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

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Andrew Stark 2020-11-6

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/638035-how-can-i-fix-index-in-position-2-exceeds-array-bounds-in-for-loop-if-statements

评论： Andrew Stark 2020-11-6

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.

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Andrew Stark 2020-11-6

at the time of error

ii=10,000 (final p value)

P2 is an empty array

W2 is a 10000x1 cell matrix

Andrew Stark 2020-11-6

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

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Answer 1

Sindar 2020-11-6

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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