Incompatible Indices when updating matrix in the loop

Question

Mouath Abu-Daoud 2021-12-15

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评论： William Rose 2021-12-16

Hey everyone!

I am doing some ECG analysis. I am trying to create a 2D-array that is updated for every loop iteration, but I get the following error when I run the code "Unable to perform assignment because the indices on the left side are not compatible with the size of the right side." Any idea on how to fix it?

Note: I know it is bad practice to not preallocate memory for matrices, but I am dealing with very large amounts of data and I cannot find its size.

%% Frequency Domain Analysis
fs3 = 4; % 4 hertz sampling frequency
xdft = [];
PowerSpectralDensity = [];
for i = 0: 60: (60-1)*240
    starttime2 = i;
    finishtime2 = starttime2 + 60;
    StartElement2 = starttime2 *fs+1;
    FinishElement2 = finishtime2 * fs;
    minuteSubset2 = ECG (StartElement2 : FinishElement2);
    t3 = starttime2: 1/fs3: finishtime2 - 1/fs3;
    %resampling the data to frequency = 4
    minuteSubset2_rs = resample(minuteSubset2, fs3, fs);
    N = length(minuteSubset2_rs);
    xdft = fft(minuteSubset2_rs');
    PowerSpectralDensity = (1/(fs3*N))* abs(xdft).^2;
    PowerSpectralDensity = PowerSpectralDensity(1:N/2);
    for j = 1: 240
    PDarray(i/60 +1, j) = xdft; %error in this line
    end
end

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Answer 1

William Rose 2021-12-15

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@Mouath Abu-Daoud,

On the line in question, you are assigning vector xdft (right hand side) to one element of array PDarray(). That won't work. You must assign a vector to a row or column of an array, and the sizes must match.

Please provide values for fs and array ECG(), if you want others to be able to run this code.

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Mouath Abu-Daoud 2021-12-15

编辑：Mouath Abu-Daoud 2021-12-15

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Thank you for your answer. How can I fix this though?

Here's my full code in case anyone needs it.

%This program analyzes ECG data over a period of 4 hours to detect if apnea
%was present
%% Opening file and extracting data
file = fopen('a01.dat');
ECG = fread(file,'int16');
fs = 100; %sampling frequency of orginal data
fs2 = 500; %new sampling frequency
t = 1/fs: 1/fs: 60;
t = t.';
% Take only the first 4 hours
ECG = ECG(1:240*60*fs)./200;
time = 1/fs:1/fs:240*60;
%% Processing and plotting data for the first minute
firstMinuteSubset = ECG (0*fs+1:60*fs);
firstMinuteSubset_rs = resample(firstMinuteSubset, fs2,fs);
    firstMinuteT1= 0: 1/fs: 60 -1/fs;
    firstMinuteT2 = 0: 1/fs2: 60 - 1/fs2;
%resampling the data to frequency = 500
    firstmMinuteSubset_rs = resample(firstMinuteSubset, fs2,fs);
%filtering the data
    d = designfilt('lowpassfir', 'filterOrder', 5, 'cutOffFrequency', 10, 'SampleRate', fs2);
    filteredFirstMinute = filter(d, firstmMinuteSubset_rs);
figure;
hold on;
plot(firstMinuteT2, filteredFirstMinute, 'r');
plot(firstMinuteT2, firstmMinuteSubset_rs, 'b');
title('Filtered and Unfiltered ECG Data');
xlabel('Time (s)');
ylabel('ECG');
%% Processing 4 hours of data
averageHRmatrix = zeros(1, 240); %preallocated matrix as in 4 hours there are 240 minutes
minimumHRmatrix = zeros(1,240);
maximumHRmatrix = zeros(1,240);
HeartRateMatrix = [];
for i = 0: 60: (60-1)*240
    starttime = i;
    finishtime = starttime + 60;
    StartElement = starttime *fs+1;
    FinishElement = finishtime * fs;
    minuteSubset = ECG (StartElement : FinishElement);
    t1= starttime: 1/fs: finishtime -1/fs;
    t2 = starttime: 1/fs2: finishtime - 1/fs2;
    %resampling the data to frequency = 500
    minuteSubset_rs = resample(minuteSubset, fs2,fs);
    %filtering the data
    d = designfilt('lowpassfir', 'filterOrder', 5, 'cutOffFrequency', 10, 'SampleRate', fs2);
    filteredECG = filter(d, minuteSubset_rs);
   
    %finding the length of RR peaks
    RRpeaks = [];
    for i2=2:length(minuteSubset) - 1 
       if (minuteSubset(i2)>1 && (minuteSubset(i2)>minuteSubset(i2+1)) && (minuteSubset(i2)>minuteSubset(i2-1)))
          RRpeaks = [RRpeaks, i2];
       end
    end
   
   
    %finding the Heart Rate
    RR_array_60s = diff(RRpeaks)/fs; 
    HR_array_60s = 1./ RR_array_60s.*60;
    HR_2 = mean(HR_array_60s)/60;
   
    HeartRateMatrix(i/60+1) = HR_2;
    minimumHRmatrix(i/60+1) = min(HR_2);
    maximumHRmatrix(i/60+1) = max(HR_2);
end
figure;
plot(HeartRateMatrix);
xlabel('Time (min)');
ylabel('Average Heart Rate (bpm)');
%% Frequency Domain Analysis
fs3 = 4; % 4 hertz sampling frequency
xdft = [];
PowerSpectralDensity = [];
for i = 0: 60: (60-1)*240
    starttime2 = i;
    finishtime2 = starttime2 + 60;
    StartElement2 = starttime2 *fs+1;
    FinishElement2 = finishtime2 * fs;
    minuteSubset2 = ECG (StartElement2 : FinishElement2);
    t3 = starttime2: 1/fs3: finishtime2 - 1/fs3;
    %resampling the data to frequency = 4
    minuteSubset2_rs = resample(minuteSubset2, fs3, fs);
    N = length(minuteSubset2_rs);
    xdft = fft(minuteSubset2_rs');
    PowerSpectralDensity = (1/(fs3*N))* abs(xdft).^2;
    PowerSpectralDensity = PowerSpectralDensity(1:N/2);
    for j = 1: 240
    PDarray(i/60 +1, j) = xdft;
    end
end