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

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Andrew Stark on 6 Nov 2020
Commented: Andrew Stark on 6 Nov 2020
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.
Andrew Stark on 6 Nov 2020
Currently it is a temporary variable but eventually I will use Z later! Thank you so much for your help!

Sindar on 6 Nov 2020
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));
% 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 on 6 Nov 2020
That works great! Thank you! I guess the last value/index in the for loop was causing the problem

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