Thread: Problem writing to and reading from external text file

Hi,

I am trying to develop a small programme that writes a three field "record" to an external text file and then reads that same data back from that external file.

So far, the programme does write the "records" to the external file in a one-field-per-line format whereas I would prefer to have all three fields of a "record" on one line in the external file. The contents of the external file are shown below.

Fred Zodiac
12345
3.8
Barney Tempest
23456
3.5
Wilma Shore
34567
3.7

This clearly shows that I am having a problem understanding how to write data to an external file in the required three-fields-per-line format, in the function saveStudentInformation().

The programme also does find a target "record" in the external file but displays it on-screen in the one-field-per-line format. Again this shows that I am having trouble understanding how to get printf() in function showStudentInformation() to display the data in the required three-fields-per-line format.

The code is shown below.

Code:

/* Create a program that uses a menu with options to enter student information*
 * (name, ID, GPA), print student information, or quit the program. Use data      *
 * files and FILE pointers to store and print information entered.                         */

#include <stdio.h>
#include <stdlib.h>  // For fgets()
#include <string.h>  // For strcpy()
#include <ctype.h>   // For toupper()


static void showMenu();
static void enterStudentInformation(char [], char [], char []);
static void saveStudentInformation(char [], char [], char []);
static void findStudentInformation(char [], char [], char []);
static void showStudentInformation(char [], char [], char []);


int main()
{
   char studentName[20] = "";
   char id[20] = "";
   char gpa[20] = "";
   char response;
   char newline;

   showMenu();
   (void) scanf("%c%c", &response, &newline);  // second %c swallows \n
   response = toupper(response);

   while (response != 'Q')
   {
      switch (response)
      {
         case 'A': enterStudentInformation(studentName, id, gpa);
                   saveStudentInformation(studentName, id, gpa);
                   break;
         case 'B': findStudentInformation(studentName, id, gpa);
                   showStudentInformation(studentName, id, gpa);
                   break;
      }

      showMenu();
      (void) scanf("%c%c", &response, &newline);
      response = toupper(response);
   }
   return 0;
} // end main()


void showMenu()
{
   printf("\tMenu\n");
   printf("\nA. Enter student information. <A/a>: ");
   printf("\nB. Show student information. <B/b>: ");
   printf("\nC. Quit the program. <Q/q>: ");
}


void enterStudentInformation(char aStudentName[], char anID[], char aGPA[])
{
   int stringSize = 19;

   printf("\nEnter student name (firstname lastname): ");
   (void) fgets(aStudentName, stringSize, stdin);
   printf("\nEnter student ID: ");
   (void) fgets(anID, stringSize, stdin);
   printf("\nEnter student gpa: ");
   (void) fgets(aGPA, stringSize, stdin);
}


void saveStudentInformation(char aStudentName[], char anID[], char aGPA[])
{
   FILE * ptrWriteRecord;

   ptrWriteRecord = fopen("studentInformation.dat", "a");
   if (ptrWriteRecord != NULL)
   {
      fprintf(ptrWriteRecord, "%s%s%s", aStudentName, anID, aGPA);
      (void) fclose(ptrWriteRecord);
   }
   else
   {
      printf("\nFile studentInformation.dat cannot be opened.\n");
   }
}


void findStudentInformation(char aStudentName[], char anID[], char aGPA[])
{
   FILE * ptrReadRecord;
   char temp[20] = "";
   int stringSize = 20;

   ptrReadRecord = fopen("studentInformation.dat", "r");
   if (ptrReadRecord != NULL)
   {
      printf("\nEnter student name (firstname lastname): ");
      (void) fgets(temp, stringSize, stdin);

      while (feof(ptrReadRecord) != EOF)
      {
         (void) fgets(aStudentName, stringSize, ptrReadRecord);
         if (strcmp(aStudentName, temp)== 0)
         {
            (void) fgets(anID, stringSize, ptrReadRecord);
            (void) fgets(aGPA, stringSize, ptrReadRecord);
            fclose(ptrReadRecord);
            return;  // student information has been found
         }
      }  // end while()
   }
   else
   {
      printf("\nFile studentInformation.dat cannot be opened.\n");
      fclose(ptrReadRecord);
   }
}  // end findStudentInformation()


void showStudentInformation(char aStudentName[], char anID[], char aGPA[])
{
   printf("\nName\t\tID\t\tGPA\n");
   printf("%s%s%s\n", aStudentName, anID, aGPA);
}

How can I write a "record" to the external file in a three-fields-per-line format and then display the data on-screen in that same format. For reading the external file in function findStudentInformation() I have tried using both fgets() and fscanf(), to no avail.

