Thread: Reading values from external text file

Well post the code you have that uses fscanf.

Originally Posted by Salem

Well post the code you have that uses fscanf.

Nevermind, I figured that out. My problem with the code now is the calculations are off. I initially had the calculations within the main function and they worked fine, but moved them to user defined functions below the main. Initially when I did this I got a ton of errors so changed some variable names in the user defined functions. Now the numbers are off, and I can't figure out why. They should read:

ROCKET MOTOR TEST RESULTS


Total Impulse: 217.1 lb-sec
Specific Impulse: 141.0 sec
Average Thrust: 103.4 lb

But are now reading:

ROCKET MOTOR TEST RESULTS


Total Impulse: 217.0 lb-sec
Specific Impulse: 140.0 sec
Average Thrust: 206.0 lb

I didn't change the calculations, just moved them. This is my first time attempting the user defined functions and call backs, so I'm not sure what in the mix is throwing this off.

Code:

/* Preprocessor directives */
#include <stdio.h>
#include <math.h>
#define prop_mass 1.54
#define infile "C:\\Users\\tommc\\OneDrive\\Documents\\ENGR 200\\\
time-thrust.txt"
#define outfile "C:\\Users\\tommc\\OneDrive\\Documents\\ENGR 200\\\
impulse-thrust.txt"

/* function prototypes */
int get_i_t(double);
int get_i_sp(double);
int get_f_avg(double);

/* Main function */
int main(void)
{
   /* Declare variables */
   double i_t, calc_it, calc_sp, calc_f_avg, i_sp, f_avg, delta_t, t, F, f_tot,
   t_tot;
   int i, ndata;
   FILE *time=NULL, *impulse=NULL;
   
   /* open files */
   
   time=fopen(infile, "r");
   impulse=fopen(outfile, "w");

   
   /* Print headings */
   printf("********************************************");
   printf("\n         ROCKET MOTOR TEST RESULTS");
   fprintf(impulse,"********************************************");
   fprintf(impulse,"\n         ROCKET MOTOR TEST RESULTS");
   
   /* Verify input file and terminate program if file not found */
   
   if(time==NULL)
   {
       printf("\n\n\n\n ERROR OPENING INPUT FILE.");
       printf("\n\n PROGRAM TERMINATED.\n\n\n");
       return 1;
   }
   
   /* Read control number */
   
   fscanf(time, "%i", &ndata);
         
       /* Calculate specific impulse and average thrust */
       
       i_t = get_i_t(calc_it);
       i_sp = get_i_sp(calc_sp);
       f_avg = get_f_avg(calc_f_avg);
       
   /* Print output values */
   
   printf("\n\nTotal Impulse:      %5.1lf", i_t);
   printf(" lb-sec");
   printf("\nSpecific Impulse:   %5.1lf", i_sp);
   printf(" sec");
   printf("\nAverage Thrust:     %5.1lf", f_avg);
   printf(" lb");
   printf("\n********************************************\n\n\n");
   fprintf(impulse, "\n\nTotal Impulse:      %5.1lf", i_t);
   fprintf(impulse, " lb-sec");
   fprintf(impulse, "\nSpecific Impulse:   %5.1lf", i_sp);
   fprintf(impulse, " sec");
   fprintf(impulse, "\nAverage Thrust:     %5.1lf", f_avg);
   fprintf(impulse, " lb");
   fprintf(impulse, "\n********************************************\n\n\n");
   
   /* Close the files */
   fclose(time);
   fclose(impulse);

   /* Exit the program */
   return 0;
}

get_i_t(double calc_it)
{
double calc_i_t, delta_t, F, t;
int i, ndata;
FILE *time=NULL;
time=fopen(infile, "r");

fscanf(time, "%i", &ndata);

for (i = 0; i <= ndata; i++)
    {
    fscanf(time, "%lf,%lf,%lf", &t, &delta_t, &F);
    calc_it += delta_t * F;
    }
    
fclose(time);

return calc_it;    
}

get_i_sp(double calc_sp)
{
 /* Declare variables */
   double i_t, i_sp, delta_t, F, t;
   int i, ndata;
   FILE *time=NULL;
   
   time=fopen(infile, "r");
   
   fscanf(time, "%i", &ndata);
   
for (i = 0; i <= ndata; i++)
   {
   fscanf(time, "%lf,%lf,%lf", &t, &delta_t, &F);
   i_t += delta_t * F;
   }
calc_sp = i_t / prop_mass;
fclose(time);
return calc_sp;
}

get_f_avg(double f_avg)
{
/* Declare variables */

   double i_t, delta_t, t, F, t_tot, calc_f_avg;
   int i, ndata;
   FILE *time=NULL;
   time=fopen(infile, "r");
   fscanf(time, "%i", &ndata);
   
for (i = 0; i <= ndata; i++)
    {
    fscanf(time, "%lf,%lf,%lf", &t, &delta_t, &F);
    i_t += delta_t * F;
    t_tot = t;
    }
calc_f_avg = i_t / t_tot;
fclose(time);
return calc_f_avg;
}

