applications in linked lists in c programming - arrays

Question

The question is at follows
The structure below represents information about students and their list of grades in
all modules.
typedef struct {
char* module; //module, e.g. "COMP1028"
int grade; //numerical grade
} module_grade;
typedef struct {
unsigned int student_ID;
char* student_name;
module_grade* grades; //array of grades
unsigned int grades_len; //length of the array grades
} student;
You are required to implement THREE functions for managing students’ grades.
a) Write a C function called create_student_list that takes a student name
and a student ID and returns a pointer to a new_student. The grades of the
new_student will be initialized to NULL and grade_len is set to 0.
The definition of the function is:
student* create_student_list (char* stu_name, int stu_id) {
b) Write a C function search_grade that takes a pointer to a student and a module
and returns the grade of the student for this module. If the array of grades is
NULL or the module is not in the array of grades, the function shall return -1.
The definition of the function is:
int search_grade(student* s, char* module) {
c) Write a C function adding_grade that takes a pointer to a student and a module
grade and adds the grade to the student’s array of grades.
If student->grades already contain a similar module, the function does nothing.
If student->grades==NULL, you need to create an array of length 1 and add the
new grade into the array.
If student->grades!=NULL, you need to increase the size of the array by ONE,
and add new_grade into the new entry in the array of grades.
The function shall return 1 if a grade is successfully added. Otherwise, the
function shall return 0; this can be because the module is already there or malloc
fails.
The definition of the function is:
int adding_grade(student* s, module_grade new_grade)
in question a) I don't understand if he want it to be a linked list so we would need a loop but i can't manage to make a node and link it together so i only assigned the data to the pointer but it is not reading it correctly. outputting only one character and not the whole string.
typedef struct {
char* module; //module, e.g. "COMP1028"
int grade; //numerical grade
} module_grade;
typedef struct {
unsigned int student_ID;
char student_name[50];
module_grade* grades; //array of grades
unsigned int grades_len; //length of the array grades
} student;
student* create_student_list (const char *stu_name, int stu_id);
int main(){
student Student;
module_grade Module;
printf("enter the student name?\t");
scanf("%s",Student.student_name);
printf("%s",Student.student_name);
printf("\nenter the student ID?\t");
scanf("%d",&Student.student_ID);
printf("%d",Student.student_ID);
create_student_list( &Student.student_name, Student.student_ID);
//search grade
printf("enter the module wanted to know the grade?");
scanf("%s",Module.module);
search_grade(&Student,Module.module);
return 0;
}
student* create_student_list (const char *stu_name, int stu_id) {
student student1 = {.student_ID = stu_id,.student_name =
*stu_name};
student *newstudent = &student1;
printf("\n%s\t%d",&newstudent->student_name,newstudent->student_ID);
newstudent->grades = NULL;
newstudent->grades_len = 0;
}
for question b and c i can't manage to make it work because i don't know if i need a linked list or how to link it with the given structures in the question.
please i need help my future and progress in uni depends on it so i will really appreciate any answers or explanation.

Answer 1

Whether to use a linked list or not
You are right that the objects of type module_grade could be stored as a linked list. However, this would require the struct definition of module_grade to contain a pointer to the next node, i.e. the struct definition would have to be changed to the following:
typedef struct module_grade
{
char* module;
int grade;
//pointer to the next node in the linked list
struct module_grade *next;
} module_grade;
However, your teacher probably does not want you to change the struct definition, since it is part of your assignment to use the struct definitions provided by your teacher. Also, in the text of the assignment, the teacher uses the word "array". This probably means that the grades member of the struct definition of student should point to the start of an array, not the first node of a linked list.
The information above only applies to storing the module_grade objects. Whether your teacher wants you to store the individual student objects in a linked list is another question. Since the information you provided in the question does not provide any indication that you need to create more than one student object, there is no reason to create such a linked list. However, I suspect that your teacher may ask you to create such a linked list in a future assignment.
Errors in your code
Errors in the function main
The line
scanf("%s",Student.student_name);
will not work, because scanf requires you to pass a pointer to the memory address of a memory buffer which has sufficient space to store the string. However, you are instead passing a pointer which is not pointing to anything, because it has an indeterminate value.
The simplest way to allocate sufficient memory for storing the string would be to declare a local array.
You are making the same error in the following line:
scanf("%s",Module.module);
Also, in the line
create_student_list( &Student.student_name, Student.student_ID);
it is wrong to pass &Student.student_name, as that will pass the address of the pointer Student.student_name, but you instead want to pass its value. Also, you are discarding the return value of that function call, which you should also not do. Instead, you should use it.
You should also not declare a variable
student Student;
in the function main, because it is the responsibility of the function crate_student_list to create that object.
In order to fix these bugs, you could use the following code:
int main()
{
char student_name[100];
int student_ID;
char module_name[100];
student* p_student;
printf( "Enter the student name: " );
scanf( "99%s", student_name );
printf( "%s\n", student_name );
printf( "Enter the student ID: ");
scanf( "%d", &student_ID );
printf( "%d\n", student_ID );
p_student = create_student_list( student_name, student_ID );
if ( p_student == NULL )
{
fprintf( stderr, "Error creating student!\n" );
exit( EXIT_FAILURE );
}
printf("enter the module wanted to know the grade?");
scanf( "%99s", module_name );
search_grade( p_student, module_name );
return 0;
}
Note that I am limiting the number of matched characters by scanf to 99 characters, so that scanf will not try to write more than 100 characters (99 matched characters plus the terminating null character) to any of the buffers. Otherwise a buffer overflow will occur if the user enters too many characters, which will invoke undefined behavior (i.e. your program may crash).
It is also worth noting that it does not make much sense to call search_grade before calling adding_grade, because if there are no grades stored in Student, then the search will always fail.
Errors in the function create_student_list
The function create_student_list is supposed to return a value. You probably forgot to write
return newstudent;
at the end of this function.
However, this would be wrong, because the lifetime of the object student1, to which the pointer newstudent is pointing, will end as soon as the function create_student_list returns, because you declared it as a variable with automatic storage duration. Therefore, if you return a pointer to that object at the end of the function, then you will be returning a pointer to an object that no longer exists, i.e. a dangling pointer. Any attempt to dereference this pointer in the function main will invoke undefined behavior.
In order to prevent the lifetime of the object to automatically end when the function returns, you should not declare the object as a local variable. Instead, you should use malloc and free to have full control of the lifetime of the object.
Also, you will probably want to create a copy of the string that stu_name is pointing to. Otherwise, if the memory containing this string is later modified (for example used for something else), then you will lose the string. However, if the student object has its own copy of the string, then there is no danger of this happening.
Therefore, it would be better to write the function create_student_list like this:
student* create_student_list ( const char* stu_name, int stu_id )
{
//allocate memory for new_student
student* new_student = malloc( sizeof *new_student );
if ( new_student == NULL )
{
fprintf( stderr, "Warning: malloc failed!\n" );
return NULL;
}
//set struct members except for student_name
new_student->student_ID = stu_id;
new_student->grades = NULL;
new_student->grades_len = 0;
//allocate memory for student_name and copy it
new_student->student_name = malloc( strlen(stu_name + 1 ) );
if ( new_student->student_name == NULL )
{
fprintf( stderr, "Warning: malloc failed!\n" );
free( new_student );
return NULL;
}
strcpy( new_student->student_name, stu_name );
return new_student;
}