I want to do matrix multiplication with variable sized arrays in C, however I can't initialize the matrix variable.
Here is the code:
printf("Enter dimensions of matrix A: ");
scanf("%d %d", &p, &q);
printf("Enter number of columns of matrix B: ");
scanf("%d", &r);
int A[p][q] = {{}};
int B[q][r] = {{}};
int C[p][r] = {{}};
I searched for similar queries online however, most deal with one dimensional arrays or two dimensional square arrays. What should I do?
Variable Length Arrays (VLA) cannot be initialized in C. (C23 may allow zero initializations.)
Instead assign values.
printf("Enter dimensions of matrix A: ");
if (scanf("%d %d", &p, &q) == 2 && p > 0 && q > 0) {
int A[p][q]; // Cannot initialize here.
// Zero the whole array.
memset(A, 0, sizeof A);
// ... or assign individual elements.
for (int ip = 0; ip < p; ip++) {
for (int iq = 0; iq < q; iq++) {
A[ip][iq] = rand(); // Assign a value.
}
}
... // Use `A[][]`
}
Array pointers come to the rescue :)
int (*A)[q] = calloc(p, sizeof(*A));
int (*B)[r] = calloc(q, sizeof(*B));
int (*C)[r] = calloc(p, sizeof(*C));
It allocates 2D array (as a chunk of memory) and initialises it to 0. You access the array the normal way - ie A[x][y]
Why don't try malloc?
printf("Enter dimensions of matrix A: ");
scanf("%d %d", &p, &q);
printf("Enter number of columns of matrix B: ");
scanf("%d", &r);
int *A[p];
for (int i=0; i<p; ++i)
A[i] = malloc(sizeof(int)*q);
int *B[q];
for (int i=0; i<q; ++i)
B[i] = malloc(sizeof(int)*r);
int *C[p];
for (int i=0; i<p; ++i)
C[i] = malloc(sizeof(int)*r);
Do not forget to free it before the function finished :)
for (int i=0; i<p; ++i)
free(A[i]);
for (int i=0; i<q; ++i)
free(B[i]);
for (int i=0; i<p; ++i)
free(C[i]);
As others have pointed out, memory allocation is possibly the way to go.
It might help to organize things a bit more clearly. For example, with the datatypes
typedef int Cell;
typedef Cell* Row;
typedef struct {
int rows;
int columns;
Row* data;
} Matrix;
we could initialize and destroy matrices using
void initMatrix(Matrix* p) {
p->data = malloc(sizeof(Row) * p->rows);
for (int r = 0; r < p->rows; r++) {
p->data[r] = calloc(p->columns, sizeof(Cell));
}
}
void destroyMatrix(Matrix* p) {
for (int r = 0; r < p->rows; r++) {
free(p->data[r]);
}
free(p->data);
}
which might make working with the matrices simpler; for example:
int main() {
int p, q;
printf("Enter dimensions of matrix A: ");
scanf("%d %d", &p, &q);
Matrix A = {.rows = p, .columns = q};
initMatrix(&A);
A.data[2][2] = 5; // Assuming, of course...
// ...
destroyMatrix(&A);
}
{{0}}. For VLA, after declaration, you can usememset()to zero the array, e.g.memset (A, 0, sizeof A);