How to efficiently replace value at the n last column?

Question

balandong 2018-9-4

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/417392-how-to-efficiently-replace-value-at-the-n-last-column

评论： balandong 2018-9-4

Dear all, The objective was to replace the value at the end of a column. I have make a simple case but wonder if there is another way to make it more compact and efficient?

Thanks in advance

f_sbj=2
sA=23;
A =1:sA;
appndOut=cell(sA,1);
for f_x=1:sA
Out = nchoosek(A,  f_x);
[rows, columns] = find(Out==f_sbj);
Out(rows, :) = [];
[m,n] = size(Out );
newNan=nan(m,(sA-n));
appndOut{f_x}=[newNan Out];
end

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Walter Roberson 2018-9-4

That code appears to insert at the beginning of columns, not replace at the end of columns.

The code can be made more compact, but not necessarily more efficient.

balandong 2018-9-4

Thanks for the quick response, Appreciate if you can some idea to make it more compact

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Answer 1

Walter Roberson 2018-9-4

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/417392-how-to-efficiently-replace-value-at-the-n-last-column#answer_335350

在 MATLAB Online 中打开

lastNcol = @(M, N) M(:,end-N+1:end);
NanPrePad = @(M, N) lastNcol( [nan(size(M,1), N), M], N);
select_and_prepad = @(M, N) NanPrePad( M(all(M~=f_sbj, 2),:) );
appndOut = arrayfun(@(f_x) select_and_prepad(nchoosek(A, f_x), sA), (1:sA).', 'uniform', 0);

You can use shorter names for the anonymous functions if you find the above to not be compact enough.

Though I wonder why you do not do something like

A = setdiff(1:sA, f_sbj);

That would eliminate the possibility of f_sbj being generated by the nchoosek, so it would not be necessary to delete any rows containing f_sbj.

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Walter Roberson 2018-9-4

在 MATLAB Online 中打开

f_sbj=2
sA=23;
A =1:sA;
lastNcol = @(M, N) M(:,end-N+1:end);
NanPrePad = @(M, N) lastNcol( [nan(size(M,1), N), M], N);
select_and_prepad = @(M, N) NanPrePad( M(all(M~=f_sbj, 2),:), N );
appndOut = arrayfun(@(f_x) select_and_prepad(nchoosek(A, f_x), sA), (1:sA).', 'uniform', 0);

This takes a moment to execute.

balandong 2018-9-4