Thread: How to make this "for loop"

Hi. I made a function in Matlab for image processing (with 256x256 arrays), but it contains a "for loop" that make it slow. So I think it'd be faster in c++, but i'm just a beginner and i don't know how to work with 2D arrays.

My Matlab code is:

Code:

[data2]=functionx(data,fx,fy) //data2 is an output array 256x256; data,fx,fy are 256x256 inputs
[Lx,Ly]=size(data); //Lx is number of rows, Ly columns
data2 = zeros(Lx,Ly); //preallocate an array 2D

for x = 1:Lx-1
   for y = 1:Ly-1       
      
      dx = x + fx(x,y);
      dy = y + fy(x,y);
      
      ix = floor(dx); //floor rounds the element dx to the nearest integer towards minus infinity
      iy = floor(dy);
      
      ax = 1 - (dx - ix);
      ay = 1 - (dy - iy);
      
      
      if ix>0 & iy>0 & ix<Lx & iy<Ly
         data2(x,y) = ax*ay*data(ix,iy) + (1-ax)*ay*data(ix+1,iy) + ax*(1-ay)*data(ix,iy+1) + (1-ax)*(1-ay)*data(ix+1,iy+1);
      end
   end
end

Well, it isn't a difficult code and i don't ask for all it, only how to work with 2D arrays (create 2D arrays, arrays sum, access to a value). Thanks a lot for your time.

This doesn't really look like C++ imho.

Originally Posted by matsp

And you would declare data[] and data2[] as this:

Code:

double data[Lx][Ly];
double data2[Lx][Ly];

--
Mats

Hi Mats. Thanks for your answer. The problem is that data is an input array, so I have to put their values to a new array, or do i have to use a pointer to this array?

Here are some basic things you might want to know:
Create:
1) You create the array by declaring it (like mastp noted). You should keep in mind that you might need to dynamically allocate it (with new), but that is a different story, which I don't think you mind at the moment.
Assign:
2) You access a value with the [][] symbols. Like array[10][2] is the element on the 11th line and 3nd row. Note that arrays in C/C++ start from ZERO!
Sum:
3) To sum the values of an array you just have to make a function. If you want something particular ask. I don't know exactly what you mean by sum array (sum every value, sum every value of each line and return a vector?). In any case here is a function that sums every value and returns a double (floating number with double precision) with the result

Code:

double sum2DArray(double **array, int rows, int columns) {
   double sum = 0;
   for (int i=0; i<rows; ++i)
      for (int j=0; j<columns; ++j)
            sum += array[i][j];
   return sum;
}

//Use it like this
double data[100][10]; //declare the 2D array
double sum; //declare the integer that holds the sum
sum = sum2DArray9(data, 100, 10); //assign to sum the sum of the array

Originally Posted by matsp

Depends on whether you want the original data to remain the same, or it's ok to write back the result to the old array. Looking at this:

Code:

data2(x,y) = ax*ay*data(ix,iy) + (1-ax)*ay*data(ix+1,iy) + ax*(1-ay)*data(ix,iy+1) + (1-ax)*(1-ay)*data(ix+1,iy+1);

, it looks like you MAY use the same positions in data[x][y] more than once, so you have to retain the old value until you are completely finished.

--
Mats

lol, thanks for your answer and for your correction (have to -> may, i may improve my english).
Well, data2 is an output array and its values are added each "for loop" iteration, and data is only for reading values. So, how could I write this line code?

Another question. I'm writing a Mexfile, so code works with MxArray. To pass the values of an input MxArray to an array2D, is it possible this code?

Code:

typedef double TIPO;       // macros before main
typedef TIPO** IMAGEN;


//rowsT and columnsT are the size

data2 = new TIPO* [rowsT];       // It creates output array
for (i = 0; i < rowsT; ++i)       
data2[i] = new TIPO [columnsT];

//Write mxArray data to array2D data2
salida = mxGetPr(Tu_ptr); //I think this isn't correct, because i'm very bad with pointers