Question

I need help with this C code

Can you explain line by line?
Also can you explain to me the flow of the code, like the flow of how the compiler reads it. I need to present this and I want to make sure I understand every single line of it.

Thank you!

/*
* Converts measurements given in one unit to any other unit of the same
* category that is listed in the database file, units.txt.
* Handles both names and abbreviations of units.
*/
#include <stdio.h>
#include <string.h>
#define NAME_LEN 30       /* storage allocated for a unit name */
#define ABBREV_LEN 15       /* storage allocated for a unit abbreviation */
#define CLASS_LEN 20       /* storage allocated for a measurement class */
#define NOT_FOUND -1       /* value indicating unit not found */
#define MAX_UNITS 20       /* maximum number of different units handled */

typedef struct
{               /* unit of measurement type */
char name[NAME_LEN];       /* character string such as "milligrams" */
char abbrev[ABBREV_LEN];   /* shorter character string such as "mg" */
char class[CLASS_LEN];   /* character string such as "pressure",
               "distance", "mass" */
double standard;       /* number of standard units equivalent to this unit */
} unit_t;

int fscan_unit (FILE * filep, unit_t * unitp);
void load_units (int unit_max, unit_t units[], int *unit_sizep);
int search (const unit_t units[], const char *target, int n);
double convert (double quantity, double old_stand, double new_stand);

int
main (void)
{
unit_t units[MAX_UNITS];   /* units classes and conversion factors */
int num_units;       /* number of elements of units in use */
char old_units[NAME_LEN],   /* units to convert (name or abbrev) */
new_units[NAME_LEN];   /* units to convert to (name or abbrev) */
int status;           /* input status */
double quantity;       /* value to convert */

int old_index,       /* index of units element where
               old_units found */
new_index;           /* index where new_units found */

/* Load units of measurement database */
load_units (MAX_UNITS, units, &num_units);

/* Convert quantities to desired units until data format error
(including error code returned when q is entered to quit) */
printf ("Enter a conversion problem or q to quit.\n");
printf ("To convert 25 kilometers to miles, you would enter\n");
printf ("> 25 kilometers miles\n");
printf (" or, alternatively,\n");
printf ("> 25 km mi\n> ");

for (status = scanf ("%lf%s%s", &quantity, old_units, new_units);
status == 3;
status = scanf ("%lf%s%s", &quantity, old_units, new_units))
{
printf ("Attempting conversion of %.4f %s to %s . . . \n",
   quantity, old_units, new_units);
old_index = search (units, old_units, num_units);
new_index = search (units, new_units, num_units);
if (old_index == NOT_FOUND)
   printf ("Unit %s not in database\n", old_units);
else if (new_index == NOT_FOUND)
   printf ("Unit %s not in database\n", new_units);
else if (strcmp (units[old_index].class, units[new_index].class) != 0)
   printf ("Cannot convert %s (%s) to %s (%s)\n",
       old_units, units[old_index].class, new_units,
       units[new_index].class);
else
   printf ("%.4f%s = %.4f %s\n", quantity, old_units,
       convert (quantity, units[old_index].standard,
           units[new_index].standard), new_units);
printf ("\nEnter a conversion problem or q to quit.\n> ");
}
return (0);
}

/*
* Gets data from a file to fill output argument
* Returns standard error code: 1 => successful input, 0 => error,
* negative EOF value => end of file
*/
int
fscan_unit (FILE * filep,   /* input - input file pointer */
   unit_t * unitp)   /* output - unit_t structure to fill */
{
int status;

status = fscanf (filep, "%s%s%s%lf", unitp->name,
       unitp->abbrev, unitp->class, &unitp->standard);

if (status == 4)
status = 1;
else if (status != EOF)
status = 0;

return (status);
}

/*
* Opens database file units.txt and gets data to place in units until end
* of file is encountered. Stops input prematurely if there are more than
* unit_max data values in the file or if invalid data is encountered. */

void
load_units (int unit_max,   /* input - declared size of units */
   unit_t units[],   /* output - array of data */
   int *unit_sizep)   /* output - number of data values
               stored in units */
{
FILE *inp;
unit_t data;
int i, status;

/* Gets database of units from file */
inp = fopen ("units.txt", "r");
i = 0;

for (status = fscan_unit (inp, &data);
status == 1 && i < unit_max; status = fscan_unit (inp, &data))
{
units[i++] = data;
}
fclose (inp);

/* Issue error message on premature exit */
if (status == 0)
{
printf ("\n*** Error in data format ***\n");
printf ("*** Using first %d data values ***\n", i);
}
else if (status != EOF)
{
printf ("\n*** Error: too much data in file ***\n");
printf ("*** Using first %d data values ***\n", i);
}

