How do I use the cat function?

HI everyone!

I created the following code to obtain certain data points (peaks). I'm trying to use the the cat function to get all of the peaks into one array (Allpeaks). However when i did this I got the erorr below:

fieldNames = fieldnames(MCR_full.MIB037.Reaches);

for k = 1 : 10
    thisFieldName = fieldNames{k};
    thisArray = MCR_full.MIB037.Reaches.(thisFieldName).kin;
    x = thisArray(:, 1);  
    y = thisArray(:, 3);
    Allpeaks=cat(peaks)
    [peaks, locs]= findpeaks (y);
    reachLimitArray= find (peaks >= 0.12);
    if length(reachLimitArray )> 1
        disp('There is at least two value above the limit.');
        for i = 1 : length(reachLimitArray)
 	       
        end
    else 
        disp('All values are below the limit.');
    end
end

Error using cat

Dimension argument must be a real, positive, integer scalar.

If anyone knows what this means and how to fix it your help would be greatly appreciated!

[Mackenzie Maher]

Thanks for both of those tips and what you suggested worked. However, I think concatenate is the wrong function. I’m looking for one that stores all the data I need and prevents the loop from overwriting every time it cycles through. Any ideas on how to do this?

[Stephen23]

"Any ideas on how to do this?"

The standard approach is to use indexing into an array:

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

Answer 1

[Jan]

Whenever you have an error for a specific command, read the help section at first:

help cat
% Or more exhaustive:
doc cat

You will see, that the first input must be the dimension, along which the list of folloing arrays is concatenated.

In your case cat(peaks) appears, before peaks is defined. This looks strange. I guess you want to replace:

    Allpeaks=cat(peaks)
    [peaks, locs]= findpeaks (y);

by

    [peaks, locs]= findpeaks(y);
    Allpeaks = cat(1, Allpeaks, peaks);   % Or cat(2, ...) ?

[Mackenzie Maher]

Hi again, I've tried everything and i can't seem to get my peaks into one matrix I did the display sizie fuction so i know they are there but when i tried preallocating but no mater what I do my loop keep keeps overwriting data every time it cycles through.

FieldNames = fieldnames(MCR_full.MIB037.Reaches);
allPeaks = zeros(10,10); %empty vector

for k = 1 : 10
    thisFieldName = fieldNames{k};
    thisArray = MCR_full.MIB037.Reaches.(thisFieldName).kin;
    x = thisArray(:, 1);  
    y = thisArray(:, 3);
    peaks =  findpeaks(y);
    allPeaks = peaks; %adding new peaks to end of vector
    disp(size(peaks));
    
    reachLimitArray= find (peaks >= 0.12);
    if length(reachLimitArray )> 1
        disp('There is at least two value above the limit.');
        
        for i = 1 : length(reachLimitArray)
            index = reachLimitArray(i);
            disp(peaks(index));
            
        end
    else
        disp('All values are below the limit.');
    end
end
disp(allPeaks);

If any one has any ideas please let me know!

Thanks:)

[Image Analyst]

@Mackenzie Maher, do all the signals have the same number of peaks, as returned from findpeaks, or can there be different numbers of peaks?

Please attach 'MCR_full' so we can run your code:

save('answers.mat', 'MCR_full');

[Walter Roberson]

allPeaks = zeros(10,10); %empty vector

That is not an empty vector.

    allPeaks = peaks; %adding new peaks to end of vector

and that does not add anything to the end of anything.

findpeaks() typically finds a variable number of peaks, so you would typically expect a different length of peaks each iteration of k . Are you sure you want to put them into a vector? You would not be able to tell them apart.

Consider using a cell array

FieldNames = fieldnames(MCR_full.MIB037.Reaches);
allPeaks = cell(10,1);

for k = 1 : 10
    thisFieldName = fieldNames{k};
    thisArray = MCR_full.MIB037.Reaches.(thisFieldName).kin;
    x = thisArray(:, 1);  
    y = thisArray(:, 3);
    peaks =  findpeaks(y);
    allPeaks{k} = peaks(:).'; 
    disp(size(peaks));
    
    reachLimitArray= find (peaks >= 0.12);
    if length(reachLimitArray )> 1
        disp('There is at least two value above the limit.');
        
        for i = 1 : length(reachLimitArray)
            index = reachLimitArray(i);
            disp(peaks(index));
            
        end
    else
        disp('All values are below the limit.');
    end
end
celldisp(allPeaks);

[Mackenzie Maher]

Thank you so much you are wonderful it worked!