为什么我不能在不声明像const这样的矩阵的情况下进行编译

Question

My doubt is: why in this code:

/*Asignacion de valores en arreglos bidimensionales*/
#include <stdio.h>

/*Prototipos de funciones*/
void imprimir_arreglo( const int a[2][3] );

/*Inicia la ejecucion del programa*/
int main()
{
  int arreglo1[2][3] = { { 1, 2, 3 }, 
                     { 4, 5, 6 } };                         
  int arreglo2[2][3] = { 1, 2, 3, 4, 5 };
  int arreglo3[2][3] = { { 1, 2 }, { 4 } };

  printf( "Los valores en el arreglo 1 de 2 filas y 3 columnas son:\n" );
  imprimir_arreglo( arreglo1 );

  printf( "Los valores en el arreglo 2 de 2 filas y 3 columnas son:\n" );
  imprimir_arreglo( arreglo2 );

  printf( "Los valores en el arreglo 3 de 2 filas y 3 columnas son:\n" );
  imprimir_arreglo( arreglo3 );

  return 0;
}  /*Fin de main*/

/*Definiciones de funciones*/
void imprimir_arreglo( const int a[2][3] )
{
  int i;  /*Contador filas*/
  int j;  /*Contador columnas*/

  for (i = 0; i <=1; i++)
  {
    for (j = 0; j <= 2; j++)
    {
      printf( "%d ", a[i][j] );
    }

    printf( "\n" );
  }
} /*Fin de funcion imprime_arreglo*/

I can't compile without declaring the matrix variables like const, and in a vector i can... Why this behavior occurs? Sorry if my english its bad, i'm speak spanish. I'll be very thankful with your answers.

Answer 1

Remove const from

void imprimir_arreglo( const int a[2][3] );

and

void imprimir_arreglo( const int a[2][3] )
{

and your code will work.

Answer 2

There is a real mess about this subject. You should not use constant modifier for indirect pointers, like const int** , as there could be a mess, like:

1. Is it an int ** that the values cannot be modified?

2. Or, is it a pointer (or even array) for const int * ?

There is a topic about it on C-faq .

Example:

const int a = 10;
int *b;
const int **c = &b; /* should not be possible, gcc throw warning only */
*c = &a;
*b = 11;            /* changing the value of `a`! */
printf("%d\n", a);

It shouldn't allow to change a 's value, gcc does allow, and clang runs with warning but doesn't change the value.

Therefore, I'm not sure why the compilers (tried with gcc and clang ) complain (with warnings, but works) about const T[][x] , as it is not exactly the same as above. But, in general, I may say that this kind of problem is solved on different ways depending on your compiler (as gcc and clang ), so never use const T[][x] !

The best alternative, in my opinion, is to use a direct pointer:

void imprimir_arreglo( const int *a, int nrows, int ncols )
{
  int i;  /*Contador filas*/
  int j;  /*Contador columnas*/

  for (i = 0; i < nrows; i++)
  {
    for (j = 0; j < ncols; j++)
    {
      printf( "%d ", *(a + i * ncols + j) );
    }

    printf( "\n" );
  }
}

And to call:

imprimir_arreglo( arreglo1[0], 2, 3 );

This way, your function is more dynamic and more portable.