CODEWITHSUNDEEP
cstruct
user5925844
user5925844

Reputation:

initialize struct of C

typedef struct {
    int num_rows;
    int num_cols;
    int** data;
} BinaryMatrix;


BinaryMatrix *ConstructBinaryMatrix(int num_rows, int num_cols) {
    BinaryMatrix matrix = {
            .num_rows = num_rows,
            .num_cols = num_cols,
            .data = (int **) malloc((num_rows) * sizeof(int *)),
    };

    int i;
    for (i = 0; i < num_cols; i++) {
        matrix.data[i] = (int *) malloc(num_cols * sizeof(int));
    }
    return &matrix;

}

Is this the correct way to define a BinaryMatrix, and how to initialize it?

Thanks for your help.

I got the following error.

BinaryMatrix* M;
M = ConstructBinaryMatrix(2, 2);
printf("%d:%d", M->num_rows, M->num_cols);

The output is: 4198012:0

Upvotes: 0

Views: 144

Answers (3)

Steve Summit
Steve Summit

Reputation: 48043

You are returning a pointer to local data, which is bad form and doesn't work.

You should probably call malloc to allocate space for a BinaryMatrix structure, something like this:

BinaryMatrix *ConstructBinaryMatrix(int num_rows, int num_cols) {
    BinaryMatrix *matrix = malloc(sizeof(BinaryMatrix));
    matrix->num_rows = num_rows;
    matrix->num_cols = num_cols,
    matrix->data = malloc(num_rows * sizeof(int *));

    int i;
    for (i = 0; i < num_rows; i++) {
        matrix->data[i] = malloc(num_cols * sizeof(int));
    }
    return matrix;
}

(Also I have fixed the loop bounds, as M.M. pointed out.)

Upvotes: 2

Arif Burhan
Arif Burhan

Reputation: 505

Unless its very large I would use:

#define M 3
#define N 3

int matrix[M][N]={{0,0,0},{0,0,0},{0,0,0}};
/* above possibly static */

Simples !

If the matrix was large simply use for loops to initialize.

Upvotes: -2

M.M
M.M

Reputation: 141648

i < num_cols should be i < num_rows.

Currently your code returns the address of a local variable. Local variables are destroyed when the function returns, so you in fact return a dangling pointer, not a good idea.

Instead you should return a copy of the local variable. Remove the * from the function prototype and from declaration of M, and remove the & from the return statement.

Upvotes: 1

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