MATLAB: Possible to modify only one portion of an array in one line

arrayelementmodifymultiple conditions

I have large arrays (10 to 100k elements) and would like to change some values that are too high, but only after certain point. What I have in mind is something like this:

x = [100 80 70 20 40 10 50 1 60] ;
xMax = 30 ;
transition = 3 ;
x(x>xMax & x(i)>transition) = xMax

I would like to know if there is a more efficient and elegant way of doing it instead of with a loop, like this:

x = [100 80 70 20 40 10 50 1 60] ;
xMax = 30 ;
transition = 3 ;
for i = 1 :length(x)
    if i > transition & x(i) > xMax
        x(i) = xMax ;
    end
end

Is it possible to include two conditions in this manner?

Thanks

Best Answer

x(x > xMax & 1:length(x) > transition) = xMax

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MATLAB: How do you loop across rows of an array without overwriting calculated values

The loop version:

increment = 30;
numblocks = ceil(size(A, 1)/increment);
avg = zeros(numblocks, 1);
for idx = 1 : numblocks
  M = A(idx*increment-increment+1 : min(size(A, 1), idx*increment), 2); %The min operation makes sure you don't select element past the end of the array for the last block which may have less than increment elements
  avg(idx) = nanmean(M);
end

The better matlab non-loop version:

increment = 30;
avg = nanmean(reshape(A(1:end-mod(end, increment), 2), increment, [])).'; %deal with all blocks that are exactly the length of increment
avg = [avg; nanmean(A(end-mod(end, increment)+1:end, 2))]; %deal with last block that is too short

Note that since R2015a (I think) nanmean(x) can be replaced by mean(x, 'omitnan') which does not require any toolbox.

MATLAB: How to create an array of fixed length

Maybe you want to pre-allocate z and c like this:

z = zeros(size(z0)) ;
c = zeros(size(z0)) ;

Best Answer

Related Solutions

MATLAB: How do you loop across rows of an array without overwriting calculated values

MATLAB: How to create an array of fixed length

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