fwrite printing garbage values like @@@@@

Question

double *array_out;
.......
FILE * pFile;
pFile = fopen ("Q1_n_iter.bin", "wb");
fwrite(array_out,sizeof(double),n*n,pFile);
fclose (pFile);

I have checked array_out is correctly populated, I am not able to figure out why it prints garbage instead of values in it.

Answer 1

The values fwrite writes into a file is not to be read directly with human eyes. You should let computer interpret them for you with fread or something. For example, you could do the following:

#include <stdio.h>
#include <stdlib.h>

char * filename = "Q1_n_iter.bin";

int main( ) {
    double * array_out;
    double * array_in;
    FILE * pFile;

    array_out = malloc( 3 * sizeof * array_out );

    array_out[0] = 0.12;
    array_out[1] = 1.35;
    array_out[2] = 2.80;

    pFile = fopen( filename, "wb" );
    fwrite( array_out, sizeof * array_out, 3, pFile );
    fclose( pFile );
    free( array_out );

    array_in = malloc( 3 * sizeof * array_in );
    pFile = fopen( filename, "rb" );
    fread( array_in, sizeof * array_in, 3, pFile );

    for ( int i = 0; i < 3; i++ )
        printf( "%d: %.2f\n", i, array_in[i] );

    free( array_in );

    return 0;
}

And would receive the following output:

0: 0.12
1: 1.35
2: 2.80

If you want to print formatted data into the file, for a human to read, then you should rather use the function fprintf. It works just like the printf, with an additional field for specifying the output stream.

fprintf( FILE * stream, const char * FormatString, ... );

In your case, you should do it in the following manner:

// ...

for ( int i = 0; i < n * n; i++ )
    fprintf( pFile, "%.2f ", array_out[i] );

// ...

Remove ".2" if you do not want rather 6 digits after the decimal point as default, specify another number if you want any other precision.

Edit:

To create the file with the filename determined with command-line arguments, you could do the following:

#include <stdio.h>
#include <stdlib.h>

char * filename;

int main( int argc, char * argv[] ) {

    // declarations
    // and what not

    filename = malloc( 256 );
    sprintf( filename, "Q1_%d_%d.bin", n, iter );

    // whatever
    free( filename );

    return 0;
}

Or, if you don't want to use the sprintf, include the string.h library and:

    // ...
    strcpy( filename, "Q1_" );
    strcat( filename, argv[1] );
    strcat( filename, "_" );
    strcat( filename, argv[2] );
    strcat( filename, ".bin" );
    // ...