (with new dim value).

[pkiskool]

Sorry, I actually do but just left it out.

[source]
indx = ivector(1, dim); /* holds pivot info from ludcmp for lubskp
aa = matrix(1, dim, 1, dim); /* a copy of the matrix to send to ludcmp
x = vector(1, dim); /* Copy rhs to x and send to lubksp */
[/source]

The code is really lengthy so thus I didn't bother putting up the whole thing.

[Mitsakos]

I don't know about matrix... I can't find anywhere the dmatrix() function...
I'll post an example on how to do the same thing with new() instead of dmatrix():

#include <iostream>
#include <fstream>
using namespace std;
int main(){
   int dim, ns, s, d;
   double **matr;
   char * inname = "matrix.txt";
   ifstream infile(inname);

   if (!infile) {
      cout << "Cannot open file " << inname << " for reading." << endl;
      exit(EXIT_FAILURE);
   }
   infile >> dim >> ns >> s >> d; //Reads the first line (4 variables)
   //Create the array
   matr = new double*[dim];
   for(int i=0; i<dim; i++){
      matr[i] = new double[dim];
   }
	
   //Read from file to array
   for(int i=0; i<dim; i++){
      for(int j=0; j<dim; j++){
         infile >> matr[i][j];
      }
   }
   //Previewing the array
   for(int i=0; i<dim; i++){
      for(int j=0; j<dim; j++){
	      cout <<  matr[i][j] << ' ';
      }
      cout << '\n';
   }
   //Deleting contents
   for(int i=0; i<dim; i++){
      delete [] matr[i];
   }
   delete [] matr;
}

You can put together the first to for-loops so you won't do the same loop twice.

Anybody who knows better about it can help you more. This is just how I would do it.

[pkiskool]

Thanks, I'll see what I can do with this.
It's end of the day so I might try it out tomorrow morning.
Could you check back again around tomorrow noon to see if I have any question?
I'll let you know how it went.

Thanks.

[Mitsakos]

sure, no problem

[shereif102]

i think the problem is

if (!infile) {
cout << "Cannot open file " << inname << " for reading." << endl;
exit(EXIT_FAILURE);
}

i think infile is an object so it will not = false or null :D

and as u said the program allows extining ithink that is the error

[pkiskool]

I think you are absolutely right shereif102...
infile is an object!

and thanks to you as well Mitsakos , I think your approach seems to be working.

Thanks for the help everyone, despite me putting up insufficient information regarding the code.

[Mitsakos]

The if(!infile) works fine. But if you want to be completely accurate you can use:

if(!infile.is_open()){}

The is_open() returns true if the file is open and false otherwise.

[EDIT]
The infile is a stream. When trying to open the file it will have some contents (different than 0) so it will act as a True in the if.
If it returns an error then it is gonna have the value of 0 which is also a value for the False value.