How can I fix "Index in position 2 exceeds array bounds" in for loop/ if statements?

W=cell(p,1);
for j=1:(p-1);
    w=A(index(j):index(j+1)-2);
    W{j}=w;
end
W2=cell(p,1);
for j=1:(p-1);
    w2=A(index(j):index(j+1)-2);
    W2{j}=w2;
end
Z=cell(p,1);
for ii=1:p
    P1=W{ii,1};
    Z{ii,1}=P1(3:3:end);
end
for ii=1:p
    P2=Z{ii,1};
    if P2(end,1)>0
        W2(ii,1)={[]};
    end
end

I am getting the error "Index in position 2 exceeds array bounds" on the line if P2(end,1)>0

The W2 matrix looks how I want it to look I just dont know how to stop it from producing an error.

[Sindar]

try debug mode

dbstop if error

and figure out whether everything has the sizes you expect. It looks like P2 is empty, but it's not obvious where that happened

[Sindar]

Specifically, it would be useful to know the following at the time of the error:

• the value of ii
• the size of P2 (my guess is that it is an empty array)
• the size of W{ii,1}

Extra question: do you use Z for anything later, or is it purely a temporary variable in this chunk?

[Andrew Stark]

at the time of error

ii=10,000 (final p value)

P2 is an empty array

W2 is a 10000x1 cell matrix

[Andrew Stark]

Currently it is a temporary variable but eventually I will use Z later! Thank you so much for your help!

Answer 1

[Sindar]

Okay, I believe this code is equivalent to what you have, cleaner and without the error

% pre-allocate with cell arrays of []
W=cell(p,1);
W2=cell(p,1);
Z=cell(p,1);
for ii=1:p
    % for all but the last index,
    % load in part of A
    if ii<p
        P1=A(index(ii):(index(ii+1)-2));
    % for the last one, not loading anything leaves it empty
    % but, we want P1 defined
    else
        P1=[];
    end
    W{ii}=P1;
    % be explicit about copying W
    % there may also be internal Matlab tricks that save some memory with this way
    W2(ii)=W(ii);
    Z{ii}=P1(3:3:end);
    if ~isempty(P1) && Z{ii}(end)>0
        W2(ii)={[]};
    end
end

A few tricks:

• instead of separate for loops, combine into one
• in the last, extra iteration for W1/W2, explicitly leave the cell empty (you may want something else here)
• avoid packing and unpacking the same cell data
• you can index into a cell of Z, avoiding the P2 temporary variable
• isempty check avoids the original error. In this case, the cell will already be empty, anyway

[Andrew Stark]

That works great! Thank you! I guess the last value/index in the for loop was causing the problem