Obtaining values from a loop

How can I store all the data in a matrix from this loop?

for i = 1:size(q,2)
[Maxima(:),MaxIdx(:)] = findpeaks(q(:,i));
[Minima,MinIdx] = findpeaks(-q(:,i));
end


figure; hold on 
plot(t(MaxIdx),Maxima,'o');
plot(t(MinIdx),-Minima,'o');
plot(t,q)

[Rohit Kulkarni]

Hi Mads,

In my understanding you want to obtain all the information from a matrix using loops.

Here is an example that shows the use of code provided by you on a sample matrix "q" and sample time vector "t":

% Sample time vector
t = linspace(0, 10, 1000); % 0 to 10 seconds, 1000 points

% Sample matrix q with sinusoidal columns of different frequencies
q = [sin(2 * pi * 1 * t); sin(2 * pi * 1.5 * t); sin(2 * pi * 2 * t)]';
% Note: q is transposed to match the expected dimensions (rows are time points)

% Initialize cell arrays to store maxima and minima information
Maxima = cell(1, size(q, 2));
MaxIdx = cell(1, size(q, 2));
Minima = cell(1, size(q, 2));
MinIdx = cell(1, size(q, 2));

% Loop through each column of q
for i = 1:size(q, 2)
    [maxima, maxIdx] = findpeaks(q(:,i));
    [minima, minIdx] = findpeaks(-q(:,i)); % To find minima
    
    % Store in cell arrays
    Maxima{i} = maxima;
    MaxIdx{i} = maxIdx;
    Minima{i} = -minima; % Convert back to positive values
    MinIdx{i} = minIdx;
end

% Plotting
figure; hold on 
% Iterate over each column's results to plot
for i = 1:size(q, 2)
    plot(t(MaxIdx{i}), Maxima{i}, 'o', 'MarkerEdgeColor', 'r');
    plot(t(MinIdx{i}), Minima{i}, 'o', 'MarkerEdgeColor', 'b');
end
plot(t, q) % Plot the original sinusoidal waves
xlabel('Time (s)');
ylabel('Amplitude');
title('Maxima and Minima of Sinusoidal Waves');
legend({'Maxima', 'Minima', 'Waveforms'}, 'Location', 'best');
grid on;

Refer to the following documentation to know more about cell arrays:

https://in.mathworks.com/help/matlab/cell-arrays.html

Hope this resolves your query!

Thanks

[Dyuman Joshi]

If you want the plot for every iteration, why not include the plot command inside the loop?

[mads skibsted]

Thanks a lot!

Another problem is that i wanted to store all the minima and maxima in one large matrix for all the sine curves at the end of the loop.

Can you maybe help me with that?

[Dyuman Joshi]

Pre-allocate a cell array and store the values in each iteration - Preallocate arrays, https://in.mathworks.com/help/matlab/matlab_prog/preallocate-memory-for-a-cell-array.html

Answer 1

[Rohit Kulkarni]

Hi Mads,

In my understanding you want to obtain all the information from a matrix using loops.

Here is an example that shows the use of code provided by you on a sample matrix "q" and sample time vector "t":

% Sample time vector
t = linspace(0, 10, 1000); % 0 to 10 seconds, 1000 points

% Sample matrix q with sinusoidal columns of different frequencies
q = [sin(2 * pi * 1 * t); sin(2 * pi * 1.5 * t); sin(2 * pi * 2 * t)]';
% Note: q is transposed to match the expected dimensions (rows are time points)

% Initialize cell arrays to store maxima and minima information
Maxima = cell(1, size(q, 2));
MaxIdx = cell(1, size(q, 2));
Minima = cell(1, size(q, 2));
MinIdx = cell(1, size(q, 2));

% Loop through each column of q
for i = 1:size(q, 2)
    [maxima, maxIdx] = findpeaks(q(:,i));
    [minima, minIdx] = findpeaks(-q(:,i)); % To find minima
    
    % Store in cell arrays
    Maxima{i} = maxima;
    MaxIdx{i} = maxIdx;
    Minima{i} = -minima; % Convert back to positive values
    MinIdx{i} = minIdx;
end

% Plotting
figure; hold on 
% Iterate over each column's results to plot
for i = 1:size(q, 2)
    plot(t(MaxIdx{i}), Maxima{i}, 'o', 'MarkerEdgeColor', 'r');
    plot(t(MinIdx{i}), Minima{i}, 'o', 'MarkerEdgeColor', 'b');
end
plot(t, q) % Plot the original sinusoidal waves
xlabel('Time (s)');
ylabel('Amplitude');
title('Maxima and Minima of Sinusoidal Waves');
legend({'Maxima', 'Minima', 'Waveforms'}, 'Location', 'best');
grid on;

Refer to the following documentation to know more about cell arrays:

https://in.mathworks.com/help/matlab/cell-arrays.html

Hope this resolves your query!