Reading file data to array of structures while allocating memory dynamically

Question

I need help solving the code below. After the code is compiled using gcc it can be run like ./compiledFile inputFile.txt It should take the inputFile.txt read it while allocating memory dynamically for each variable in this case name and courseID, but my code is not working. The place that I don't understand the most and need help is allocating memory, inserting data into structures and printing the data like the example given below. By the look of this code you could tell that I am a newbie to c and dynamic memory allocation and structure in all.

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

struct people
{
  char* name[10];
  char* courseID[15];
  int grade;
};

void printData(struct people student[], int count);
int main(int argc, char* argv[])
{
  FILE *in_file;
  char buffer[30];
  char *token, *del=",";
  int count=0;

  struct people student[20];

  if(( in_file = fopen(argv[1], "r")) == NULL)
  {
     printf("unable to open the file");
  }
  while (fgets(buffer, sizeof(buffer), in_file))
  {
     student = malloc(sizeof(struct people));
     token = strtok(buffer, del);
     strcpy(student[count].name, token);
     count++;
  }
  fclose(in_file);
  printData(student, count);
}
void printData(struct people student[], int count)
{
  int i;
  for(i=0; i<count; i++)
  {
    printf("%s", student[i].courseID);
    if (strcmp((student[i].name, student[i].courseID) > 0))
    {
      printf("%s  %s", student[i].name, student[i].grade)
    }
  }
}

the data.txt file has the following content separated by a comman:

John,MATH 1324,90 David,SCI 1401,88 Omondi,MATH 1324,89 David,MATH 1324,90

when printed out it should look like the following:

MATH 1324
John 90
Omondi 89
David 90

SCI 1401
David 88

Answer 1

1. Change the definition of people to:

   struct people
   {
     char name[10];
     char courseID[15];
     int grade;
   };
   

   This assumes that name won't be longer than 9 characters and coursID won't be longer than 14 characters. If that is not true, change them accordingly.

2. The line:

   student = malloc(sizeof(struct student);
   

   is wrong in couple of ways.

   • student is already declared to be an array of people. You cannot assign it to point to memory allocated by malloc.

   • struct student is not a type.

   That line can be removed.

3. The line

   strcpy(student[count].name, token);
   

   can cause problems if the length of token is longer than 10 (or whatever size you choose for name in people). The safer thing to do is use strncpy.

   strncpy(student[count].name, token, 10);
   student[count].name[9] = '\0';
   

4. You have not set the value of courseID anywhere. Yet, you are trying to print it in printData. You are doing the same thing for grade. You need to update the processing of input lines to set those correctly.

   Change the while loop to:

   while (fgets(buffer, sizeof(buffer), in_file))
   {
      token = strtok(buffer, del);
      strncpy(student[count].name, token, 10);
      student[count].name[9] = '\0';
      token = strtok(NULL, del);
      strncpy(student[count].courseID, token, 15);
      student[count].courseID[14] = '\0';
      token = strtok(NULL, del);
      student[count].grade = atoi(token);
      count++;
   }
   

5. There are couple of syntax errors in printData. However, fixing those syntax errors does not take care of your printing requirements. It will be easier to print the data in the order that you want to if you sort the data. The following functions will help you sort the data.

   int compareStudent(const void* ptr1, const void* ptr2)
   {
      struct people* p1 = (struct people*)ptr1;
      struct people* p2 = (struct people*)ptr2;
      return (strcmp(p1->courseID, p2->courseID));
   }
   
   void sortData(struct people student[], int count)
   {
      qsort(student, count, sizeof(struct people), compareStudent);
   }
   

   You can call sortData before calling printData or call sortData first in printData. The logic for printing the data needs to be updated a little bit. Here's an updated printData.

   void printData(struct people student[], int count)
   {
     int i;
     int j;
   
     sortData(student, count);
   
     for(i=0; i<count; ++i)
     {
       printf("%s\n", student[i].courseID);
       printf("%s %d\n", student[i].name, student[i].grade);
       for ( j = i+1; j < count; ++j )
       {
          if (strcmp(student[i].courseID, student[j].courseID) == 0)
          {
             printf("%s %d\n", student[j].name, student[j].grade);
          }
          else
          {
             i = j-1;
             break;
          }
       }
     }
   }
   

Answer 2

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

struct people {
    char name[10];//char *name[10] is array of pointer 
    char courseID[15];
    int grade;
};

void printData(struct people student[], int count);

int main(int argc, char* argv[]){
    FILE *in_file;
    char buffer[30];
    char *token, *del=",";
    int count=0;

    struct people student[20];

    if((in_file = fopen(argv[1], "r")) == NULL){
        printf("unable to open the file");
        return -1;//It is not possible to continue the process
    }

    while (fgets(buffer, sizeof(buffer), in_file)){
        //student = malloc(sizeof(struct people));//It is by securing an array already
        token = strtok(buffer, del);
        strcpy(student[count].name, token);
        token = strtok(NULL, del);
        strcpy(student[count].courseID, token);
        token = strtok(NULL, del);
        student[count].grade = atoi(token);
        count++;
    }
    fclose(in_file);
    printData(student, count);
}

int cmp(const void *a, const void *b){
    const char *x = ((const struct people*)a)->courseID;
    const char *y = ((const struct people*)b)->courseID;
    return strcmp(x, y);
}

void printData(struct people student[], int count){
    qsort(student, count, sizeof(struct people), cmp);//sort by courseID
    char *prev = "";
    int i;
    for(i=0; i<count; i++){
        if(strcmp(prev, student[i].courseID)!=0){
            prev = student[i].courseID;
            printf("\n%s\n", prev);
        }
        printf("%-9s %d\n", student[i].name, student[i].grade);
    }
}

Answer 3

Several issues with the Question code...

