Best way to initialize a matrix inside a struct in c.

[robgeek]

Hi!

What is the best way to initialize an 2d array inside a struct in c?
I tried this but I'm getting an error message:

#include <stdio.h>

struct node {
	int x[3][3] = {{3, 5, 2},
		       {1, 7, 4},
		       {0, 3, 9}};
};


int main(int argc, const char *argv[])
{
	

	return 0;
}

main.c:4:14: error: expected ‘:’, ‘,’, ‘;’, ‘}’ or ‘__attribute__’ before ‘=’ token
    4 |  int x[3][3] = {{3, 5, 2},

[jlb]

Since this is C you can't initialize structure variables in the structure declaration, you'll need to initialize the variable after the definition, manually.

By the way your code will compile with C++.

[robgeek]

I was wondering if there is a way to do that like the following code or I'll have to do like the comment one.

void init_node(struct node *n)
{
	n->x = {{3, 5, 2},
                {1, 7, 4},
	        {0, 3, 9}};

	// n->x[0][0] = 3;
	// n->x[0][1] = 5;
	// if the matrix is too large i'll have too many lines
}

[salem c]

Is x the only member of the node, or is that just for clarity?

This is about the best you can manage in standard C.

void init_node(struct node *n)
{
    static const int dummy[3][3] = {{3, 5, 2},
                {1, 7, 4},
	        {0, 3, 9}};
    memcpy(n->x, dummy, sizeof(dummy));
}

[robgeek]

Is x the only member of the node, or is that just for clarity?

Actually "struct node" has more variables including x, but the problem is the 2d array.

[salem c]

The dummy + memcpy should suffice then.

[robgeek]

I'm curious. If my answer was "just for clarity" how would you do that?

[salem c]

Because then you could do

void init_node(struct node *n)
{
    static const node dummy = {{3, 5, 2},
                {1, 7, 4},
	        {0, 3, 9}};
    *n = dummy;
}