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C FAQ - Can we initialize structure members within structure definition?

C programming FAQ: Can we initialize structure members within structure definition? Here, we will also learn how we can initialize the members of structure?
Submitted by IncludeHelp, on May 18, 2018

Can we initialize structure members within structure definition?

No!
We cannot initialize a structure members with its declaration, consider the given code (that is incorrect and compiler generates error).

struct numbers{
	int a=10;
	int b=20;
};

Here, we are initializing a with 10 and b with 20 that will not be executed by the compiler and compiler will return an error.

Example

Let's see the example and error returns by the compiler

#include <stdio.h>

struct numbers{
	int a=10;
	int b=20;
};

int main()
{
	//structure variable declaration
	struct numbers num;
	
	//printing the structure members 
	printf("a=%d, b=%d\n",num.a, num.b);
	
	return 0;
}

Output

prog.c:4:7: error: expected ‘:', ‘,', ‘;', ‘}' or ‘__attribute__' before ‘=' token
  int a=10;
       ^
prog.c: In function ‘main':
prog.c:14:27: error: ‘struct numbers' has no member named ‘a'
  printf("a=%d, b=%d\n",num.a, num.b);
                           ^
prog.c:14:34: error: ‘struct numbers' has no member named ‘b'
  printf("a=%d, b=%d\n",num.a, num.b);
                                  ^
prog.c:11:17: warning: variable ‘num' set but not used [-Wunused-but-set-variable]
  struct numbers num;
                 ^~~

How to initialize structure members?

While declaring structure's variable (object to the structure), we can assign the values of the members.

Syntax:

struct structure_name structure_variable_name ={value1, value2};

Example:

struct numbers num ={10,20};

Correct program:

#include <stdio.h>

struct numbers{
	int a;
	int b;
};

int main()
{
	//structure variable declaration
	struct numbers num ={10,20};
	
	//printing the structure members 
	printf("a=%d, b=%d\n",num.a, num.b);
	
	return 0;
}

Output

    a=10, b=20