Dane Parchment - 1 year ago 62
C Question

# C: (Working But Not As Intended) My determinate method is editing the provided matrix (as a parameter) but I just want it to provide the determinate

Here is my code for the determinate function:

double getDeterminate(Matrix m) {
//We don't want to change the given matrix, just
//find it's determinate so use a temporary matrix
double **temp = m.matrix;
int rows = m.rows, cols = m.cols;
//If the matrix is not square then the derminate does not exist
if(rows == cols) {
//If the matrix is 2x2 matrix then the
//determinate is ([0,0]x[1,1]) - ([0,1]x[1,0])
if(rows == 2 && cols == 2) {
return (temp[0][0] * temp[1][1]) - (temp[0][1] * temp[1][0]);
}
//Otherwise if it is n*n...we do things the long way
//we will be using a method where we convert the
//matrix into an upper triangle, and then the det
//will simply be the multiplication of all diagonal
//indexes
int i, j, k;
double ratio;
for(i = 0; i < rows; i++){
for(j = 0; j < cols; j++){
if(j>i){
ratio = temp[j][i]/temp[i][i];
for(k = 0; k < rows; k++) {
temp[j][k] -= ratio * temp[i][k];
}
}
}
}
double det = 1; //storage for determinant
for(i = 0; i < rows; i++) {
det *= temp[i][i];
}
return det;
}
return 0;
}

And here is my struct for the Matrix (in an external header file):

#ifndef MATRIX_H
#define MATRIX_H

typedef struct Matrix {
int rows;
int cols;
double ** matrix;
} Matrix;

/** Build/Destroy/Display Methods
-------------------------------------------*/
Matrix buildMatrix(int r, int c);

Matrix displayMatrix(Matrix m);

/** Matrix Get and Set
-------------------------------------------*/
void setValue(Matrix m, double val, int r, int c);

double getValue(Matrix m, int r, int c);

/** Matrix Multiplication
-------------------------------------------*/
Matrix multByValue(double val, Matrix m);

Matrix multByMatrix(Matrix mat1, Matrix mat2);

/** Matrix Inversion
-------------------------------------------*/
Matrix invertMatrix(Matrix m);

/** Matrix Determinate
-------------------------------------------*/
double getDeterminate(Matrix m);

/** Matrix Transpose
-------------------------------------------*/
Matrix transpose(Matrix m);

#endif

Currently it is doing exactly what I want it to do, finding the determinate by turning the matrix into an upper triangle. However, when I run the determinate method, it still changes the matrix I provided as a parameter, even though I believe I used temporary variables to do the calculations without messing with given Matrix.

Example In my main.c I have this:

Matrix mat2 = buildMatrix(3, 3);
setValue(mat2, 12, 0, 0); setValue(mat2, 4, 0, 1); setValue(mat2, 3, 0, 2);
setValue(mat2, 4, 1, 0); setValue(mat2, 13, 1, 1); setValue(mat2, 25, 1, 2);
setValue(mat2, 78, 2, 0); setValue(mat2, 8, 2, 1); setValue(mat2, 9, 2, 2);
displayMatrix(mat2);
printf("Determinate of mat2 is %4.2f", getDeterminate(mat2));
Matrix mat4 = transpose(mat2);
displayMatrix(mat4);

And when I run it I get:

Mat2 Before Transpose
| 12.00 4.00 3.00|
| 4.00 13.00 25.00|
| 78.00 8.00 9.00|

Determinate of mat2 is 3714.00

Mat4 After Transpose of Mat2
| 12.00 0.00 0.00|
| 4.00 11.67 0.00|
| 3.00 24.00 26.53|

As you can see the transpose function is doing it's job correctly, but mat2 has been changed by the determinate method. I don't want that to occur, so what have I done to cause this to happen, and what can I do to fix it? I think it has something to do with pointer, I am new to C, so I don't really know what to do.