Question

I have a question about how can I construct a loop. I have a double array and I want to code that process.

This is the array where I want to apply the process and the length is 4×4 (I’m doing only the first, but the original was 4x4x3).

b1= (:,:,1);

I want to apply for every value this process:

1.- Create a vector every time, with the information of every value of the array.

ma= 0;
 for p=(136:136)
 ma(p)=b1(1,1,1);
 end
 for p=(312:2151)
     ma(p)=0
 end
 ma= ma';

Then, I have to do with the last result the following process (I’ve defined the variables before).
spout1_a= spb1y.ma;
spout1_b= spout1_a./spsum_pesos1;
spout1_c= sum(spout1_b);

The problem is I know how can to do it for one value (the first one), but not with all of them. How can I do it?

Greetings and thank you so much,

Emma

EDIT

b1= Refl(:,:,1);

load sp1.txt;

spb1y= sp1(:,1);
spsum_pesos1= sum(spb1y);
output = cell(length(banda1), 5); % this works well

 for i = 1:numel(b1)
    ma = zeros(2151,1);
    ma(136) = output(i);
    spout1_a= spb1y.*ma;
    spout1_b= spout1_a./spsum_pesos1;
    spout1_c= sum(spout1_b); % I want to save that result on every value of the matrix
end 

Am I writing this last part well?

Answer 1

Here are a few pointers for coding in MATLAB:

1. Preallocate: allocate memory prior to the loop so that variables don’t grow in dimensions inside the loop. This causes MATLAB to dynamically allocate memory in each iterations, which can often slow execution time significantly.
2. Vecotrize: try to use loops only where you have to. In many cases, you can use vectorized operations instead, which are a lot faster.

With that said, you can try this:

% # Iterate over each value in 'b1'
for i = 1:numel(b1)

    % # Create a vector 'ma'
    ma = zeros(2151, 1);
    ma(136) = b1(i);

    % # Do some more calculations with 'ma' ...
    spout1_a = ma;
    spout1_b= spout1_a ./ spsum_pesos1;
    spout1_c= sum(spout1_b);
end

I’m not sure what you want to achieve, but this code does exactly what the question describes.