CODEWITHSUNDEEP
cpointersmatrix
user3339703
user3339703

Reputation: 73

Printing a pointer to a matrix without brackets

I am taking in an input from a text file that looks something like this:

3 
2 
4 
1 2 
3 4 
5 6 
7 8 9 10 
11 12 13 14

With the first line being the number of rows in matrix A stored in m, 2nd row being the number of columns in matrix A and number of rows in Matrix B stored in n, and 3rd row being the number of columns in matrix B stored in p.

I am grabbing the information from the text file using this function:

 void read_matrices(int **A, int **B, int **C, int *m, int *n, int *p, char *file) {
    FILE *fp = fopen(file, "r");
    if (!fp) {
        fprintf(stderr, "\n Error: file open failed for file '%s'\n\n", file);
        exit(0);
    }

    /* read & output m, n, p */
    fscanf(fp, "%d\n%d\n%d\n", m, n, p);

    /* allocate memory for A and set values to null */
    *A = (int*)calloc(*m * *n, sizeof(int));

    /* read A */
    int i, j;
    for (i = 0; i < *m; i++) {
        fscanf(fp, "\n");
        for (j = 0; j < *n; j++) {
            fscanf(fp, "%d", (A + i * *n + j));
        }
    }

    /* allocate memory for B and set values null */
    *B = (int*)calloc(*n * *p, sizeof(int));

    /* read B */
    for (i = 0; i < *n; i++) {
        fscanf(fp, "\n");
        for (j = 0; j < *p; j++) {
            fscanf(fp, "%d", (B + i * *p + j));
        }
    }

    /* allocate memory for C and set values null */
    *C = (int*)calloc(*m * *p, sizeof(int));

    /* close FP & free allocated memory */
    fclose(fp);
}

I am printing the matrix using the function:

void print_matrix(int *mtx, int r, int c) {
    int i, j;
    for (i = 0; i < r; i++) {
        printf("\n");
        for (j = 0; j < c; j++) {
            printf("%5d", (mtx + i * c + j));
        }
    }
}

When I print out the matrix's I am getting the wrong numbers out. But when I try printing inside the read function I get the correct result. I am getting the result:

Matrix A contents:

    8   12
   16   20
   24   28
Matrix B contents:

    7   11   15   19
   23   27   31   35

Also when I change fscanf(fp, "%d", (A + i * *n + j)); to fscanf(fp, "%d", *(A + i * *n + j)); I get a Bus Error, but the way I have it now I get warning: int format, pointer arg.

Upvotes: 2

Views: 363

Answers (1)

WhozCraig
WhozCraig

Reputation: 66194

Both your targets for writes in your element-reads are using the wrong pointer values.

fscanf(fp, "%d", (A + i * *n + j));

should be 

fscanf(fp, "%d", (*A + i * *n + j));
// here ----------^

Similar problem with the second matrix:

fscanf(fp, "%d", (B + i * *p + j));

should be 

fscanf(fp, "%d", (*B + i * *p + j));
// here ----------^

Most modern compilers will warn you about this, and if you're doesn't either jack up the compiler warnings or get a toolchain with brains. clang, for example, emits:

main.c:24:30: Format specifies type 'int *' but the argument has type 'int **'

Upvotes: 1

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