MATLAB: Stuck with simple matrix calculation, can you help

matrixmatrix manipulationproductsum

So I have two matrixes, both 44×72, let's call them A and V. What I need to do is multiply each value in row/column "ij" by the same value in the other matrix. Then I need to sum all these values up and divide by the sum of all the values in matrix A. Here's another way to think about what I need to do:

Answer = Sum(Aij x Vij) / Sum(Aij) over all i,j grid cells

Sounds easy to do but I can't figure it out. Thanks in advance for the help!

Aaron

Best Answer

B=A.*V;
SumA=sum(sum(A));
if SumA~=0
  C=sum(sum(B))/SumA;
end

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MATLAB: How to replace zeros with other value in a big matrix fast

Don't double call these functions (MAX,MIN,SUM,MEAN,etc.). Learn to use the colon, it is your friend!

A(~A(:))=mean(A(:));

This will be somewhat faster. If you need even more speed, replace the call to mean with its definition.

A(~A(:)) = sum(A(:))/numel(A);

The thing is, with a very large array, it might just take some time!

Here is how I timed these, BTW. You can play around with other options as well! Run the function 3 times (the first time warms it up after a save). The time will print to the command window.

function [] = write_ones()
T1 = 0;  % Time two different approches
T2 = 0;
for ii = 1:5
    A = round(rand(3320))>.75;
      tic
      A(A==0)=mean(mean(A));
      T1 = T1 + toc;
      clear A
      A = round(rand(3320))>.75;  % A new A.
      tic
      A(~A(:)) = sum(A(:))/numel(A);
      T2 = T2 + toc;
  end
[T1 T2]

MATLAB: Using subs function on a syms matrix

Assign it to ‘t’:

syms t1 t2 t3 
t = [t1 t2 t3]; %t1 t2 and t3 are syms variables
t = subs(t,[t1 t2 t3],[0 0 45])

producing:

t =
[ 0, 0, 45]

Best Answer

Related Solutions

MATLAB: How to replace zeros with other value in a big matrix fast

MATLAB: Using subs function on a syms matrix

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