Array indices must be positive integers or logical values

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Jonas Damsbo
Jonas Damsbo 2020-1-8
评论： the cyclist 2020-1-9
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:
a.png
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Jonas Damsbo
Jonas Damsbo 2020-1-9
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
Rik 2020-1-9
Look at the values of ptpert at that point. Use the debugging tools to step through your code line by line.

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回答（1 个）

the cyclist
the cyclist 2020-1-8
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.
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Jonas Damsbo
Jonas Damsbo 2020-1-9
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
the cyclist 2020-1-9
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.

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