Stuart

The fgets() function reads a line including the new-line character. If you read from stdin, hitting the Enter key will create a new line in this case. Determine the length of the string, and overwrite the last character with '\0'.

You may want to handle NULL and zero-length strings.

Code:

void delnl(char *const str){
  const size_t len = str ? strlen(str) : 0;


  if (len && str[len - 1] == '\n')
  {
    str[len - 1] = '\0';
  }
}

Originally Posted by aGerman

You may want to handle NULL and zero-length strings.

Code:

void delnl(char *const str){
  const size_t len = str ? strlen(str) : 0;


  if (len && str[len - 1] == '\n')
  {
    str[len - 1] = '\0';
  }
}

I would handle both cases in the calling function when I capture the length of the string input, If NULL or only newline, no need to call delnl().

Originally Posted by rstanley

I would handle both cases in the calling function when I capture the length of the string input, If NULL or only newline, no need to call delnl().

Plus there's a simpler way to set the trailing newline to a null terminator than how delnl does it: str[strcspn(str, "\n")] = '\0';

Not standard, but if the '\n' appears only at the end of the string:

Code:

char *p = strrchr( str, '\n' );
if ( p ) *p = '\0';

Before rstanlay updated his comment I've briefly seen hin posting a piece of code that ignores characters that have not been read. This is actually also required for fgets if the input is longer than buffer size - 2. User input can be complicated

Code:

#include <stdbool.h>#include <stddef.h>
#include <stdio.h>
#include <string.h>


/// @brief Remove a trailing new line character from a null-terminated string.
///
/// @param str       Pointer to the first character of the string. (NULL does
///                  not crash)
/// @param pNlFound  Pointer to a variable retrieving the information whether
///                  `str` had an appended new line character. (can be NULL)
///
/// @return The new length of `str` is returned.
static size_t delnl(char *const str, bool *const pNlFound)
{
  if (pNlFound)
    *pNlFound = false;


  const size_t len = str ? strlen(str) : 0;
  if (!len)
    return len;


  char *const pLast = str + len - 1;
  if (*pLast == '\n')
  {
    if (pNlFound)
      *pNlFound = true;


    *pLast = '\0';
    return len - 1;
  }


  return len;
}


#define STR_SIZ 20 // size for a string of 19 characters + terminating null


int main(void)
{
  char myBuf[STR_SIZ] = {0};
  size_t len = 0;
  bool nlFound = false;


  // Iterate as long as Enter has been pressed without any other input.
  do
  {
    fputs("Enter something: ", stdout);
  } while (!(len = delnl(fgets(myBuf, STR_SIZ, stdin), &nlFound)));


  printf(
    "\n"
    "String content: '%s'\n"
    "%zu character(s) read.\n",
    myBuf, len
  );


  // Not finding the new line char at the end of the string is a strong
  // indicator that the input has been larger than the buffer.
  len = 0;
  if (!nlFound)
    while (getchar() != '\n') // Ignore the remaining characters in stdin.
      ++len; // We count them only for testing purposes.


  printf(
    "%zu character(s) ignored.\n",
    len
  );
}

Originally Posted by Salem

strcspn - cppreference.com
Bear in mind that because the second parameter is also a string, strcspn has a nested loop.
Sure, it runs only once, but it's a loop nonetheless.

Yep, though because n is a fixed value (2 if you count the null terminator), it's really just O(1) rather than O(n) in time complexity.

Here's my quick-and-dirty (and untested) version of strcspn() that takes a single character for the second parameter:

Code:

size_t str_length_or_match(const char *str, int c)
{
    const char *s = str;
    while (*s != '\0' && *s != c) {
        ++s;
    }
    return s - str;
}

// use thusly:
// str[str_length_or_match(str, '\n')] = '\0';

Then again, it's possible that the system's implementation of strcspn() is still faster than this, even with the loop on the second parameter.

Originally Posted by flp1969

Not standard, but if the '\n' appears only at the end of the string:

Code:

char *p = strrchr( str, '\n' );
if ( p ) *p = '\0';

As far as I know, strrchr() is standard so that's a portable way to do it.

Sorry... my mistake strrchr() is, in fact, listed on ISO 9899.