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A two dimensional array can be treated as a one dimensional array of pointers, i.e. a one dimensional array of addresses, where each element of the pointer array points to a one dimensional array of elements. Each one dimensional array can be created by dynamic memory allocation and its base addresses assigned to one of the pointers in the 1-dimensional array of pointers. The syntax used for declaring an array of pointers is:
The advantage associated with this approach, as compared to the conventional 2-dimension array declaration, is that a fixed block of memory need not be reserved in advance. Dynamic allocation of memory is carried out using the malloc function.
Consider the following declaration:
which says x is an array of 5 pointers. Each of these pointers can be used to create a one dimensional array as explained above. The difference in the declaration between pointer to an array and array of pointers can be seen from the missing braces ( ) around *x.
To allocate adequate memory for the array, you can use a for loop construct so that in each pass space for a one dimensional array having a given number of elements (ncols) is created using the malloc function. See illustration below:
/* Demonstration of memory allocation for 2-dimensional array */
#include <stdio.h>
#include <malloc.h>
main()
{
int i, *x[5];
for(i=0; i < 5; i++)
x[i] = (int *)malloc(ncols * sizeof(int));
...
}
Here is a realistic program to demonstrate usage of array of pointers to create two dimensional matrices and perform matrix multiplication.
#include <stdio.h>
#include <malloc.h>
main()
{
int *a[10], *b[10], *c[10];
int i, r1, r2, c1, c2;
void getMatrixElements();
void displayMatrix();
void multiplyMatrix();
printf("Size of Matrix A (r1, c1)?: ");
scanf("%d,%d",&r1,&c1);
printf("Size of Matrix B (r2, c2)?: ");
scanf("%d,%d",&r2,&c2);
if(c1 != r2)
{
printf("\n\nError! Matrix multiplication not possible.\n");
printf("No. of cols in matrix A should be equal to no. of rows in matrix B\n");
exit();
}
/* Memory allocation */
for(i=0; i < r1; i++)
{
a[i] = (int *)malloc(c1 * sizeof(int));
c[i] = (int *)malloc(c2 * sizeof(int));
}
for(i=0; i < r2; i++)
b[i] = (int *)malloc(c2 * sizeof(int));
printf("Matrix A elements?\n");
getMatrixElements(a, r1, c1);
displayMatrix(a, r1, c1);
printf("\nMatrix B elements?\n");
getMatrixElements(b, r2, c2);
displayMatrix(b, r2, c2);
multiplyMatrix(a, b, c, r1, c1, c2);
printf("\nResultant matrix C -(%d X %d)\n", r1, c2);
displayMatrix(c, r1, c2);
}
void getMatrixElements(int *x[10], int r, int c)
{
int i, j;
for(i=0; i < r; i++)
{
for(j=0; j < c; j++)
{
scanf("%d",(*(x + i) + j)); fflush(stdin);
printf("\t");
}
printf("\n");
}
}
void displayMatrix(int *x[10], int r, int c)
{
int i, j;
for(i=0; i < r; i++)
{
for(j=0; j < c; j++)
{
printf("%d\t",*(*(x + i) + j));
}
printf("\n");
}
}
void multiplyMatrix(int *a[10],int *b[10],int *c[10],int rw,int m,int cl)
{
int i, j, k;
for(i=0; i < rw; i++)
{
for(j=0; j < cl; j++)
{
*(*(c + i) + j) = 0;
for(k=0; k < m; k++)
*(*(c + i) + j) += (*(*(a + i) + k)) * (*(*(b + k) + j));
}
}
}
Distinguish between the following:
i) int *a[10]; ii) int (*a)[10];
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