CODEWITHSUNDEEP
carrays
Danny Nitman
Danny Nitman

Reputation: 17

Initialize multi-dimensional array

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 ?

Upvotes: 0

Views: 80

Answers (4)

Lundin
Lundin

Reputation: 214810

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 } }
};

Upvotes: 2

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

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...).

Upvotes: 1

jfMR
jfMR

Reputation: 24788

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];

Upvotes: 1

Codor
Codor

Reputation: 17605

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;
    }
}

Upvotes: 0

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