# How to replace two for loops with matrix expression

3 views (last 30 days)
Fei Lai on 18 May 2016
Answered: Nobel Mondal on 18 May 2016
the original code is like this:
%
for i=2:nhx-1
for j=2:nhy-1
Unew(i,j)= U(i,j)-dt*(P(i+1,j)-P(i-1,j))/(2*hx)...
+nu*dt*(1/(hx*hx)*(U(i+1,j)-2.*U(i,j)+U(i-1,j))...
+1/(hy*hy)*(U(i,j+1)-2.*U(i,j)+U(i,j-1)))...
-dt*U(i,j)/(hx)*(U(i,j)-U(i-1,j))...
-dt*V(i,j)/(2*hy)*(U(i,j+1)-U(i,j-1));
end
end
how can I replace two for loops to increase the calculation speed?
Todd Leonhardt on 18 May 2016
Edit your post to format the code by indenting all code by at least 2 spaces. That will make it more readable.

Nobel Mondal on 18 May 2016
You may directly use "vectorization" and "element-wise operation to solve this :
Here is an example code -
%%Assumptions - as I don't have the actual data
U = magic(100);
P = magic(length(U));
V = magic(length(U));
dt = 0.1;
hx = rand;
hy = rand;
nu = rand;
nhx = size(U,1);
nhy = size(U,2);
Unew = zeros(nhx, nhy);
%%Old algo
for i=2:nhx-1
for j=2:nhy-1
Unew(i,j)= U(i,j)-dt*(P(i+1,j)-P(i-1,j))/(2*hx)...
+nu*dt*(1/(hx*hx)*(U(i+1,j)-2.*U(i,j)+U(i-1,j))...
+1/(hy*hy)*(U(i,j+1)-2.*U(i,j)+U(i,j-1)))...
-dt*U(i,j)/(hx)*(U(i,j)-U(i-1,j))...
-dt*V(i,j)/(2*hy)*(U(i,j+1)-U(i,j-1));
end
end
%%Vectorized algo
myUnew = zeros(nhx, nhy);
myUnew(2:nhx-1, 2:nhy-1)= U(2:nhx-1, 2:nhy-1)-dt.*(P(3:nhx,2:nhy-1)-P(1:nhx-2,2:nhy-1))./(2*hx)...
+nu.*dt.*(1/(hx*hx).*(U(3:nhx,2:nhy-1)-2.*U(2:nhx-1, 2:nhy-1)+U(1:nhx-2,2:nhy-1))...
+1/(hy*hy).*(U(2:nhx-1,3:nhy)-2.*U(2:nhx-1, 2:nhy-1)+U(2:nhx-1,1:nhy-2)))...
-dt.*U(2:nhx-1, 2:nhy-1)/(hx).*(U(2:nhx-1, 2:nhy-1)-U(1:nhx-2,2:nhy-1))...
-dt.*V(2:nhx-1, 2:nhy-1)/(2*hy).*(U(2:nhx-1,3:nhy)-U(2:nhx-1,1:nhy-2));
%%Now verify functionality
success = isequal(Unew, myUnew);
if success
disp('Test passed')
else
disp('Test failed. Please check the algorithm.')
end

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