/* Send back size of used portion of array */
*unit_sizep = i;
}

/*
* Searches for target key in name and abbrev components of first n
* elements of array units
* Returns index of structure containing target or NOT_FOUND
*/
int
search (const unit_t units[],   /* array of unit_t structures to search */
   const char *target,   /* key searched for in name and abbrev
               components */
   int n)           /* number of array elements to search */
{
int i, found = 0,       /* whether or not target has been found */
where;           /* index where target found or NOT_FOUND */

/* Compare name and abbrev components of each element to target */
i = 0;
while (!found && i < n)
{
if (strcmp (units[i].name, target) == 0 ||
   strcmp (units[i].abbrev, target) == 0)
   found = 1;
else
   ++i;
}
/* Return index of element containing target or NOT_FOUND */
if (found)
where = i;

else
where = NOT_FOUND;
return (where);
}

/*
* Converts one measurement to another given the representation of both
* in a standard unit. For example, to convert 24 feet to yards given a
* standard unit of inches: quantity = 24, old_stand = 12 (there are 12
* inches in a foot), new_stand = 36 (there are 36 inches in a yard),
* result is 24 * 12 / 36 which equals 8 */
double
convert (double quantity,   /* value to convert */
   double old_stand,   /* number of standard units in one of
               quantity's original units */
   double new_stand)   /* number of standard units in 1 new unit */
{
return (quantity * old_stand / new_stand);
}

units.txt FILE:

miles       mi           distance       1609.3
kilometers   km           distance       1000
yards       yd           distance       0.9144
meters        m           distance       1
quarts       qt           liquid_volume       0.94635
liters       l           liquid_volume       1
gallons       gal           liquid_volume       3.7854
milliliters   ml           liquid_volume       0.001
kilograms   kg           mass           1
grams       g           mass           0.001
slugs        slugs           mass           0.14594
pounds       lb           mass           0.43592

Answer 1

Hi, I entered comment in all lines. just go through the code. now you can understand it properly.

- This the program of the unit converter.

- if you check main(): we are loading units array fist. using load_units function.

- Then we are taking data from the user.

- and after that, we are checking if any errors are there or not.

- and then if all things are ok are converting it to a new unit from the old unit.

/*

* Converts measurements given in one unit to any other unit of the same

* category that is listed in the database file, units.txt.

* Handles both names and abbreviations of units.

*/

#include <stdio.h>

#include <string.h>

#define NAME_LEN 30 /* storage allocated for a unit name */

#define ABBREV_LEN 15 /* storage allocated for a unit abbreviation */

#define CLASS_LEN 20 /* storage allocated for a measurement class */

#define NOT_FOUND -1 /* value indicating unit not found */

#define MAX_UNITS 20 /* maximum number of different units handled */

typedef struct

{ /* unit of measurement type */

   char name[NAME_LEN]; /* character string such as "milligrams" */

   char abbrev[ABBREV_LEN]; /* shorter character string such as "mg" */

   char class[CLASS_LEN]; /* character string such as "pressure",

             "distance", "mass" */

   double standard; /* number of standard units equivalent to this unit */

} unit_t;

/* function diclaration */

int fscan_unit (FILE * filep, unit_t * unitp); /* read data from file */

void load_units (int unit_max, unit_t units[], int *unit_sizep); /* load data from file to array */

int search (const unit_t units[], const char *target, int n); /* serch unit from array of units */

double convert (double quantity, double old_stand, double new_stand); /* convert data */

   int

main (void)

{

   unit_t units[MAX_UNITS]; /* units classes and conversion factors */

   int num_units; /* number of elements of units in use */

   char old_units[NAME_LEN], /* units to convert (name or abbrev) */

    new_units[NAME_LEN]; /* units to convert to (name or abbrev) */

   int status; /* input status */

   double quantity; /* value to convert */

   int old_index, /* index of units element where

            old_units found */

    new_index; /* index where new_units found */

   /* Load units of measurement database */

   load_units (MAX_UNITS, units, &num_units);

   /* Convert quantities to desired units until data format error

    (including error code returned when q is entered to quit) */

   printf ("Enter a conversion problem or q to quit.\n"); /* give option to user */

   printf ("To convert 25 kilometers to miles, you would enter\n"); /* give usage to user */

   printf ("> 25 kilometers miles\n");

   printf (" or, alternatively,\n");

   printf ("> 25 km mi\n> ");

   for (status = scanf ("%lf%s%s", &quantity, old_units, new_units); /* get data from user quantity old_units and new_units */

status == 3; /* status shoud be 3 because data we are reading is 3 */

status = scanf ("%lf%s%s", &quantity, old_units, new_units)) /* get data from user quantity old_units and new_units */