Also, this is the contents of the external file

22
0.0,0.0,0.0
0.1,0.1,50.0
0.2,0.1,100.0
0.3,0.1,140.0
0.4,0.1,150.0
0.5,0.1,155.0
0.6,0.1,155.0
0.7,0.1,154.0
0.8,0.1,150.0
0.9,0.1,149.0
1.0,0.1,148.0
1.1,0.1,140.0
1.2,0.1,138.0
1.3,0.1,132.0
1.4,0.1,120.0
1.5,0.1,100.0
1.6,0.1,80.0
1.7,0.1,50.0
1.8,0.1,30.0
1.9,0.1,20.0
2.0,0.1,10.0
2.1,0.1,0.0

> for (i = 0; i <= ndata; i++)
Every one of your for loops should use <, not <=

You're going round the loops 23 times instead of 22, resulting in an extra calculation with potentially garbage values from the fscanf.

There are multiple warnings that need to be fixed, like using uninitialised variables and mis-matched return types.
In fact, it looks like all your functions should return doubles, not ints.

Code:

$ gcc -Wall foo.c
foo.c: In function ‘main’:
foo.c:21:8: warning: unused variable ‘i’ [-Wunused-variable]
   21 |    int i, ndata;
      |        ^
foo.c:20:4: warning: unused variable ‘t_tot’ [-Wunused-variable]
   20 |    t_tot;
      |    ^~~~~
foo.c:19:74: warning: unused variable ‘f_tot’ [-Wunused-variable]
   19 |    double i_t, calc_it, calc_sp, calc_f_avg, i_sp, f_avg, delta_t, t, F, f_tot,
      |                                                                          ^~~~~
foo.c:19:71: warning: unused variable ‘F’ [-Wunused-variable]
   19 |    double i_t, calc_it, calc_sp, calc_f_avg, i_sp, f_avg, delta_t, t, F, f_tot,
      |                                                                       ^
foo.c:19:68: warning: unused variable ‘t’ [-Wunused-variable]
   19 |    double i_t, calc_it, calc_sp, calc_f_avg, i_sp, f_avg, delta_t, t, F, f_tot,
      |                                                                    ^
foo.c:19:59: warning: unused variable ‘delta_t’ [-Wunused-variable]
   19 |    double i_t, calc_it, calc_sp, calc_f_avg, i_sp, f_avg, delta_t, t, F, f_tot,
      |                                                           ^~~~~~~
foo.c: At top level:
foo.c:80:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
   80 | get_i_t(double calc_it)
      | ^~~~~~~
foo.c: In function ‘get_i_t’:
foo.c:82:8: warning: unused variable ‘calc_i_t’ [-Wunused-variable]
   82 | double calc_i_t, delta_t, F, t;
      |        ^~~~~~~~
foo.c: At top level:
foo.c:100:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
  100 | get_i_sp(double calc_sp)
      | ^~~~~~~~
foo.c: In function ‘get_i_sp’:
foo.c:103:16: warning: unused variable ‘i_sp’ [-Wunused-variable]
  103 |    double i_t, i_sp, delta_t, F, t;
      |                ^~~~
foo.c: At top level:
foo.c:121:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
  121 | get_f_avg(double f_avg)
      | ^~~~~~~~~
foo.c: In function ‘main’:
foo.c:51:14: warning: ‘calc_it’ may be used uninitialized [-Wmaybe-uninitialized]
   51 |        i_t = get_i_t(calc_it);
      |              ^~~~~~~~~~~~~~~~
foo.c:52:15: warning: ‘calc_sp’ may be used uninitialized [-Wmaybe-uninitialized]
   52 |        i_sp = get_i_sp(calc_sp);
      |               ^~~~~~~~~~~~~~~~~
foo.c:53:16: warning: ‘calc_f_avg’ may be used uninitialized [-Wmaybe-uninitialized]
   53 |        f_avg = get_f_avg(calc_f_avg);
      |                ^~~~~~~~~~~~~~~~~~~~~

> t_tot = t;
I think you meant += here, if you want a total.

Also, there is no longer any point to opening the input file in main.
Each function now handles the input file.