1) Definition of main():

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

It must return an integer. Add a return statement at the end of main(), and make a proper "CLEANUP" section:

   printData(student, count);

CLEANUP:

   if(in_file)
      fclose(in_file);

   return(0);
}

---

2) Better handling of fopen() error condition:

  if(( in_file = fopen(argv[1], "r")) == NULL)
  {
     printf("unable to open the file");
     goto CLEANUP;
  }

And, initialize the in_file pointer:

int main(int argc, char* argv[])
{
  FILE *in_file = NULL;

---

3) Next, the exact definition of student needs to be established. Is it a static array, or is a pointer to a dynamically allocated array? I will assume that you would like to use a dynamic array (given the question text). However, this assumption conflicts with the following line, which defines student as a static array:

  struct people student[20];

Change it to:

  struct people *student = NULL;

---

4) Now, the following question code allocates a new (separate) chunk of memory for each student:

     student = malloc(sizeof(struct people));

However, what is needed is all the student records in one array, in the same chunk of memory. So, what is needed is to expand a chunk of memory to include student records as they are read, like this:

  while (fgets(buffer, sizeof(buffer), in_file))
  {
     void *tmp = realloc(student, sizeof(struct people) * (count + 1));
     if(NULL == tmp)
     {
        printf("realloc() failed.\n");
        goto CLEANUP;
     }
     student = tmp;

     token = strtok(buffer, del);

---

5) Take a look at the people structure:

struct people
{
  char* name[10];
  char* courseID[15];
  int grade;
};

It appears that the question code has some difficulty when it comes to pointers & arrays. The code is attempting to define the name and courseID fields as both pointers, and arrays. Given that the question is to do with dynamically allocating stuff, I elect to go that direction. Hence, this structure should be changed to the following:

struct people
{
  char *name;
  char *courseID;
  int   grade;
};

---

6) So, each time through the loop, the student name will be placed in allocated storage, and pointed to by the .name field. So, change this:

     token = strtok(buffer, del);
     strcpy(student[count]->name, token);
     count++;
  }

to this:

     token = strtok(buffer, del);
     student[count].name = strdup(token);
     count++;
  }

---

7) I don't understand the intent of this line:

   if (strcmp((student[i].name, student[i].courseID) > 0))

I am inclined to eliminate it.

---

8) The following line has flaws:

      printf("%s  %s", student[i].name, student[i].grade)

Change it to this to print the integer grade (and don't forget the ending semicolon):

      printf("%s  %d\n", student[i].name, student[i].grade);

The '\n' makes the output look better, one record per line.

---

9) Since student is a pointer to dynamically allocated memory (not a static array), change this:

void printData(struct people student[], int count)

to this:

void printData(struct people *student, int count)

---

10) Now, finish the task of parsing the data; from this:

     token = strtok(buffer, del);
     strcpy(student[count].name, token);
     count++;
  }

to this:

     token = strtok(buffer, del);
     student[count].name = strdup(token);
     token = strtok(NULL, del);
     student[count].courseID = strdup(token);
     token = strtok(NULL, del);
     student[count].grade = strtol(token, NULL, 10);
     count++;
  }

---

11) Now, to make life easier, sort the array. First by courseID, then by name:

 ...  
     count++;
  }

  /** Sort the array by coursID, then by name. **/
  qsort(student, count, sizeof(*student), CmpStudentRecs);

  printData(student, count);
 ...

Which will require an additional "Compare Student Recs" function:

int CmpStudentRecs(const void *recA, const void *recB)
{
  int result = 0;
  struct people *stuRecA = (struct people *)recA;
  struct people *stuRecB = (struct people *)recB;

  /** First compare the courseIDs **/    
  result=strcmp(stuRecA->courseID, stuRecB->courseID);

  /** Second (if courseIDs match) compare the names **/
  if(!result)
     result=strcmp(stuRecA->name, stuRecB->name);

  return(result);
}

---

12) Some finishing touches with the printData() function:

void printData(struct people *student, int count)
{
  int i;
  char *courseID = "";

  for(i=0; i<count; i++)
  {
    if(strcmp(courseID, student[i].courseID))
      {
      printf("%s\n", student[i].courseID);
      courseID = student[i].courseID;
      }

    printf("\t%s  %d\n", student[i].name, student[i].grade);
  }
}

---

Finished. Output:

SLES11SP2:~/SO> ./test data.txt
MATH 1324
   David  90
   John  90
   Omondi  89
SCI 1401
   David  88
SLES11SP2:~/SO> 

---

SPOILER

Answer 4

first of all, it would be great if you could also share what is the actual output or error you get while running this program.

most of the time dynamic memory allocation is used when we do not know the actual size of data elements, but here you have already fixed the size of struct people student as 20

 struct people student[20];

this is absolutely fine, but then you do malloc in while loop

     student = malloc(sizeof(struct student);

you have already alloted 20 locations using array declaration, now malloc is not required. if you want to use dynamic memory allocation using pointers for learning purpose then you should first declare student as pointer to type struct people

struct people* student;

allocate memory dynamically in while loop

student=(struct people*) malloc(sizeof(struct people));

then access it

*(student+count)

hope this helps, if you still have doubts/problems edit the question and include the output/error you get while compiling/running this program.