#include <stdio.h>


void add_poly(double *z, double *x, double *y, int n)
{
    int i;
    for (i = 0; i <= n; i++)
        z[i] = x[i] + y[i];
}


/*
 * The x and y arrays are of size n+1 (so that the maximum array index is n),
 * but the z array is of size 2n+1 to hold the product.
 */
void mult_poly(double *z, double *x, double *y, int n)
{
    int i, j;
    for (i = 0; i <= 2*n; i++)
        z[i] = 0;
    for (i = 0; i <= n; i++)
        for (j = 0; j <= n; j++)
            z[i+j] += x[i] * y[j];
}


void print_poly(double *x, int n)
{
    int i, printed_something = 0;
    for (i = n; i >= 0; i--) {
        if (x[i]) {
	    if (printed_something)
		printf(" + ");
            printf("%gx**%d", x[i], i);
	    printed_something = 1;
	}
    }
    if (!printed_something)
	printf("0");
    printf("\n");
}


int main()
{
    static double x[] = { 7, 0, 3, 1 };
    static double y[] = { 6, 2, 0, 0 };
    static double z[] = { 9, 5, 0, 4 };
    static double w[] = { 7, 0, 0, 8 };
    static double prod1[7], prod2[7];

    mult_poly(prod1, x, y, 3);
    mult_poly(prod2, z, w, 3);
    add_poly(prod1, prod1, prod2, 6);
    print_poly(prod1, 6);

    return 0;
}