   {

      printf ("Attempting conversion of %.4f %s to %s . . . \n",

            quantity, old_units, new_units); /* print data that user entered */

      old_index = search (units, old_units, num_units); /* serch old unit index in units array */

      new_index = search (units, new_units, num_units); /* serch new unit index in units array */

      if (old_index == NOT_FOUND) /* if old unit not found print error */

         printf ("Unit %s not in database\n", old_units); /* print error message for old unit */

      else if (new_index == NOT_FOUND) /* if new unit not foud */

         printf ("Unit %s not in database\n", new_units); /* print error message for new unit */

      else if (strcmp (units[old_index].class, units[new_index].class) != 0) /* check if old and new unit are same print error message */

         printf ("Cannot convert %s (%s) to %s (%s)\n",

               old_units, units[old_index].class, new_units,

               units[new_index].class); /* error message for old and new unit is same */

      else

         printf ("%.4f%s = %.4f %s\n", quantity, old_units,

               convert (quantity, units[old_index].standard,

                  units[new_index].standard), new_units); /* if user entered data is same convert old to new unit and print it */

      printf ("\nEnter a conversion problem or q to quit.\n> "); /* ask user to enter again or quit by entering q */

   }

   return (0); // return to _start with 0

}

/*

* Gets data from a file to fill output argument

* Returns standard error code: 1 => successful input, 0 => error,

* negative EOF value => end of file

*/

   int

fscan_unit (FILE * filep, /* input - input file pointer */

      unit_t * unitp) /* output - unit_t structure to fill */

{

   int status; /* define variable to store fscanf status */

   status = fscanf (filep, "%s%s%s%lf", unitp->name,

         unitp->abbrev, unitp->class, &unitp->standard); /* read file using fscanf store data in structure membters */

   if (status == 4) /* status shoud be 4 because we are reading 4 data from file */

      status = 1; /* make status 1 */

   else if (status != EOF) /* if status is not end of file */

      status = 0; /* make status 0 */

   return (status); /* return status to calling function */

}

/*

* Opens database file units.txt and gets data to place in units until end

* of file is encountered. Stops input prematurely if there are more than

* unit_max data values in the file or if invalid data is encountered. */

   void

load_units (int unit_max, /* input - declared size of units */

      unit_t units[], /* output - array of data */

      int *unit_sizep) /* output - number of data values

             stored in units */

{

   FILE *inp; /* File pointer */

   unit_t data; /* unit variable data */

   int i, status; /* integer variable */

   /* Gets database of units from file */

   inp = fopen ("units.txt", "r"); /* open file units.txt for read */

   i = 0; /* initialize i with 0 */

   for (status = fscan_unit (inp, &data); /* read unit in data and get status in status */

         status == 1 && i < unit_max; /* if status is 1 then enter in side the for part and i shoud less than unit_max */

status = fscan_unit (inp, &data)) /* read unit in data and get status in status */

   {

      units[i++] = data; /* store data in units array with ith index and increment i after storing data */

   }

   fclose (inp); /* after storing all data close file */

   /* Issue error message on premature exit */

   if (status == 0) /* if status is 0 print error message */

   {

      printf ("\n*** Error in data format ***\n"); /* print data formate error */

      printf ("*** Using first %d data values ***\n", i); /* print in which index there is formate problem */

   }

   else if (status != EOF) /* if status is not EOF(end of file) means there are too much data and units array is full print error message */

   {

      printf ("\n*** Error: too much data in file ***\n"); /* print too much data error */

      printf ("*** Using first %d data values ***\n", i); /* print index */

   }

   /* Send back size of used portion of array */

   *unit_sizep = i; /* store size of array in unit_sizep referance variable */

}

/*

* Searches for target key in name and abbrev components of first n

* elements of array units

* Returns index of structure containing target or NOT_FOUND

*/

int

search (const unit_t units[], /* array of unit_t structures to search */

      const char *target, /* key searched for in name and abbrev

                components */

      int n) /* number of array elements to search */

{

   int i, found = 0, /* whether or not target has been found */

   where; /* index where target found or NOT_FOUND */

   /* Compare name and abbrev components of each element to target */

   i = 0;

   while (!found && i < n) /* If found is zero and i is less than n it will enter in while */

   {

      if (strcmp (units[i].name, target) == 0 ||

            strcmp (units[i].abbrev, target) == 0) /* check name and abbrev perameter with targes is any one is same found will 1 */

         found = 1;

      else

         ++i; // if not same i will be increment by 1

   }

   /* Return index of element containing target or NOT_FOUND */

   if (found) /* if found is 1 then give that unit idex to where */

      where = i;

   else

      where = NOT_FOUND; /* if unit not found where will have NOT_FOUND */

   return (where); /* return where(index) */

}

/*

* Converts one measurement to another given the representation of both

* in a standard unit. For example, to convert 24 feet to yards given a

* standard unit of inches: quantity = 24, old_stand = 12 (there are 12

* inches in a foot), new_stand = 36 (there are 36 inches in a yard),

* result is 24 * 12 / 36 which equals 8 */

double

convert (double quantity, /* value to convert */

      double old_stand, /* number of standard units in one of

             quantity's original units */

      double new_stand) /* number of standard units in 1 new unit */

{

   return (quantity * old_stand / new_stand); /* (24*12/36) */

}