Indexing a variable using the slice of another matrix

Question

Alexander James 2021-5-29

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/842985-indexing-a-variable-using-the-slice-of-another-matrix

评论： Alexander James 2021-6-4

Hello, I'm struggling to wrap my head around what I think it quite a simple problem My digi-bog model is taking an awfully long time to complete (5hours), I checked the profiler and over 3 hours of that runtime was spent on this line

%% convert Q to water stage
for i = 1:numel(river_discharge_per_channel) % 4.4e6 values (very long daily time series)
    [~,index] = min(abs(Q_list-river_discharge_per_channel(i))); %Q_list changing length 2000-7000
    water_depth(i) = d(index);
end 

This step was being perfomed for every 20 interation of the main loop (12000 iterations, with 365 sub steps). I realised that this was not necassary and that I only needed to calculate the water stage for the next 20 years so I rewrote the above code to:

%% convert Q to water stage
        for i = 1:20    % not nessary to go through the entire daily discharge list, we will only do it for 20years  
            river_discharge_per_channel_year = river_discharge_per_channel(lookup_year==t-1+i); %lookup_year is the year corresponding to the daily values (I made a lookup table for this) t = the main yearly loop
            for j = 1:numel(river_discharge_per_channel_year)
                [~,index] = min(abs(Q_list-river_discharge_per_channel_year(j)));
                water_depth(river_discharge_per_channel(lookup_year==t-1+i(j))) = d(index); % this is where I'm going wrong, I want to reference the day within the yearly slice I made earlier.
            end    
        end

It's when I try to assign the water_depth values (d) back into the daily list of water_depths, I'm not managing the index it correctly. I attempt this with this line:

water_depth(river_discharge_per_channel(lookup_year==t-1+i(j))) = d(index);

I'm not sure this is possible the way I'm trying to accomplish it, suggestions most welcome. I'm not sure the issue will be understood easily by readers, not least of becuase of my dyslexic brain, so questions most welcome.

Thanks in advance

Alex

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Alexander James 2021-5-29

编辑：Alexander James 2021-5-29

it is producing an error, it's why I created this post. I'm looking for alternative ways of accomplishing what I'm attempting in that line.

Matt J 2021-5-29

编辑：Matt J 2021-6-4

在 MATLAB Online 中打开

Well, I think I've already given it to you in my Answer below. Once you know how to rewrite the first version of your loop, it is straightforward to rewrite your double for-loop in a similar way (however, I don't see any benefit to this):

%% convert Q to water stage
     
          [Q_list, isort]=sort(Q_list); 
           d=d(isort);
  
       for i = 1:20    % not nessary to go through the entire daily discharge list, we will only do it for 20years  
           
            index=(lookup_year==t-1+i);
 
            water_depth(index) = interp1(Q_list,d, river_discharge_per_channel(index),  'nearest');
        end

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Answer 1

Matt J 2021-5-29

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/842985-indexing-a-variable-using-the-slice-of-another-matrix#answer_712105

编辑：Matt J 2021-6-4

在 MATLAB Online 中打开

Your original loop,

for i = 1:numel(river_discharge_per_channel) % 4.4e6 values (very long daily time series)
    [~,index] = min(abs(Q_list-river_discharge_per_channel(i))); %Q_list changing length 2000-7000
    water_depth(i) = d(index);
end 

can be replaced with the following,

[Q_list, isort]=sort(Q_list); 
d=d(isort);
water_depth = interp1(Q_list,d, river_discharge_per_channel,  'nearest');

You then showed us a modified version with a double for-loop. Adapting the above to this is straightforward,

for i = 1:20    % not nessary to go through the entire daily discharge list, we will only do it for 20years  
   
    index=(lookup_year==t-1+i);
    
    water_depth(index) = interp1(Q_list,d, river_discharge_per_channel(index),  'nearest');
end

but might be unnecessary and sub-optimal. I don't see why you wouldn't use the original version, which requires no loops.

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Matt J 2021-5-29

编辑：Matt J 2021-5-29

在 MATLAB Online 中打开

wouldn't sort(Q) result in a distorted matix?

Yes, it would sort it. But nothing in your original loop depends on the order of Q_list (only that it is ordered consistent with d). If other parts of your code not shown do depend on it, you are free of course to make a back-up copy.

A simple test shows that your original loop and my proposal produce the same results,

[river_discharge_per_channel,Q_list,d, water_depth]=deal(rand(1,10));
%% convert Q to water stage
for i = 1:numel(river_discharge_per_channel) % 4.4e6 values (very long daily time series)
    [~,index] = min(abs(Q_list-river_discharge_per_channel(i))); %Q_list changing length 2000-7000
    water_depth(i) = d(index);
end 
water_depth
water_depth = 1×10
    0.4557    0.8646    0.3858    0.6567    0.4446    0.8195    0.2802    0.5644    0.0991    0.1318
[Q_list, isort]=sort(Q_list); 
indices=1:numel(Q_list);
d=d(isort);
water_depth2 = interp1(Q_list,d, river_discharge_per_channel,  'nearest')
water_depth2 = 1×10
    0.4557    0.8646    0.3858    0.6567    0.4446    0.8195    0.2802    0.5644    0.0991    0.1318