A null assignment can have only one non-colon index

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Chathurika Jayasundara
评论： Walter Roberson 2025-9-21，20:19
B=zeros(23,23)
for r=1:23;
for n=1:23;
if r==n
B(r,n)=A(r,n);
else
B(r,n)=[];
end
end
end
A is a symmetric 23*23 matrix and I want to remove all the other values except r=n. It gives me an "A null assignment can have only one non-colon index" error

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KSSV
KSSV 2018-9-11
B=zeros(23,23)
for r=1:23;
for n=1:23;
if r==n
B(r,n)=A(r,n);
else
B(r,n)=NaN;
end
end
end
  6 个评论
显示 4更早的评论
Amy
Amy 2025-9-21，18:09
That's the same code that was giving them an error before tho?
Walter Roberson
Walter Roberson 2025-9-21，20:19
B(r,n)=[]; attempts to delete a single element from the middle of an array. There is no way to leave a "hole" in the middle of an array, so either the request would have to be denied, or else the array would have to be reshaped as a single column with the one element left out...
like
Bnew = B(:);
Bnew(sub2ind(size(B),r,n)) = [];
B = Bnew;
MATLAB chooses to refuse the deletion request.
What is your expectation for B(r,n) = []; ? Are you expecting to get back an array with a hole in it? Are you expecting to get back an array that has the same leading rows as B before n but then has a "shorter" column n missing element #r, followed by trailing rows that are the full size? Are you expecting to get back a rectangular array that is missing the bottom right corner, with the other elements "flowing" into the empty space?

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