Machine Learning onramp section 4.6

Having trouble with the further practice section:

1. how can i have the for loop iterate through all the variables in the sampleletters.mat file without creating an array of them manually?
2. can i concatenate these results of the new function 'extract' without first creating a table outside of the for loop?
3. how do i vertically concatenate outputs of extract? when i use either a semicolon (as shown) or [ ] , i get the error 'all tables being horizontally concatenated must have the same number of rows.'

screenshots of the question and my current work below!

Answer 1

[Asvin Kumar]

For 2 and 3, have a look at https://www.mathworks.com/help/matlab/matlab_prog/add-and-delete-table-rows.html#AddAndDeleteTableRowsExample-2

Since the outputs of extract are already tables you can concatenate them without creating an empty table.

For 1, you could approach it in a slightly different manner:

T = whos('-file','sampleletters.mat'); 

BigTable = extract(T(1).name);  
for i = 2:size(T,1)  
    BigTable = [BigTable ; extract(T(i).name)];  
end  
BigTable 
 
function feat = extract(var_name)  
    letter = getfield(load('sampleletters.mat',var_name),var_name); 
    
    aratio = range(letter.Y)/range(letter.X);  
    idxmin = islocalmin(letter.X,"MinProminence",0.1);  
    numXmin = nnz(idxmin);  
    idxmax = islocalmax(letter.Y,"MinProminence",0.1);  
    numYmax = nnz(idxmax);  
    dT = diff(letter.Time);  
    dXdT = diff(letter.X)./dT;  
    dYdT = diff(letter.Y)./dT;  
    avgdX = mean(dXdT,"omitnan");  
    avgdY = mean(dYdT,"omitnan");  
    corrXY = corr(letter.X,letter.Y,"rows","complete");  
    
    featurenames = ["AspectRatio","NumMinX","NumMinY","AvgU","AvgV","CorrXY"];  
    feat = table(aratio,numXmin,numYmax,avgdX,avgdY,corrXY,'VariableNames',featurenames);  
end