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Monday, 4 November 2013

Pointers to structures

Structure pointers
What is structure pointer?
If the pointer refers to the address of the structure variable, then the pointer is called structure pointer or pointer to structure.
     struct student {
char name[32];
int rollno, marks[5];
float average;
     } obj, *ptr;

     ptr = &obj;

Here, pointer ptr refers to the address of the structure variable obj.

C program using structure pointer:


  #include <stdio.h>
  /* structure st with three different members */
  struct st {
        char ch;
        int num;
        float val;
  } obj, *ptr; // one structure object and a pointer to structure
    int main() {
        /* assigning address of structure variable to pointer */
        ptr = &obj;

        /* assigning values to structure elements using pointer */
        ptr->ch = 'a';
        ptr->num = 10;
        ptr->val = 20.22;

        /* printing the result */
        printf("obj.ch : %c\n", obj.ch);
        printf("obj.num: %d\n", obj.num);
        printf("obj.val: %.2f\n", obj.val);
        return 0;
  }



  Output:
  jp@jp-VirtualBox:~/$ ./a.out
  obj.ch : a
  obj.num: 10
  obj.val: 20.22



How to access and assign values to structure elements using pointers?
Structure dereference operator is used to assign value to or access structure elements.
          strcpy(ptr->name, "JP");
          ptr->rollno = 5;
          ptr->mark[0] = 100, ptr->mark[1] = 99;
          ptr->average = (100 + 98) / 2;

So, we have assigned values to structure elements using structure dereference operator. The above set of statements is equivalent to 
          strcpy((*ptr).name, "JP");
          (*ptr).rollno = 5;
         (*ptr).mark[0] = 100, (*ptr).mark[1] = 99;
         (*ptr).average = 99.5;

Passing structure pointer to function:
Like other data types, structure pointer can also be passed as function argument.  Below program explains how to pass structure pointer as function argument.


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

  /* structure with 4 data members */
  struct student {
        char name[32];
        int rollno, marks[2];
        float average;
  };

  /* updates value for the given structure object using pointer */
  void update(struct student *ptr) {
        strcpy(ptr->name, "Barack Obama");
        ptr->rollno = 110011;
        ptr->marks[0] = 99;
        ptr->marks[1] = 100;
        ptr->average = (ptr->marks[0] + ptr->marks[1]) / 2.0;
  }

  int main() {
        struct student obj;
        /* passing address of a structure variable as argument */
        update(&obj);

        /* printing the result */
        printf("Name: %s\n", obj.name);
        printf("Roll Number: %d\n", obj.rollno);
        printf("Marks in subject 1: %d\n", obj.marks[1]);
        printf("Marks in subject 2: %d\n", obj.marks[2]);
        printf("Average: %f\n", obj.average);
        return 0;
  }



  Output:
  jp@jp-VirtualBox:~/$ ./a.out
  Name: Barack Obama
  Roll Number: 110011
  Marks in subject 1: 100
  Marks in subject 2: 1120337920
  Average: 99.500000



How to return structure pointer from function?
Below program explains how to return structure pointer from function.


  #include <stdio.h>
  #include <stdlib.h>
  struct op {
        int sum;
        float diff;
  };

  /* finds sum and diff with given input and returns structure pointer */
  struct op * calculate(int x1, int y1, float x2, float y2) {
        struct op *ptr;
        /* dynamic memory allocation for structure */
        ptr = (struct op *)malloc(sizeof(struct op));

        /* assigning values to structure elements */
        ptr->sum = x1 + y2;
        ptr->diff = x2 - y2;
        return ptr;
  }

  int main() {
        struct op *ptr; /* structure pointer declaration */
        ptr = calculate(10, 20, 40.33, 33.22);
        /* printing the result */
        printf("ptr->sum : %d\n", ptr->sum);
        printf("ptr->diff: %.2f\n", ptr->diff);
        return 0;
  }



  Output:
  jp@jp-VirtualBox:~/$ ./a.out
  ptr->sum : 43
  ptr->diff: 7.11



Dynamic memory allocation for structure:
Let us see how to perform dynamic memory allocation for a structure.
     struct student {
char name[32];
int rollno, marks[5];
float average;
     } *ptr;

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

The above statement performs dynamic memory allocation for a structure object using pointers.

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