As we know, a pointer to type T is analogous to an array of type T.A pointer can be used to represent a vector, as illustrated in Fig. When a pointer is declared, the memory is allocated only for the pointer variable. The memory for the array elements is usually allocated separately using dynamic memory allocation functions, namely, malloc or calloc.
The function ivec_alloc, that dynamically allocates an integer vector containing n elements and returns a pointer to the allocated memory block, is given below. Note that the function return type is int*.
int *ivec_alloc(int n)
{
int *tmp = (int*) malloc(n * sizeof(int));
if (tmp == NULL) {
printf(“Error: Out of memory …\n”);
exit(l);
}
return tmp;
}
This function accepts an array size (n) and allocates memory for the array, i. e., n * sizeof (int)bytes, to a temporary integer pointer tmp. This pointer is returned to the calling function. However, if the memory allocation is unsuccessful, it displays an error message and exits with error code 1. Thus, we require only a single line call to allocate a dynamic array as shown below.
int *a;
a= ivec_alloc(100); /* allocate 100 element int array */
Note that the pointer variable tmp is local to the function and will be destroyed when the function returns. However, the address of the allocated memory block is returned to the calling function, and we can continue to use the block as a vector (through pointer a in above example) until we explicitly free its memory using free function.
Finally note that the above function is written specifically for the allocation of an integer vector and we need to write a separate function for the vector of each type required in the program. For this, we should first change the function name, say fvec_alloc for float type, cvec_alloc for char type, sivec_alloc for short int type, etc. Also, we should replace each int keyword (other than that in the function parameter declaration, int n) with the desired type. For example, function fvec_alloc for allocation of a float vector is given below.
float *fvec_alloc(int n)
{
float *tmp = (float*) malloc(n * sizeof(float));
if (tmp == NULL) {
printf(“Error: Out of memory …\n”);
exit(1);
}
return tmp;
}
C Program for read a dynamic vector and print it in reverse order
A program to read a dynamic vector of type float and print it in reverse order is given below. It first reads the array size (n) from the keyboard and allocates a dynamic vector of that size using the fvec_alloc function. Then a for loop is used to read array elements from the keyboard and another for loop is used to print the array in reverse order. Observe that the elements of the dynamic array are accessed using the usual subscript notation, a [i ]. Finally, the memory allocated to the array is freed and the program ends. Note that the program includes the stdlib.h header file.
#include <stdio.h>
#include <stdlib.h>
float * fvec_alloc(int n);
void main ()
{
float *a; /* pointer for dynamic array */
int i, n;
printf(“Enter vector size: “);
scanf(“%d”, &n);
a= fvec_alloc(n); /* allocate vector */
/* read a vector */
printf(“Enter vector elements: “);
for (i = 0; i < n; i++)
scanf(“%f”, &a[i]);
/* print a vector */
printf(“Given vector in reverse order: “);
for (i = n – 1; i >= 0; i–)
printf(“%5.2f “, a[i]);
printf(“\n”);
free(a); /* free vector */
getch();
}
/* include definition of fvec alloc function here */
The program output is given below.
Enter vector size: 2
Enter vector elements: 1.2 2.3 3.4 4.5 5.6
Given vector in reverse order: 5.60 4.50 3.40 2.30 1.20
Dinesh Thakur holds an B.C.A, MCDBA, MCSD certifications. Dinesh authors the hugely popular