Access elements of a matrix through pointer arithmetic

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Access elements of a matrix through pointer arithmetic

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#1 by (3 votes)

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How can I access the elements of an integer array using pointer arithmetic?

I am clear that to access those of a vector is such that *(vector+"posicion") but for a matrix I do not know how I can do it.

I know that they can be written in the normal way, matriz[i][j]; but not with pointers and for more that I have searched I have not found a site where they explain it correctly.

I have been practicing and for this I have made a super basic program that asks the user the number of rows and columns that are going to be required and that I fill the matrix manually and then show it but I would like to implement it with pointer arithmetic.

 int main() {
 int **matriz;
 int i , filas , columnas;

 printf("\nIntroduce el numero de filas de la matriz: ");
 fflush(stdin);
 scanf("%i",&filas);

 printf("\nIntroduce el numero de columnas de la matriz: ");
 fflush(stdin);
 scanf("%i",&columnas);

 matriz = (int**)malloc(filas*sizeof(int));  //Reservamos el espacio para las filas

 if(matriz == NULL)
 {
     printf("\nError, no se ha podido reservar el espacio...");
     exit(1);
 }

 for(i = 0; i < filas; i++)
 {
     matriz[i] = (int*)malloc(columnas*sizeof(int));    //Reservamos espacio para las columnas
     if(matriz[i] == NULL)
     {
         printf("\nError, no se ha podido reservar el espacio...");
         exit(1);
     }
 }

 IntroducirDatos(matriz , filas , columnas);
 MostrarMatriz(matriz , filas , columnas);

 return 0; }

And the functions of Enter Data and Show Matrix:

 void IntroducirDatos(int **matriz , int filas , int columnas) {

   int i, j;

 for (i = 0; i < filas; i++)     //Introducimos datos
 {
     for(j = 0; j < columnas; j++)
     {
         printf("\nIntroduce el valor para la posicion (%i , %i): ", i+1, j+1);
         scanf("%i",&matriz[i][j]);
     }
 }
}

Show Matrix

 void MostrarMatriz(int **matriz , int filas , int columnas) {

  int i, j;

 printf("\nLa matriz es: \n\n");

 for(i = 0; i < filas; i++)
 {
     for(j = 0; j < columnas; j++)
     {
         printf("|%i ", matriz[i][j]);

     }
     printf("\n");
 } }

Any ideas on how to do it?

c punteros matrices

asked by Josemanuu 15.06.2017 в 11:08

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1 answer

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score 3 · Answer 1

As your code is, you can not.

What you have is not so much a "matrix" as "a vector of vector pointers of int ". You do not have any information on where matriz[0][0] will be relative to matriz[1][0] .

Take a look at this sample program that dynamically creates a matrix. At the end, the different pointers that make up the matrix are printed, and you will see that there is no direct relationship 1 .

As you comment, if the variable is defined as a matrix (and not as pointer pointers), for example int[5][4] , then yes you know that, the order of the elements will be sequential ( [0,0], [0,1], [0,2], [0,3], [1, 0] ... [4,3] ) and you can use arithmetic of pointers. The formula for [i, k] is

i * [número de filas] + k,

with the example above, the element [2,3] is

matriz + (2 * 4 + 3);

1 In the middle of the loop I added an additional malloc to make sure that malloc does not return consecutive memory blocks. But it is not a trap, because malloc returns consecutive memory blocks is a detail of implementation and there is nothing in the standard that ensures that acts like this.