Initialize multi-dimensional array

Question

I have this matrix that holds a 4x4 array of double:

typedef struct {
 double data[4][4];
} mat;

And here I declare my mat

mat m1;

And i want to initialize my mat with zeros:

double data[4][4] = { { 0,0,0,0 },{ 0,0,0,0 },{ 0,0,0,0 },{ 0,0,0,0 } };
MAT_A.data = data;

And got this error:

expression must be a modifiable lvalue

And:

'=': left operand must be l-value

I also tried this:

MAT_A->data = data;

EDIT:

Another question is how to initiate mat ?

• Basile Starynkevitch: You'll better improve your question to give an actual minimal reproducible example

Answer 1

The problem is that you cannot assign an array to a struct, nor an array to an array. The C syntax does not allow it.

And if you want to initialize a variable, you should not use assignment. These are different things; MAT_A.data = data; is assignment. This is initialization:

mat m =
{
 .data = { { 0,0,0,0 },{ 0,0,0,0 },{ 0,0,0,0 },{ 0,0,0,0 } }
};

As for assignment, what you can do is assign a struct to a struct, for example by using a compound literal:

m = (mat) 
{
 .data = { { 0,0,0,0 },{ 0,0,0,0 },{ 0,0,0,0 },{ 0,0,0,0 } }
};

Answer 2

The problem is not in your initialization of the array data, but in the following assignment:

MAT_A.data = data;

In order to copy an array you need to copy the elements of the array individually:

for (int i = 0; i < 4; i++)
 for (int j = 0; j < 4; j++)
 MAT_A.data[i][j] = data[i][j];

Answer 3

i want to initiate my mat with zeros:

Then you could simply code:

mat m1 = {0};

or

double data[4][4] = {0};

(and {} instead of {0} would also practically work, even if non-standard)

and you could even use memset to clear the zone (with <string.h> standard header):

double data[4][4];
memset (&data, 0, sizeof(data));

or

memset (&m1, 0, sizeof(m1));

BTW, optimizing compilers like GCC (invoked as gcc -O2 -march=native) are likely to generate in practice the same machine code in all cases, if m1 or data is an automatic variable (on your call stack).

However, assignments of arrays (m1.data = data;) is illegal (because arrays decay into pointers).

And your title is wrong: C has no multi-dimensional arrays, only (mono-dimensional) arrays (perhaps arrays of arrays, arrays of structures, etc...).

• Lundin: I take it you mean {0} since {} would be invalid C.
• Danny Nitman: And i need to initiate first my mat ? how can i do that ?
• Basile Starynkevitch: Didn't I answer to that in my answer? Please improve first your question by giving an minimal reproducible example. Then I could improve my answer !

Answer 4

You could explicitly initialize the array as follows.

for (int i = 0; i < 4; i++)
{
 for (int j = 0; j < 4; j++)
 {
 MAT_A.data[i][j] = 0;
 }
}

• Lundin: This is assignment, not initialization.