C: assign 2d array of struct

Question

Background

I have a struct with two 2D arrays...

typedef struct v
{
    int *b;
    int *a;
} v;

Inside my main function I have...

int A [M][K] = {{2,2},{2,2}, {3,2} };
int B [K][N] = {{2,2,2},{2,2,2}};
struct v *data= (struct v *) malloc(sizeof(struct v));
data.a->A;
data.b->B;
pthread_create(&multiplicationWorker, NULL, (void *) &alterAarrays,  data);

...and a private function...

void alterArrays ( v* coords)
{
    ...
}

Question:

I want to pass references to the 2D arrays inside alterArrays. Also it won't let me assign the values of the 2D arrays like this. Any suggestions?

data.a->A; That isn't valid and wouldn't do anything useful even if it were — Ed Swangren
– Ed Swangren, Commented Oct 21, 2012 at 21:12

Geoff Montee · Accepted Answer · 2012-10-21 21:34:18Z

2

Define sizes:

#define K 2
#define M 3
#define N 3

If you want to use arrays in your struct:

Define struct (did you mean to call these a and b? You refer to them as this in the code. Also, the dimensions originally differed from the arrays in the code:

typedef struct v
{
    int a [M][K];
    int b [K][N];
} v;

Then the copies:

int A [M][K] = {{2,2},{2,2}, {3,2} };
int B [K][N] = {{2,2,2},{2,2,2}};
struct v *data= (struct v *) malloc(sizeof(struct v));

memcpy(data->a, A, M * K * sizeof(int));
memcpy(data->b, B, K * N * sizeof(int));

If you want to use pointers in your struct:

Define struct:

typedef struct v
{
    int (*a)[K];
    int (*b)[N];
} v;

And you need to make A and B global, so they are not on the stack. So define them at the top of your source file:

int A [M][K] = {{2,2},{2,2}, {3,2} };
int B [K][N] = {{2,2,2},{2,2,2}};

Then the assignment:

struct v *data= (struct v *) malloc(sizeof(struct v));

data->a = A;
data->b = B;

edited Oct 21, 2012 at 21:34

answered Oct 21, 2012 at 21:07

Geoff Montee

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2 Comments

James Over a year ago

Will this change the values of A and B? because that is what I want. Seems to me like a copy would not do so

Geoff Montee Over a year ago

No. If you want to change A and B, then you need to use pointers in the struct, rather than arrays. Also, you should either make A and B global, or put them on the heap.

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