For loop taking only last column

Hi,

I am trying a code that creates two vector columns (36x1) and then store the two columns in a matrix. The problem is that the code is repeating the second column sector when storing it in a vector. I tried several option with the foor loops but the results doesn't change. Matrice is matrix in english if it can help!

 for i=1:5:6 
 %here i start from a big matrix and I take 4 rows for each iteration, all columns
 x_kalman_rolling_matrice_for = x_kalman_rolling_matrice(i+2:i+5,1:36);
for i=1:2
    %here i take all rows from a matrix, but only the first 36 columns (in second iteration from the 2nd to the 37th column);
    regressori_girati_for = regressori_girati(:,i:i+35);  
for i=1:36
%here i compute a number given by the transpose of a columns from the x_kalman_rolling_matrice_for, so a row, and I multiply it by a column

    fitted_kalman_rolling = transpose(x_kalman_rolling_matrice_for(:,i))*regressori_girati_for(:,i);
    fitted_kalman_rolling_matrice(i,1)=fitted_kalman_rolling; %here I put one numnber under the other to create a vector
    
end
end
for i=1:2
    fitted_kalman_rolling_matrice_grande(:,i) = [fitted_kalman_rolling_matrice]; %here I would like the two vector to be written one column after the other
    %but printing the results shows me that the code is taking only the second vector
 end
 end

[Mara]

Hey Davide, if you have nested for-loops, you cannot call the looping variable i in all loops. Otherwise i will be overwritten and your loops won't work the way you expect them to work.

[Davide Martintoni]

Hi Mara,

thank you for the suggestion. I tried to modify the looping variable in the iteration to this:

for j=1:5:6 
 
 x_kalman_rolling_matrice_for = x_kalman_rolling_matrice(j+2:j+5,1:36);
for k=1:2
    regressori_girati_for = regressori_girati(:,k:k+35);  
for i=1:36

    fitted_kalman_rolling = transpose(x_kalman_rolling_matrice_for(:,i))*regressori_girati_for(:,i);
    fitted_kalman_rolling_matrice(i,1)=fitted_kalman_rolling;
    
end
end

    fitted_kalman_rolling_matrice_grande(:,j) = [fitted_kalman_rolling_matrice];

end

Now it works, giving two different columns vector. But, shall I always (when doing nested loop) change the looping variable? I used the same "i" variable in previous cases and it worked fine.

[Mara]

Good to hear it works now! Maybe you should recheck these previous loops in detail, whether they behave correctly, because as far as I know you should never use the same looping variable in nested loops.

Have a look at this discussion, too

https://de.mathworks.com/matlabcentral/answers/64124-why-does-matlab-confuse-index-variables-in-a-for-loop

Best,

Mara

[Davide Martintoni]

Yes, I have seen that the values generated by the code change when changing the looping variable. Thanks for the reference to the previous discussion. Really appreciate the help. Thanks

[Mara]

Sure, glad I could help

[Adam Danz]

Using smart indentation (ctrl+a, ctrl+i) really helps in identifying these types of problems.

Your code with smart indentation,

for i=1:5:6
    %here i start from a big matrix and I take 4 rows for each iteration, all columns
    x_kalman_rolling_matrice_for = x_kalman_rolling_matrice(i+2:i+5,1:36);
    for i=1:2
        %here i take all rows from a matrix, but only the first 36 columns (in second iteration from the 2nd to the 37th column);
        regressori_girati_for = regressori_girati(:,i:i+35);
        for i=1:36
            %here i compute a number given by the transpose of a columns from the x_kalman_rolling_matrice_for, so a row, and I multiply it by a column
            
            fitted_kalman_rolling = transpose(x_kalman_rolling_matrice_for(:,i))*regressori_girati_for(:,i);
            fitted_kalman_rolling_matrice(i,1)=fitted_kalman_rolling; %here I put one numnber under the other to create a vector
            
        end
    end
    for i=1:2
        fitted_kalman_rolling_matrice_grande(:,i) = [fitted_kalman_rolling_matrice]; %here I would like the two vector to be written one column after the other
        %but printing the results shows me that the code is taking only the second vector
    end
end

[Bjorn Gustavsson]

Another helpful notation convention is to number the loop-variables i1, i2, i3 etc just to keep track of which loop-variable is which in the "nesting". This has helped me tremendously.

[Adam Danz]

That's an interesting approach, Bjorn. I reserve i,j,k for loop variables and rarely have more than 3 nested loops but when I do, I arrive to a mental impasse on coming up with loop var names. I haven't thought of numbering them....

[Bjorn Gustavsson]

That was what I always used to (but then my memory went somewhere I didn't follow), but I too often forgot which order I had i, j and k. Now I only rely on remembering the order of the natural numbers, when that goes I figure people will tell me to stop messing up programming?

[Adam Danz]

😄