Storing Doubles from File to Array. Better Method?

Question

I'm working on a segment of a program where I have to read doubles from a .dat file I created which holds 12 doubles which looks like:

10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
100.0
110.0
120.0

Here is my code:

double readSales(FILE *input, double sales[])
{
    int count = 0, n;
    double s;
    n = fscanf(input, "%lf", &s);
    sales[0] = s;

    while(n == 1)
    {
    n = fscanf(input, "%lf", &s);
    count++;
    sales[count] = s;
    }
}

This works fine but I don't think it is well written due to having to declare the first element of the array. The issue is without the declaration, I just get a 0.0 as the first element of the array. Thank you for any feedback.

Welcome to StackOverflow! Be sure to take the tour and visit the help center. Could you post the code that exhibits the problem you mention? Then you could get help with fixing it. — user6564029
– user6564029, Commented Mar 16, 2019 at 18:28

Weather Vane · Accepted Answer · 2019-03-16 18:19:22Z

1

You are writing to the array before checking if the input is valid. I suggest

int readSales(FILE *input, double sales[])      // change the type
{
    int count = 0;
    double s;
    while(fscanf(input, "%lf", &s) == 1) 
    {
        sales[count] = s;
        count++;
    }
    return count;                               // number of elements
}

answered Mar 16, 2019 at 18:19

Weather Vane

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4 Comments

Neil Over a year ago

Maybe something like if(ferror(input)) return perror("input"), 0; at the end would signal an error.

Neil Over a year ago

Also, if the file has 13 doubles, this programme is going to crash -- I would check the count before assigning.

Weather Vane Over a year ago

@NeilEdelman how do you know it would crash? OP has not shown the full code. Ideally it would be right to pass the maximum array length as an argument, but this answer addresses the basic question.

Neil Over a year ago

I assume that the user calls this with double[12] but you are right, there is no way to know. One may pass a pointer double (*const)[12] to turn on compile-time checks.

StoneThrow · Accepted Answer · 2019-03-16 18:37:05Z

Here's my offer on one way to implement this. I've included a main().

#include <stdio.h>

size_t readSales( FILE *input, double sales[],
                  const size_t salesSize )
{
  if ( ! input || ! sales ) { return 0; }

  size_t numRead = 0;

  while ( numRead < salesSize &&
          fscanf( input, "%lf", sales++ ) ) numRead++;

  return numRead;
}

int main( int argc, char* argv[] )
{
  double d[12];
  FILE* fp = fopen( "sales.txt", "r" );

  if ( ! fp )
  {
    printf( "No sales.txt\n" );
    return 1;
  }

  size_t numRead = readSales( fp, d, sizeof d / sizeof( *d ) );

  for ( int i = 0; i < numRead; ++i )
  {
    printf( "%f\n", d[i] );
  }

  fclose( fp );

  return 0;
}

It's a good idea to include the size of your array as an argument to readSales() to avoid buffer overrun, so I've modified the readSales() signature as such.

Do input validation for safety.

This answer addresses what you stated as your main concern in your original question: i.e. avoiding initialization of the 0th element of the array. You are correct that it is unnecessary for the task.

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