Thread: how to know dynamic size of struct

Dear all,

Suppose i have a program that is consisted of a struct. this struct will be used as an array.

Code:

typedef struct 
{
	char name[30];
	int salary;
} person;

How do i define an flexible array for this struc ? i don't know how many arrays which i need yet.

and how can i calculate the sum of all person salary ?

I wrote the simple code, but i think not solve my problem

Code:

#include <stdio.h>

typedef struct 
{
	char name[30];
	int salary;
} person;


int sumStructs(person*, int);

main(void)
{
	person a[10], *pa;                   /* I still put non flexible array*/
	pa = a;
	int i = 0;
	for(i=0; i< 10; i++)
	{
		a[i].i = i+1;
	}
	
	int d = sizeof(a)/sizeof(person);
	
	int c = sumStructs(pa, d);

	printf("sum i adalah %d \n",c);
}

int sumStructs(person*pa, int d)
{

	int sum = 0, j= 0;
	while(j<d)
	{
		sum += (*pa).i;
		pa++;
		j++;
	}
	return sum;
}

std::vector.....

Seriously though, just keep track of how big your array needs to be, and over-allocate a buffer. And use realloc() where necessary.

Code:

person *a;

a = malloc(sizeof(person) * number_of_elements);

Then you'd access the array like this:

Code:

a[i].name

or

Code:

a[i].salary

Yeah, sorry. I was gonna mention that. But I saw that

Code:

int sumStructs(person*pa, int d)

already has provisions for passing the number of elements.

Originally Posted by nonoob

Yeah, sorry. I was gonna mention that. But I saw that

Code:

int sumStructs(person*pa, int d)

already has provisions for passing the number of elements.

Hi Nonoob,

You mean, i don't need to define variable a[10] instead of just put *a ?
I follow your idea, but i got the 0 result for sumStructs function. I modify my code like this:

Code:

#include <stdio.h>

typedef struct 
{
	char name[30];
	int salary;
} person;


int sumStructs(person*, int);

main(void)
{
	person *pa;      
	pa = malloc(sizeof(student) * 100);             
	
	int i = 0;
	for(i=0; i< 10; i++)
	{
		pa[i].i = i+1;                                  //put the salary value for each person
	}
	
	int d = sizeof(pa)/sizeof(person);
	
	int c = sumStructs(pa, d);

	printf("sum i is %d \n",c);
}

int sumStructs(person*pa, int d)
{

	int sum = 0, j= 0;
	while(j<d)
	{
		sum += (*pa).i;
		pa++;
		j++;
	}
	return sum;
}

and result is :

Code:

$ ./a.exe
sum i is 0

How do i get the number of element without making the array variable ?

No...............

sizeof(pa) == 4 (or more)
sizeof(person) == 8 or whatever

So your parameter will evaluate to 0 (since .029 is not an integer value--I remember getting into a big argument about this fact once before).

Originally Posted by master5001

No...............

sizeof(pa) == 4 (or more)
sizeof(person) == 8 or whatever

So your parameter will evaluate to 0 (since .029 is not an integer value--I remember getting into a big argument about this fact once before).

You are right, however if i still define
struct a[10];
sizeof(a) == 360

I think my first question still was not answered, because the memory already prepare for 10 variables a

Originally Posted by unikgila

How do i get the number of element without making the array variable ?

You don't. You did the malloc -- did you forget between now and then how many elements you malloced? It's your responsibility to remember, not the compiler's.

This is why incrementing by a constant factor is a lovely thing

Originally Posted by master5001

This is why incrementing by a constant factor is a lovely thing

Sorry for not understand what you explain . If possible, i hope you can give me the sample code or repair the code that i have wrote above. Thanks

I could. But to be perfectly frank, I am not going to.

Here is why I am being so harsh. To be honest this question gets asked with great regularity.

Code:

int main(void)
{
	person *pa;
        int number_I_must_remember_because_its_the_size_of_my_array = 100;       
	pa = malloc(sizeof(student) * number_I_must_remember_because_its_the_size_of_my_array);
            
	
	int i = 0;
	for(i=0; i< number_I_must_remember_because_its_the_size_of_my_array; i++)
	{
		pa[i].i = i+1;                                  //put the salary value for each person
	}
	
	int c = sumStructs(pa, number_I_must_remember_because_its_the_size_of_my_array);

	printf("sum i is %d \n",c);
}