Row-major ordering in C as Command Line Argument

Question

I need to pass in a 2D area (a matrix) from the command line through a txt file that looks like this:

0 0 0 0 0
0 1 1 0 0 
0 0 1 0 0 
0 0 1 0 0
0 0 0 0 0

I'm using C and need to have it in row-major order, so I'm trying to do this:

int matrix[][] = argv[2]; // it is the second command line argument passed

This isn't working, is it because it needs to be a 1-dimensional array? Am I only allowed to use a regular 1D array for row-major ordering? The error I'm getting is "array type has incomplete element type 'int[]' "

Answer 1

I'm trying to do this:

int matrix[][] = argv[2]

This isn't working,

because this is invalid, out of the missing ';' :

• at least the second dimension must be given, this is why you have the error array type has incomplete element type 'int[]'
• matrix is a (wrongly defined) 2D array of int but in the best case argv[2] is an array of char :
  • if the program is called without argument argv[2] is undefined
  • if the program is called with one argument argv[2] is NULL
  • if the program is called with at least 2 argument argv[2] is the string containing the second arg

---

Considering argv[2] is the pathname of the file containing the 5x5 matrix the declaration of the matrix can be :

int matrix[5][5];

and you need to read the values in the file to set the matrix's content, you cannot directly map the txt file to matrix

Example :

#include <stdio.h>

int main(int argc, char ** argv)
{
  FILE * fp;

  if (argc < 3) {
    fprintf(stderr, "usage: %s ?? <file>\n", *argv);
    return -1;
  }
  else if ((fp = fopen(argv[2], "r")) == NULL) {
    perror("cannot open file");
    return -1;
  }
  else {
    int matrix[5][5];
    int i, j;

    for (i = 0; i != 5; ++i) {
      for (j = 0; j != 5; ++j) {
        if (fscanf(fp, "%d", &matrix[i][j]) != 1) {
          fprintf(stderr, "invalid file for [%d][%d]\n", i, j);
          return -1;
        }
      }
    }

    fclose(fp);

    /* debug */
    for (i = 0; i != 5; ++i) {
      for (j = 0; j != 5; ++j)
        printf("%d ", matrix[i][j]);
      putchar('\n');
    }

    putchar('\n');

    /* you can also use it as a 1D array */
    for (i = 0; i != 25; ++i) {
      printf("%d ", ((int *) matrix)[i]);
      if (i%5 == 4)
        putchar('\n');
    }

    return 0;
  }
}

Compilation and execution:

pi@raspberrypi:/tmp $ gcc -Wall a.c
pi@raspberrypi:/tmp $ cat f
0 0 0 0 0
0 1 1 0 0 
0 0 1 0 0 
0 0 1 0 0
0 0 0 0 0
pi@raspberrypi:/tmp $ ./a.out whatever f
0 0 0 0 0 
0 1 1 0 0 
0 0 1 0 0 
0 0 1 0 0 
0 0 0 0 0 

0 0 0 0 0 
0 1 1 0 0 
0 0 1 0 0 
0 0 1 0 0 
0 0 0 0 0 
pi@raspberrypi:/tmp $