Array indices must be positive integers or logical values

I have this MatLab code where I want to run a metropolis algorithm.

In my s rho is a matrix 12x12 and L is a matrix 12x12. m0 is a vector 1x12.

When I run my code I got the error 'Array indices must be positive integers or logical values' in hfuncval(1) where I want to compute s in the starting point.

How can come throught that?

Thank

s = -0.5.*(rho + L);

% (4) Metropolis algorithm

K = 10000;                      %Number of samples
pts = zeros(length(m0),K);     %Array with sample output points
hfuncval = zeros(K);            %Array with function value outputs
pts(:,1) = m0;                  %Set starting points
hfuncval(1) = s(pts(:,1));     %Compute function in starting point
step = 0.5;                    %Set step length

%Start sampling
for k = 1:K
    [ptpert] = pts(:,k-1) + (2*rand(2,1)).*step;         % propose perturbed point
    hfuncpert = hfunc(ptpert);                          % Compute function in perturbed point
    u = rand;                                           % Generate random number in [0,1]
   if u < hfuncpert/hfuncval(k-1)                       % accepting new points
   % if u < exp(log(hfuncpert)-log(hfuncval(k-1)))
        pts(:,k) = ptpert;
        hfuncval(k) = hfuncpert;
   else                                                 % Rejecting new points
        pts(:,k) = pts(:,k-1);
        hfuncval(k) = hfuncval(k-1);
    end    
end

The theory of the algorithm is:

[Rik]

What is the value of m0? That call should also return a vector, which will not fit in your scalar index. And do you intend your output matrix to be K by K? That seems very large, especially for code that you are currently writing.

[Jonas Damsbo]

Sorry, I forgot that. m0 is a vector 1x12

[Jonas Damsbo]

But, it is in the line

hfuncval(1) = s(pts(:,1));

I got the error "Array indices must be positive integers or logical values"

[Rik]

You're using the values in the first column of pts as indices into s. So m0 contains one or more values you can't use as indices. Once you fix that you will notice this line is trying to store 12 values to a single position. Note that s is not a function, it is an array.

[Jonas Damsbo]

Okay, now I have doing this.

s = @(rho,L) -0.5.*(rho + L);

rho = (m-m0)'.*Cm.*(m-m0);

L = (y-g)'.*C_d.*(y-g);

sigma = s(rho,L);

% (4) Metropolis algorithm

K = 10000;                      %Number of samples
pts = zeros(length(m0),K);     %Array with sample output points
hfuncval = zeros(1,K);            %Array with function value outputs
pts(:,1) = m0;                  %Set starting points
hfuncval(1) = sigma(1,1);     %Compute function in starting point
step = 0.5;                    %Set step length

%Start sampling
for k = 2:K
    ptpert = pts(:,k-1) + (2*rand(12,1)-1)*step;         % propose perturbed point
    hfuncpert = sigma(ptpert);                          % Compute function in perturbed point
    u = rand;                                           % Generate random number in [0,1]
   if u < hfuncpert/hfuncval(k-1)                       % accepting new points
   % if u < exp(log(hfuncpert)-log(hfuncval(k-1)))
        pts(:,k) = ptpert;
        hfuncval(k) = hfuncpert;
   else                                                 % Rejecting new points
        pts(:,k) = pts(:,k-1);
        hfuncval(k) = hfuncval(k-1);
    end    
end

But I have again the error 'Array indices must be positive integers or logical values.'

Now in this line:

hfuncpert = sigma(ptpert);

[Rik]

Look at the values of ptpert at that point. Use the debugging tools to step through your code line by line.

Answer 1

[the cyclist]

In this line:

hfuncval(k) = hfuncval(k-1);

in the first iteration of the for loop, k == 1, so you are attempting to access the "zeroth" element of the array hfuncval, which does not exist.

[Jonas Damsbo]

Thats works, thanks!

But now I got a new error:

"Invalid expression. Check for missing multiplication operator, missing or unbalanced delimiters, or other syntax error.

To construct matrices, use brackets instead of parentheses."

In this line:

[ptpert] = pts(:,k-1) + (2*rand(2,1)1)*step;

Can you see why?

[the cyclist]

This is not valid MATLAB syntax:

(2*rand(2,1)1)

I'm not sure what you intended there, so I can't offer a solution.

[Jonas Damsbo]

So, I want to make a perturbed point in that line so I can make a new randomly guess of the modelparameters which is the function s.

[the cyclist]

What did you expect from the line of code I pasted, specifically?

Maybe you intended

(2*rand(2,1)-1)

and left out the minus sign?

[Jonas Damsbo]

I got the error 'Array indices must be positive integers or logical values.' in this line again

hfuncval(1) = s(pts(:,1));

[Jonas Damsbo]

But I expect a randomly generated vector column.

[Jonas Damsbo]

So the theoy of the algorithm is:

[Jonas Damsbo]

Okay so, I have changed a little bit in my code:

s = @(rho,L) -0.5.*(rho + L);

rho = (m-m0)'.*Cm.*(m-m0);

L = (y-g)'.*C_d.*(y-g);

sigma = s(rho,L);

% (4) Metropolis algorithm

K = 10000;                      %Number of samples
pts = zeros(length(m0),K);     %Array with sample output points
hfuncval = zeros(1,K);            %Array with function value outputs
pts(:,1) = m0;                  %Set starting points
hfuncval(1) = sigma(1,1);     %Compute function in starting point
step = 0.5;                    %Set step length

%Start sampling
for k = 2:K
    ptpert = pts(:,k-1) + (2*rand(12,1)-1)*step;         % propose perturbed point
    hfuncpert = sigma(ptpert);                          % Compute function in perturbed point
    u = rand;                                           % Generate random number in [0,1]
   if u < hfuncpert/hfuncval(k-1)                       % accepting new points
   % if u < exp(log(hfuncpert)-log(hfuncval(k-1)))
        pts(:,k) = ptpert;
        hfuncval(k) = hfuncpert;
   else                                                 % Rejecting new points
        pts(:,k) = pts(:,k-1);
        hfuncval(k) = hfuncval(k-1);
    end    
end

But I have again the error 'Array indices must be positive integers or logical values.'

Now in this line:

hfuncpert = sigma(ptpert); 

[the cyclist]

I have to admit I have not reviewed your code closely.

But the error makes it pretty clear, right? ptpert is not a positive integer, so it cannot be used as an index into a vector. For example, if ptpert is 1.5, what does sigma(1.5) represent?

It looks like the line

ptpert = pts(:,k-1) + (2*rand(12,1)-1)*step;

is where you go wrong. That doesn't look like it is going to be an integer.