Multiplying Matrix by Vector

[Caeon]

This is what my program is supposed to do:

1. Print the vector (m1)
2. Print the matrix (m2)
3. Multiply the vector and matrix together and display results

The only thing wrong with my program is that I can't quite get the right results displayed.

The correct display of values should be:
30
70
110
150

My Values displayed are:
30
71
115
159

The display of the first number (A[0][0]) is correct (30). But A[1][0] is off by 1; A[2][0] is off by 5; A[3][0] is off by 9.

I noticed that x[0][0] is 1; x[0][1] is 5; and x[0][2] is 9. But I still cannot figure out why this is off.

If you change line 78 from '+=' to '=', it will print out a chart of the values without adding them together. Then if you add the columns displayed you get the right sums, but once you change it back to '+=' it becomes off by the numbers I stated above.

I have tried changing some of the values in m1 to see if the same pattern persists, and the pattern does persist. If you have any ideas or know figure out what's wrong, it would be much appreciated.

/*
   Filename: prog_5.c
   Creator: Spencer Beale
   Date: 11/8/10
   About: This program will multiply a matrix by a vector and display the results, vector, and matrix.
*/

#include <stdio.h>

int main()
{
  // Define Variables
  int n = 4;

  int m1[4] = {1, 2, 3, 4};
  int m2[4][4] = {
    {1,2,3,4},
    {5,6,7,8},
    {9,10,11,12},
    {13,14,15,16}
  };
  int multiplied_matrix[4][1] = {
    {0},
    {0},
    {0},
    {0}
  };

  // Define Functions
  void print_matrix(int n, int m[n][n]); // Prints an nxn matrix
  void print_vector(int n, int v[n]); // Prints a vector of n elements
  void matrix_x_vector(int n, int y[n], int x[n][n], int A[n][n]); // Performs the calculation

  // Execute Functions
  print_vector(n, m1);
  print_matrix(n, m2);
  matrix_x_vector(n, m1, m2, multiplied_matrix);

  printf("\n");
}

void print_matrix(int n, int m[n][n])
{
  int i, j; // i = row; j = column;
  printf("\nMatrix Given\n");

  for (i=0; i<n; i++)
  {
    for (j=0; j<n; j++)
      printf("%3i", m[i][j]);
    printf("\n");
  }
}

void print_vector(int n, int v[n])
{
  int i;
  printf("\nVector Given\n");

  for (i=0; i<n; i++)
    printf("%3i", v[i]);

  printf("\n");
}

void matrix_x_vector(int n, int y[n], int x[n][n], int A[n][n])
{
  int i, j; // i = row; j = column;

  printf("\nResulted Matrix of [M]*[V]\n");

  // Load up A[n][n]
  for (i=0; i<n; i++)
  {

    for (j=0; j<n; j++)
    {
      A[j][0] += x[j][i] * y[i];
      printf("%4i", A[j][0]);
    }
    printf("\n");

  }

  printf("\n");

  // Print A[n][n]
  for (i=0; i<n; i++)
    printf("%4i\n", A[i][0]);
}

(The document of code is included to downloaded and mess with if you so desire to do so.)

[gerard4143]

Number one, your defining your functions inside of main..Place them before main like

void print_matrix(int n, int m[n][n]); // Prints an nxn matrix
void print_vector(int n, int v[n]); // Prints a vector of n elements
void matrix_x_vector(int n, int y[n], int x[n][n], int A[n][n]);

int main()
{}

[gerard4143]

I tried rewriting you code...Please see this link on passing multi-dim arrays in C

http://www.eskimo.com/~scs/cclass/int/sx9a.html

#include <stdio.h>


void print_matrix(int n, int m[4][4]); 

void print_vector(int n, int v[4]); 

void matrix_x_vector(int n, int y[4], int x[4][4], int A[4][1]);


int main()

{  

	int n = 4;



	int m1[4] = {1, 2, 3, 4};


	int m2[4][4] = 	{

				{1,2,3,4},

				{5,6,7,8},

				{9,10,11,12},

				{13,14,15,16}

			};


	int multiplied_matrix[4][1] = 	{

						{0},

						{0},

						{0},

						{0}

					};





	print_vector(n, m1);

	print_matrix(n, m2);

	matrix_x_vector(n, m1, m2, multiplied_matrix);



	printf("\n");
	return 0;

}



void print_matrix(int n, int m[4][4])

{

	int i, j; 

	printf("\nMatrix Given\n");



	for (i=0; i<n; i++)

	{

		for (j=0; j<n; j++)

			printf("%3i", m[i][j]);

		printf("\n");

	}

}



void print_vector(int n, int v[4])

{

	int i;

	printf("\nVector Given\n");



	for (i=0; i<n; i++)

		printf("%3i", v[i]);



	printf("\n");

}



void matrix_x_vector(int n, int y[4], int x[4][4], int A[4][1])

{

	int i, j;



	printf("\nResulted Matrix of [M]*[V]\n");





	for (i=0; i<n; i++)

	{

		for (j=0; j<n; j++)

		{

			A[j][0] += x[j][i] * y[i];

			printf("%4i", A[j][0]);

		}

	
		printf("\n");

	}



	printf("\n");





	for (i=0; i<n; i++)

		printf("%4i\n", A[i][0]);

}

I never verified if the values were correct..

[Caeon]

Thank you very much! It works perfect! I will definitely read that article and look through how your code works. I very much appreciate it!

Have a good one!