code is not 'return'-ing

I want to produce a code that compares both Cholesky in-built function with calculated values, but my current code is not returning to line 1. i can't figure out why.

this is my current code

A=input('Matrix= ');
[n,m]=size(A);
if m==n
    if issymmetric(A,'nonskew')
        if eig(A)>=0
            fprintf('matlab inbuilt function "chol" is: ')
            R=chol(A)
            [n,m]=size(A);
            R1=zeros(n,m);
            h=1;
            for j=1:n
                if j==1
                    R1(j,1)=sqrt(A(j,1));
                else
                    R1(j,1:h)=((A(j,1:h))/(R1(1:h,1:h))')';
                    sum=R1(j,1:h)*(R1(j,1:h))';
                    R1(j,j)=sqrt(A(j,j)-sum);
                    h=h+1;
                end
            end
        else
            fprintf('matrix does not have positive eigenvalues, ')
            return
        end
        fprintf('Calculated value is: ')      
        Q=R1'
    else
        fprintf('the input is not a symmetrical matrix, try again')
        return
    end
else 
    fprintf('the format of input matrix is wrong')
    return
end

[Komel Kaur]

i want the code to return to line 1 if the input values is not m=n, is not symmetric and does not have eigenvalues>=0

[VBBV]

The return command just exits the loop and returns to command prompt. Read the note in reference below

https://in.mathworks.com/help/matlab/ref/return.html#:~:text=When%2520MATLAB%2520reaches%2520a%2520return,invoking%2520program%2520or%2520command%2520prompt.

[Komel Kaur]

noted with thanks. this helped me understand 'return' better

Answer 1

[Les Beckham]

return does not mean 'restart'. return in the context of a script actually means 'abort' or 'return to the command line and stop executing this script'.

If you wish this code to repeat until a valid matrix is entered, wrap it in a while(true) loop and replace all of your return statments with continue (which will restart the loop, skipping any statements after that one).

Also, add a break after the last line of code for success (Q = R1'). This will terminate the while loop.

You should also add '\n' at the end of each of your error messages so the command prompt will start on a new line.

So, this should work better. Note that this is still a rather fragile way to do this. Using the input function leaves lots of opportunities for the user to enter stuff that doesn't make sense (especially leaving off the square brackets in this case).

while (true)
    A=input('Matrix= ');
    [n,m]=size(A);
    if m==n
        if issymmetric(A,'nonskew')
            if eig(A)>=0
                fprintf('matlab inbuilt function "chol" is: ')
                R=chol(A)
                [n,m]=size(A);
                R1=zeros(n,m);
                h=1;
                for j=1:n
                    if j==1
                        R1(j,1)=sqrt(A(j,1));
                    else
                        R1(j,1:h)=((A(j,1:h))/(R1(1:h,1:h))')';
                        sum=R1(j,1:h)*(R1(j,1:h))';
                        R1(j,j)=sqrt(A(j,j)-sum);
                        h=h+1;
                    end
                end
            else
                fprintf('matrix does not have positive eigenvalues, \n')
                continue
            end
            fprintf('Calculated value is: ')
            Q=R1'
            break
        else
            fprintf('the input is not a symmetrical matrix, try again\n')
            continue
        end
    else
        fprintf('the format of input matrix is wrong\n')
        continue
    end
    
end

[Komel Kaur]

would it be better if i used a function instead of input?

[Les Beckham]

That is how I would do it.

For example, put your original code (with your original return calls) in a function. Delete the input line but keep the added '\n' that I suggested in the error/warning messages). This is not as interactive, as you lose the prompt from the input function.

You would then define your matrix on the command line and then call the function.

>> mat = [1 0; 0 2];
>> [R, Q] = compareChol(mat);

Note that this also returns R and Q into your base workspace so you can do other things with them if you wish (otherwise they would disappear when the function exited).

You can also get rid of the returns completely by using error (which will print your message and exit the function).

See example below.

function [R, Q] = compareChol(A)
    [n,m] = size(A);
    if m == n
        if issymmetric(A,'nonskew')
            if eig(A)>=0
                fprintf('matlab inbuilt function "chol" is: ')
                R = chol(A)
                [n,m]=size(A);
                R1=zeros(n,m);
                h=1;
                for j=1:n
                    if j==1
                        R1(j,1)=sqrt(A(j,1));
                    else
                        R1(j,1:h)=((A(j,1:h))/(R1(1:h,1:h))')';
                        sum=R1(j,1:h)*(R1(j,1:h))';
                        R1(j,j)=sqrt(A(j,j)-sum);
                        h=h+1;
                    end
                end
            else
                error('matrix does not have positive eigenvalues')
            end
            fprintf('Calculated value is: ')
            Q = R1'
        else
            error('the input is not a symmetrical matrix, try again')
        end
    else
        error('the format of input matrix is wrong')
    end

[Komel Kaur]

ohh.. understood. thank you for all your help

[Les Beckham]

You are welcome.