MATLAB Answers


Error: Index in position 1 is invalid. Array indices must be positive integers or logical values?

Asked by Ameer Fahmi on 18 Jul 2019
Latest activity Commented on by Walter Roberson
on 19 Jul 2019
I'm trying to use the below described code of bayesian time-varying VAR. The complete package of the code can be found here.
I have issues in estimating the TVP-VAR model with more than one lag, my data has two variables with 268 observations and the optima lag is set at 6 as indicated by information criteria.
clear; clc;
load USdata1.csv;
Y0 = USdata1(1:3,:);
shortY = USdata1(4:end,:);
[T n] = size(shortY);
Y = reshape(shortY',T*n,1);
p = 6; % no. of lags
q = n^2*p+n; % dim of states
Tq = T*q; Tn = T*n; q2 = q^2; nnp1 = n*(n+1);
nloop = 11000; burnin = 1000;
%% initialize for storage
store_Omega11 = zeros(nloop-burnin,n,n);
store_Omega22 = zeros(nloop-burnin,q);
store_beta = zeros(nloop-burnin,Tq);
%% initialize the Markov chain
Omega11 = cov(USdata1);
Omega22 = .01*ones(q,1); % store only the diag elements
invOmega11 = Omega11\speye(n);
invOmega22 = 1./Omega22;
beta = zeros(Tq,1);
%% prior
nu01 = n+3; S01 = eye(n);
nu02 = 3; S02 = .005*ones(q,1);
Vbeta = 5*ones(q,1);
%% construct and compute a few thigns
X = zeros(T,n*p);
for i=1:p
X(:,(i-1)*n+1:i*n) = [Y0(3-i+1:end,:); shortY(1:T-i,:)];
bigG = SURform([ones(n*T,1) kron(X,ones(n,1))]);
H = speye(Tq,Tq) - sparse(q+1:Tq,1:(T-1)*q,ones(1,(T-1)*q),Tq,Tq);
newnu1 = nu01 + T;
newnu2 = nu02 + (T-1)/2;
disp('Starting MCMC.... ');
disp(' ' );
start_time = clock;
for loop = 1:nloop
%% sample beta
invS = sparse(1:Tq,1:Tq,[1./Vbeta; repmat(invOmega22,T-1,1)]');
K = H'*invS*H;
GinvOmega11 = bigG'*kron(speye(T),invOmega11);
GinvOmega11G = GinvOmega11*bigG;
invP = K + GinvOmega11G;
C = chol(invP); % so that C'*C = invP
betahat = C\(C'\(GinvOmega11*Y));
beta = betahat + C\randn(Tq,1); % C is upper triangular
%% sample Omega11
e1 = reshape(Y-bigG*beta,n,T);
newS1 = S01 + e1*e1';
invOmega11 = wishrnd(newS1\speye(n),newnu1);
Omega11 = invOmega11\speye(n);
%% sample Omega22
e2 = reshape(H*beta,q,T);
newS2 = S02 + sum(e2(:,2:end).^2,2)/2;
invOmega22 = gamrnd(newnu2,1./newS2);
Omega22 = 1./invOmega22;
if loop>burnin
store_beta(i,:) = beta';
store_Omega11(i,:,:) = Omega11;
store_Omega22(i,:) = Omega22';
if ( mod( loop, 2000 ) ==0 )
disp( [ num2str( loop ) ' loops... ' ] )
disp( ['MCMC takes ' num2str( etime( clock, start_time) ) ' seconds' ] );
disp(' ' );
%% compute posterior means
betahat = mean(store_beta)';
Omega11hat = squeeze(mean(store_Omega11));
Omega22hat = mean(store_Omega22);
tempbetahat = reshape(betahat,nnp1,T)';
gamhat1 = tempbetahat(:,1:n+1);
gamhat2 = tempbetahat(:,(n+1)+1:2*(n+1));
The code gives no error when one lag is used, but when more than 1 lag is used, an error occurs reffering to the matrix "X" in line 28. The matrix X is basically constructed to include all the lags, in case of 6 lags the X matrix should have 12 columns with non-zero values, but when I use 6 lags, the Matrix "X" is having columns with zero values which generates the following error
Index in position 1 is invalid. Array indices must be positive integers or logical values.
The extended X matrix is genertated with the lines below, thus something has to be done here to avoid the error
for i=1:p
X(:,(i-1)*n+1:i*n) = [Y0(3-i+1:end,:); shortY(1:T-i,:)];
But I don't know what should be done here, could anyone please help me with this issue?


Y0 has 3 rows. p is maximum 6. for i=1:p means that i can be up to 6. Y0(3-i+1:end,:) when i reaches 4 gives 3-4+1 = 0 as a subscript.
Your p cannot exceed the number of rows in Y0. If you increase those rows then recode 3-i+1 as end-i+1
Is number of rows in Y0 is arbitrarily chosen? it can be any number porvided that the outcome of X matrix will not be zero, the authors in their original paper didn't collaborate on this point.

Sign in to comment.




1 Answer

Answer by Geoff Hayes
on 18 Jul 2019

Ameer - if the lag p is 6, then in this code
for i=1:p
X(:,(i-1)*n+1:i*n) = [Y0(3-i+1:end,:); shortY(1:T-i,:)];
when i is 4 (since iterating from 1 to 6), then
3 - i + 1 = 3 - 4 + 1 = 0
and so you are using an invalid index into Y0 and the error message makes sense. You need to adjust your code to handle this situation. Does Y0 need to be defined differently?
Y0 = USdata1(1:6,:);
And then adjust the 3 at
[Y0(3-i+1:end,:); shortY(1:T-i,:)];


It appears you'll have to decipher the implementation of the algorithm and figure out what the original author should have done to handle more lags than his code does.
I'm trying to implemente the code line by line to figure out how it should be adjusted.

Sign in to comment.