help with matrices

[wuzertheloser]

Develop a program which computes the current value of the vector {x}
based on the following forward iteration:

{x}(n+1) = [K] {x}(n), n = 0,1,2, ... ,8,9.

In other words, the next vector {x} is equal to the product of [K] and
the current vector {x}.

Perform the matrix multiplication by using the function:

void matrix_mult(dou ble a[4][4], double b[4], double c[4], int rows);

You should pass the matrix [K] and a pointer array {x} to matrix_mult()
which will pass back a new vector {y} ).

[K] is a constant 4x4 matrix defined by:

double K[4][4] = { { 2., -4., 1., 0.},
{-4., 5., -2., 1.},
{ 1., -2., 6., -3.},
{ 0., 1., -3., 7.}};

The initial components of the vector {x}(0) are specified by:

double x[4] = { 1./3., 2./3., 1./3., pow(3.,0.5)/3.};

First, print the value of the initial vector {x}(0) on the screen.
Then, perform forward iteration 10 times (for n=0 to 9). Each time
after new vector is computed from [K]{x}, normalize that vector by
using the function

double unit_vec(double vec[4], int cols)

which was discusseed in class (see Program w8-5.c; this function
transforms a vector to a unit vector - of magnitude 1). For the
matrix multiplication with the vector [K]{x}, see Program w8-3.c .

Always use the normalized vector as the vector {x}(n) for the next
iteration.

For each iteration, print the new vector, its length, and the normalized
new vector in main().


That is the assignment. I don't even know where to start. I've constructed an outline but I don't know what to put in for the C statements.

#include <stdio.h>
#include <math.h>
double unit_norm(double vec[4], int cols);
void matrix_mult(double a[4][4], double b[4], double c[4], int rows);
main()
{
 double K[4][4] = { { 2., -4.,  1.,  0.},
                    {-4.,  5., -2.,  1.},
                    { 1., -2.,  6., -3.},
                    { 0.,  1., -3.,  7.}};
 double y[4], x[4] = { 1./3., 2./3., 1./3., pow(3.,0.5)/3.};

// Enter your C statements
.........................
.........................
}

void matrix_mult(double a[4][4], double b[4], double c[4], int rows)
{
// compute c[]=a[][]*b[]
..........................
..........................
return;
}

double unit_norm(double vec[4], int cols)
{
double sum;
// normalize a vector
...........................
...........................
return sum;
}

My teacher says the output should look like:

Initial vector:
x[0] = [ 0.333333 0.666667 0.333333 0.577350]

New vector:
y[1] = [-1.666667 1.910684 -0.732051 3.708119]
The length of this vector is: 4.551322
Normalized new vector:
x[1] = [-0.366194 0.419808 -0.160844 0.814734]

New vector:
y[10] = [-3.096892 5.315900 -6.556405 6.293838]
The length of this vector is: 10.974897
Normalized new vector:
x[10] = [-0.282180 0.484369 -0.597400 0.573476]

[wuzertheloser]

Okay, here is my code now

#include <stdio.h>
#include <math.h>
double unit_norm(double vec[4], int cols);
void matrix_mult(double a[4][4], double b[4], double c[4], int rows);
main()
{
 double K[4][4] = { { 2., -4.,  1.,  0.},
                    {-4.,  5., -2.,  1.},
                    { 1., -2.,  6., -3.},
                    { 0.,  1., -3.,  7.}};
 double y[4], x[4] = { 1./3., 2./3., 1./3., pow(3.,0.5)/3.};
 int k, rows, n;
 double dot_p;
printf("\nInitial vector:\n");
printf("x[0] = [" );
for(rows=0; rows<4; rows++){
        printf(" %f", x[rows]);
        }
printf("]\n");
printf("\nNew Vector:\n");
for(n=0; n<11; n++){
        printf("y[%d] = [", n);
        for(rows = 0; rows<4; rows++){
                printf(" %f", matrix_mult(double a[4][4], double b[4], double c[4], int row));
        }
        printf("]\n");
}
}

void matrix_mult(double a[4][4], double b[4], double c[4], int row)
{
/* compute c[]=a[][]*b[]*/
int col;
double sum;
for(row=0;row<4; row++){
        for(col=0, sum=0.0;col<4; col++){
                sum +=a[row][col]*b[col];
                }
        c[row]=sum;
        }
return;
}

double unit_norm(double vec[4], int cols)
{
double sum=0;
/* normalize a vector */
int i;
for(i=0; i<cols; i++)
        sum += vec[i] * vec[i];
sum = sqrt(sum);
for(i=0; i<cols; i++)
vec[i]=vec[i]/sum;
return sum;
}

i can't get the matrix_mult function to show up when i'm trying to do the printf. anyone know how to fix this?

[REICC]

for lines 30, why are you using a,b,c? You have not declared them yet.