A null assignment can have only one non-colon index

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

Answer 1

[KSSV]

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

[Chathurika Jayasundara]

Hi Thank you for your kind reply. It works.

[Anmol Pardeshi]

but assigning a NaN replaces the cell with a <missing> because the output matrix is a string.

[Declan O'Callaghan]

I found that using NaN was not the same as removing an element. How do we remove elements?

[Jon Aztiria]

B(r,n)=[];

[Amy]

That's the same code that was giving them an error before tho?

[Walter Roberson]

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?