/* /usr/include/stdio.h, definitions for IO. */
#include <stdio.h>
#include <math.h>

/* Here are the prototypes for our procedures */
void            sum_array(double[], double *, int);
double          ddot(double[], double[], int);

int main()
{
	int             k, n, in;
	double          a[100], b[100], sum;
	/* Arrays are indexed from 0,  a[0], ..., a[99] */

	n = 100;
	printf("Type a value for in: ");
	scanf("%d", &in);	/* & = the address, the pointer */

	/* %d for int, %f for float. \n is newline */
	printf("This is how you write: in =  %d\n", in);

	for (k = 0; k < n; k++) {	/* for k = 0, ..., n-1 */
		a[k] = k + 1;	/* k++ means k = k + 1 */
		b[k] = -sin(k + 1);	/* requires -lm        */
	}

	/* Compute the inner product between a and b. */
	printf("The inner product is %f\n", ddot(a, b, n));

	/*
	 * Send the address of sum. Arrays are passed by reference
	 * automatically.
	 */
	sum_array(a, &sum, n);

	printf("The sum of the array is %f\n", sum);

        return 0; /* Return status to unix */
}

void
sum_array(double a[], double *sum, int n)
/*
 * void, i.e. not a function. double *sum means that *sum is a double. sum
 * itself is the address, the pointer to sum.
 */
{
	int             k;	/* k is local to sum_array */

	*sum = 0;		/* sum = 0 is WRONG; it will give a
				 * Segmentation Fault */

	for (k = 0; k < n; k++)
		*sum += a[k];	/* i.e. *sum = *sum + a[k] */
}

/*
 * NOT void here since this is a function (of type double) returning a
 * result.
 */
double
ddot(double x[], double y[], int n)
{
	int             k;
	double          sum;

	sum = 0.0;
	for (k = 0; k < n; k++)
		sum += x[k] * y[k];

	return sum;		/* return the result */
}