How to create answers to a calculation in an array in a forloop ?

Question

iceskating911 2022-6-29

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https://ww2.mathworks.cn/matlabcentral/answers/1750670-how-to-create-answers-to-a-calculation-in-an-array-in-a-forloop

评论： Voss 2022-6-29

Hello! I'm still fairly new to matlab and have a question to ask.

I have a dataset (labeled "data) and are using two columns of the dataset to do calcuations. The following forloop gets me the answers I am looking for:

a = Data(:,1);
b = Data(:,2);
        for l = 1 : length(edges)-1
            indexes = a > edges(l) & a <= edges(l+1);
            BinSums(l) = sum(b(indexes));
        end

Which is a 1 x 16 double with a value in each column!

Now what I want to do, is to do this same calculation 1000 times, so in the end I have an array of 1000 x 16 with each row being a new set of values.

Note: This is only part of a longer and more complicated code that inputs new data for a and b every iteration. i just want to know how I save each iteration into a new array that is 1000 x 16 if that makes sense.

I can provide more detail if needed, thank you so much for the help!!!

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Answer 1

Voss 2022-6-29

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在 MATLAB Online 中打开

Nbins = numel(edges)-1;
BinSums = zeros(1000,Nbins);
for jj = 1:1000
    
    % ...
    % different a and b each time
    a = Data(:,1);
    b = Data(:,2);
    % ...
    
    for ii = 1 : Nbins
        indexes = a > edges(ii) & a <= edges(ii+1);
        BinSums(jj,ii) = sum(b(indexes));
    end
    
end

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Voss 2022-6-29

在 MATLAB Online 中打开

You already have a loop from 1 to 1000, so all you need to do is include that loop index as the row index of BinSums when storing the result.

(There is no need for a second loop from 1 to 1000. The one I showed is replaced by the one you already have.)

clear all
close all
rawdata = xlsread('Initial001.xlsx'); %rawdata set (351 x 87 double array)
%We start off by defining our bin sizes, calling it edges
edges = 2.^(-2:14); %This defines our bins with octave spacing
Nbins = numel(edges)-1;
BinSums = zeros(1000,Nbins);
numpoints=length(rawdata(:,1)); %length of the dataset
for i=1:1000
    %First Pick a Value from Column 1 to Use at random
    Casts = unique(rawdata(:,1));
    numCasts = length(Casts);
    IndicesToUse = ceil(rand(numCasts,1)*numCasts);
    CastsToUse = Casts(IndicesToUse);
    %Next Pick the Images (Column 2) to use at random
    for j = 1:numCasts
        numImages = 20;
        ImagesToUse = ceil(rand(numImages,1)*numImages);
        SimData_temp = rawdata(find(rawdata(:,1)==CastsToUse(j)),:);
        for k=1:numImages
            if k==1 & j==1
                SimData = SimData_temp(find(SimData_temp(:,2)==ImagesToUse(k)),:);
            else
                temp = SimData_temp(find(SimData_temp(:,2)==ImagesToUse(k)),:);
                SimData = [SimData;temp]; %this stores the final dataset (228x87 double array)
            end
        end
    end
    
    a = SimData(:,78);
    b = SimData(:,80);
    
    for ii = 1 : Nbins
        indexes = a > edges(ii) & a <= edges(ii+1);
        BinSums(i,ii) = sum(b(indexes));
    end
    
end

iceskating911 2022-6-29

Okay, thank you for all the help!

Voss 2022-6-29

You're welcome!

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Answer 2

Steven Lord 2022-6-29

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https://ww2.mathworks.cn/matlabcentral/answers/1750670-how-to-create-answers-to-a-calculation-in-an-array-in-a-forloop#answer_996660

Use discretize to generate the index vector then pass those index / group indices into groupsummary as the grouping variable and your data as the data variable on which to operate.