How to select multiple rows from an array and apply some operation iteratively

This is an array with 324 Rows and 3 Columns. I want to select patch of 3 rows as (Row1= P1, Row2= P2 and Row3= P3), then apply some operation and then select next three rows as (Row4= P4, Row5= P5 and Row6= P6) apply the same operation, and so on........... Finally I want to take the sum of all the individual operations applied.

I assume that I should use for loop to get this operation done, but I'm confused!!!!!

For more assistance with the question: Actually P1, P2 and P3 will be used as vertices coordinated of a triangle, and I want to calculate the area of all triangles.

The function I am using to find area is: Triangle Area and Angles

Answer 1

[Matt J]

Well, the efficient way to do the area calculation specifically is,

A=T(1:3:end,:).';
B=T(2:3:end,:).';
C=T(3:3:end,:).';

Areas=abs(cross(B-A,C-A))/2;

To do general operations on blocks of 3 rows, you could use mat2tiles from,

https://www.mathworks.com/matlabcentral/fileexchange/35085-mat2tiles-divide-array-into-equal-sized-sub-arrays

out = cellfun(@triangle_areas, mat2tiles(T,[3,3]) )