Thread: Problem with a for loop

Hi,

I have to translate this line from Matlab to C/C++:


x = (-cols/2 : (cols/2 - 1))/(cols/2);y = -(-rows/2 : (rows/2 - 1))/(rows/2);[x,y] = meshgrid(x,y);
I try to explain better.

x is a monodimensional array of lenght=cols.
y x is a monodimensional array of lenght=rows.

Cols and rows depends from the picture I charge in the project. (EX cols=696,rows=534).

x[cols]= all the elements from -(cols/2)/(cols/2) to (cols/2)-1/(cols/2)
(example) -> x[0] = -(696/2)/(696/2)= -1;
x[1]= -(695/2)/(696/2)= .-0,99
....
x[695]= 0,99

For y it s the same. After done this I have to meshgrid. I mean:

transform the domain specified by vectors x and y into arrays X and Y , that are bidimensional arrays.
The rows of the output array X are copies of the vector x; columns of the output array Y are copies of the vector y.

I tried with :

Code:

    double x_a = -(double)cols/2;  
   double x_b = ((double)cols/2 - 1)/((double)cols/2);
   double y_a = (double)rows/2; 
   double y_b = -((double)rows/2 - 1)/((double)rows/2);
   double m_inc = (x_b-x_a) / m;  
    double n_inc = (y_b-y_a) / n;

    double X[rows][cols];  
   double Y[rows][cols];  
 
  for(i = 0 ;  i<=n; i++){     
   for(j= 0;  j<=m; j++) {         
   X[i][j] = x_a + m_inc*j;            
   Y[i][j] = y_a + n_inc*i;       
  }  
  }

I solved with:

Code:

double x[cols*2],y[rows*2];

 for(int i=0;i<cols*2;i++){
   x[i]=((double)(i-cols)/2)/((double)cols/2);
 }

The same thing for the y vector.
Now to do the meshgrid:

Code:

 double X[cols*2][rows*2];
 double Y[cols*2][rows*2];

 for(int m=0;m<cols*2;m++){
    for(int n=0;n<rows*2;n++{
        X[m][n]=x[m];
        Y[m][n]=y[n];
    }
 }

Is it right?

Originally Posted by Elysia

o_O
Why do you print code?

Physically annotating code with a red pencil is one of life's joys. Do it in moderation.

After changed the loop , print it (work good), after change the other loop, now I have a Segmentation fault a run time I see debugging. It must be about radius and theta. The code:

Code:

double** radius, **theta;
...
raidus=(double**)malloc(sizeof(double*)*rows;
  for(int i=0;i<rows;i++);
    radius[i]=(double*)malloc(sizeof(double)*cols;

theta=(double**)malloc(sizeof(double*)*rows;
  for(int z=0;z<rows;z++); 
    theta[z]=(double*)malloc(sizeof(double)*cols;

The function where they are used I modified. When I arrive to print function I have the segmentation fault debugging

Code:

void pre_filter_Computations(double **radius,double **theta,int cols,int rows){

double x[cols],y[rows];
double X[cols][rows],
 Y[cols][rows];
double epsilon=0.0001;

 printf("Entering prefilter function\n");
 
for(int i=0;i<cols*2;i++){  
    x[i]=((double)(i-cols)/2/((double)cols/2; 
}
...

The first reason is that in my University we studied just C and not C++. But my work project is about image processing. So I have to use OpenCV library that works with OpenCV. So I don' t know well the type of variable. For what I just see I suppose it' very usefull in my case. What I have to do it s to build in this function the 2 matrix theta(angle) and radius to speed up filter construction. If I use <std::vector<double> I can go to every part of the vector as it be a bidimensional array? Because after I have to square all ellement of radius and to change the center value of matrix radius to 0 and do some more stuff. Thanks helping

I have a question: Can I assign a bidimension array of double to an array of double;

double X[cols][rows];
vector<double>radius(rows*cols);