Add a known column to a matrix 'n' times without a loop?

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ken 2011-5-22

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Hi Guys!

Is there a way of qriting the script below without the loop? I'm sure there's a way but I can't find it in the help section or the internet.

I want to add a known column of numbers to a matrix 'n' times.

Thank you!

Ken

   clear all
  clc
a=zeros(5,6); % matrix a
[m,n]=size(a);
b=zeros(m,n); % matrix b
b(1:10)=[8 8 8 8 8 10 10 10 10 10];
for i=3:n;
      b(:,i)=[1 2 3 4 5];
end

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Walter Roberson 2011-5-22

These days "for loops" are not necessarily slower, and in some cases are faster.

ken 2011-5-22

Thanks for letting me know Walter

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采纳的回答

Andrei Bobrov 2011-5-22

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https://ww2.mathworks.cn/matlabcentral/answers/7975-add-a-known-column-to-a-matrix-n-times-without-a-loop#answer_10961

在 MATLAB Online 中打开

b(:,k:n) = (1:5)'*ones(1,n-k+1);

V = 1:5;
b(:,k:n) = V(ones(1,n-k+1),:).';

more more

b(:,k:n) = repmat(1:5,n-k+1,1).';

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Jan 2011-5-23

@Andrei: The ONES method is applied inside REPMAT also. Therefore I expect using it directly is faster, because the overhead of calling a function is avoided.

Usually a matrix with repeated rows wastes memory, because of the redundancy. Therefore a BSXFUN method can be faster: Do not create the matrix explicitely, but instruct BSXFUN to perform an operation by "inflating" the vector dynamically.