C: Structure Initialization (Advanced)

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C: Structure Initilaization

There is one more way by which we can initialize structures. These way utilizes the benefits of both the ways of structure initialization described here. That is we are able to initialize the elements of the structure using the set notation and also we need not remember the order of the elements of the structure.
Suppose consider the structure ‘student‘ we already used

struct student {

     int roll;

     int class;

     char name[50];

};

Now let’s declare a variable st3 and initialize using the third method

student st3={

           .name = "Mark", //Notice the equal to and the comma

           .class  = 10,

           .roll  = 1038

};

As you can see the way by which the variable st3 has been initialized. It has used the set notation and also see the order is not the same as in the case of the structure ‘student‘

The following program demonstrates this. Note the different ways by which the array of structures has been initialized

/*

*Different Ways of structure initialization

*/#include < stdio.h>

#include < string.h>

#define NAME_LEN 25

typedef unsigned short age_t;

typedef unsigned int roll_t;

typedef struct student{

    char name[NAME_LEN];

    roll_t rno;

    age_t age;

}student;

int main()

{

/* Method 3a: Just like the Method 2a, but here you do not

* need to know order of the elements in the declaration

*/

student st3={

.name = "Mark",//Notice the equal to and comma

.age  = 23,
.rno  = 1038
};

printf("%s %hi %u\n\n",st3.name,st3.age,st3.rno);

/*

* Method 3b: For Initializing an array of structures

*/

student st4[]={
{

.name = "Neil",

.age  = 23,

.rno  = 1039      },

{

.name = "Peter",

.age  = 23,

.rno  = 1040

}
};

printf("%s %hi %u\n%s %hi %u\n\n",st4[0].name,st4[0].age,

st4[0].rno, st4[1].name,st4[1].age,st4[1].rno);

/*

* Method 3c : Change the order of initialization of the

* elements of the array. Normally as seen in Method 2b,

* the 0th array element is initialized  then 1st, then

* 2nd and so on. So by using a variation  of Method 3b,

* we  can initialize the array elements in any order

*/

student st5[5]={

  [3]={

              .name = "Titus",

              .age  = 22,

              .rno  = 1041      },

[2]={

              .name = "Stephen",

              .age  = 23,

              .rno  = 1042

      }

      /*As you can see only 2nd and 3rd array elements has

       * been initialized and that too not in order. Such

       * an initialization is useful, if we wish to allocate

       * some fixed size array but only initialize some element

       */
};

printf("%s %hi %u\n%s %hi %u\n\n",st5[2].name,st5[2].age,

st5[2].rno, st5[3].name,st5[3].age,st5[3].rno);

}

The Methods 3a, 3b and 3c are the ways of structure initialization that I have seen in the kernel code. If you find any other way to do the structure initialization, kindly comment.

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C: Structure Initilaization