Thread: help regarding structs

Suppose you have an instance of the 2nd struct named foo.(not a pointer, just a usual instance)

Code:

foo.student[0].age = 20;

This will take the instance foo, inside foo, it will select the first element of student array. Because the 1st element is a struct, it will select the field named age of this struct. It will assign the value 20 to this field.

Hope this helps

One way is to make a constructor to fill in your studentsInfo. This way divides naturally into different source files, where you have one source file to take care of the studentsInfo, etc

Code:

// newStudentDefault : construct a default studentsInfo
studentsInfo newStudentDefault(void);

// newStudent : construct a studentsInfo with the given fields filled out
studentsInfo newStudent(char *studentNumber, char *famName, char *firstName,
		char *midName, char *course, int year, int age, char gender, 
		int final);

// printStudentList : print the list of students
void printStudentList(listOfStudents lis);

int main(void)
{
	// Make a lis which is uninitialized
	listOfStudents lis;
	
	// Fill element list.student index 0 and 1 using the default struct
	lis.student[0] = newStudentDefault();
	lis.student[1] = newStudentDefault();
	
	// Fill element lis.student[2] using the customized constructor
	lis.student[2] = newStudent("4567", "Doe", "Jane", "X", "History 101", 
			1980, 0, 'F', 0);
	
	// Set the size member to the proper value
	lis.size = 3;
	
	// Print the list
	printStudentList(lis);
	
	return EXIT_SUCCESS;
}

The constructors can simply return a pre-initialized struct of that type

Code:

studentsInfo newStudentDefault(void) {
	return (studentsInfo) {
		.studentNumber = "1234",
		.famName = "Doe",
		.firstName = "John",
		.midName = "X",
		.course = "DefaultCourse",
		.year = 1980,
		.age = 0,
		.gender = 'M',
		.final = 0,
	};
}

studentsInfo newStudent(char *studentNumber, char *famName, char *firstName,
		char *midName, char *course, int year, int age, char gender, 
		int final) {
	studentsInfo s = newStudentDefault();
	strcpy(s.studentNumber, studentNumber);
	strcpy(s.famName, famName);
	strcpy(s.firstName, firstName);
	strcpy(s.midName, midName);
	strcpy(s.course, course);
	s.year = year;
	s.age = age;
	s.gender = gender;
	s.final = final;
	return s;
}

void printStudentList(listOfStudents lis) {
	(void)lis;
	// TODO
}

You can also do it with pointers. The only difference is you'd malloc the struct and set the members using the -> notation instead.

Originally Posted by cipher1285

i got your point
but in mine, im using, or need to, use pointers

No problem.
Example1. No pointers.

Code:

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

#define MAX 100

struct student{
    char name[15];
    int age;
};

typedef struct student student;

struct listStudent{
    student arrayStd[MAX];
    int size;
};

typedef struct listStudent listStudent;

int main(void)
{
    int i;
    /* No pointers. Initialize */
    listStudent lStd;
    lStd.size = 0;
    
    /* Create 2 students */
    /* 1st */
    strcpy(lStd.arrayStd[0].name, "Samaras");
    lStd.arrayStd[0].age = 20;
    lStd.size++;
    
    /* 2nd */
    strcpy(lStd.arrayStd[1].name, "Midha");
    lStd.arrayStd[1].age = 21;
    lStd.size++;
    
    for(i = 0 ; i < lStd.size ; i++)
    {
        printf("Student %s is %d years old\n", lStd.arrayStd[i].name, lStd.arrayStd[i].age);
    }
    
    return 0;
}

Now for the pointers, you have to understand that in order to access a member of a normal instance of struct named myStruct, we do

Code:

myStruct.memberOfStruct

So, if myStruct is a pointer, you have to do this

Code:

(*myStruct).memberOfStruct

C offers a shortcut for this, the arrow ->. So, the above code is equivalent to this code

Code:

myStruct->memberOfStruct

So, example No.2 with pointers!

Code:

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

#define MAX 100

struct student{
    char name[15];
    int age;
};

typedef struct student student;

struct listStudent{
    student arrayStd[MAX];
    int size;
};

typedef struct listStudent listStudent;

int main(void)
{
    int i;
    /* Pointers. */
    listStudent* lStd = NULL;
    
    /* Allocate memory for where the pointer points to! */
    lStd = malloc(sizeof(listStudent));
    /* Check that malloc is successful, by checking if */
    /* pointer lStd is NULL                                     */
    
    /* Initialize */
    /* So, here we need to use the arrow, since */
    /* lStd is a pointer */
    lStd->size = 0;
    /* Create 2 students */
    /* 1st */
    /* lStd is a pointer, so we use the arrow */
    /* the array of students is of type student */
    /* ,thus we should use the dot in order to access */
    /* it's members */
    strcpy(lStd->arrayStd[0].name, "Samaras");
    lStd->arrayStd[0].age = 20;
    lStd->size++;
    
    /* 2nd */
    strcpy(lStd->arrayStd[1].name, "Midha");
    lStd->arrayStd[1].age = 21;
    lStd->size++;
    
    for(i = 0 ; i < lStd->size ; i++)
    {
        printf("Student %s is %d years old\n", lStd->arrayStd[i].name, lStd->arrayStd[i].age);
    }
    
    /* Don't forget to free!!! */
    free(lStd);
    
    return 0;
}

@the other guy (He doesn't have a name to reference him :/ ), no this is not the only difference!!!! Never forget to free the memory that you allocated with malloc