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This question was flagged by Walter Roberson

Stephen Cobeldick
on 23 Feb 2021

Original question by zahid fazal retrieved from Google Cache:

How to define a matrix which changes its size in every iteration of loop?

T = 50; %total iteration

x = randn(T,1); %input

pw =[0,0,0,0,0,1,-0.3,0.2]; % primary path TF

d = filter(pw, 1, x); % desiresd input

mu = 0.1; %step size

g = 1e-12; %gamma

%p = 2; %projection order

N = 8; %no of taps

Pmax = 4;

w = zeros(N,1); % weights of controller

y = zeros(1,T); % output of controller

%Xi = zeros(p,N); %input matrix

e_cont = zeros(T,1); %residual noise

p = zeros(T,1);

p(1)=Pmax;

for i=1:T

Xi(2:p(i),:) = Xi(1:p(i),:);

Xi(1,:) = [ x(i:-1:max(i-N+1,1)).',zeros(1,max(N-i,0)) ];

di = transpose([ d(i:-1:max(i-p(i)+1,1)).',zeros(1,max(p(i)-i,0)) ]);

Yi = Xi*w;

err= di-Yi;

sqe = err.^2;

UT = (mu*p(i)+2)*var/(2-mu);

LT = (mu*(p(i)-1)+2)*var/(2-mu);

if sqe(p(i)) > UT

p(i+1) = min(p(i)+1,Pmax);

elseif sqe(p(i))<= LT

p(i+1) = max(p(i)-1,1);

else

p(i+1) = p(i);

end

w = w + mu*Xi'*inv(g*eye(p(i))+Xi*Xi')*(di-Yi);

e_cont(i)= err(p(i));

end;

Walter Roberson
on 20 Feb 2021

It is legal to change the size of a matrix inside a loop, including being legal to let it grow one element at a time every iteration. However, it is more efficient if you can create the matrix at full size ahead of time. Sometimes much more efficient.

The problem with your code is that you do not initialize the variable Xi but you have

Xi(2:p(i),:) = Xi(1:p(i),:);

That requires that rows 1 to Pmax and all columns of Xi are initialized before the statement is executed -- but Xi is undefined here.

Also, the right hand side has p(i)-1+1 = p(i) rows, but the left hand side has p(i)-2+1 = p(i)-1 rows . This is a mismatch: you would be trying to store 4 rows of data into a location that only holds three rows.

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