How can I use a for loop to index a vector, average the indexed values, and store the averages in another vector when the subsets are of different lengths?

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Michaela Jackson
Michaela Jackson 2020-3-15
评论： Walter Roberson 2020-3-15
I have column vectors of CO2 flux values that were measured at a 5 minute frequency and correspdoning date/time vectors (Year, Month, Day, Hour, etc).
I am trying to create a for loop to subset each flux value by its corresponding hour, average the flux values for each subsetted hour (the lengths are not the same), and store the averaged values in a vector to represent the average flux for Hour 0-23.
Even just trying to subset I am getting a warning that reads: the variable 'Hour' appears to change size on every loop iteration (within a script). Consider preallocating for speed.
Is there no way to do this using a for loop? Ideally I don't want to use 24 lines of code just to index by Hour = 0-23.
Using MATLAB R2019a
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Michaela Jackson
Michaela Jackson 2020-3-15
编辑：Michaela Jackson 2020-3-15
Perhaps I am just not understanding this at all.
A = zeros(1,24);
for i=1:24
A(i) = find(Hour == i);
end
Now I am getting: Unable to perform assignment because the left and right sides have a different number of elements.
I guess that means I can't store a vector using A(i)?
Walter Roberson
Walter Roberson 2020-3-15
No, the problem is the A matrix. You find multiple entries with the same Hour value, but you try to store them in the scalar location A(i) . You could consider using a cell array
A = cell(1,24);
for i=1:24
A{i} = find(Hour == i);
end
However, considering that you are wanting to calculate average values, you should consider an approach such as
A = zeros(1,24);
for i = 1 : 24
idx = Hour == i;
A(i) = mean(flux(idx));
end

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采纳的回答

Akira Agata
Akira Agata 2020-3-15
You don't need to use for-loop.
How about the following?
load('fluxhour.mat');
[group,hourNum] = findgroups(Hour);
avgFlux = splitapply(@(x) mean(x,'omitnan'),Flux_1,group); % <- since Flux_1 contains NaN values
T = table(hourNum,avgFlux,'VariableNames',{'Hour','avgFlux'});
The output looks like:
>> T
T =
24×2 table
Hour avgFlux
____ ________
0 0.024935
1 0.024158
2 0.02483
3 0.023554
...

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