How to set value in a matrix

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Shannon
Shannon 2020-2-5
评论： Adam Danz 2020-2-6
I have a double matrix 4x5 matrix
A=[11 12 13 14 15;
21 22 23 24 25;
31 32 33 34 35;
41 42 43 44 45],
B=[3 0 2 4 1]
B indicates to set all the rows of B[i] in A(from row 1 to that position) to 0.
C = [0 12 0 0 0;
0 22 0 0 25;
0 32 33 0 35;
41 42 43 0 45]
Thanks

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采纳的回答

Adam Danz
Adam Danz 2020-2-5
for i = 1:numel(B)
A(1:B(i),i) = 0;
end
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Adam Danz
Adam Danz 2020-2-5
编辑：Adam Danz 2020-2-5
Thanks for the timing comparisons, Rik!
One way around the limitations to timeit is by increasing the size of the arrays. With the large array inputs below, both of your solutions are faster.
A = randi(10000,1000);
B=randi(10000,1,1000)-1;
ans =
0.18254 % fun1 (my loop)
ans =
0.026578 % fun2
ans =
0.026723 % funct_compact

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更多回答（1 个）

Rik
Rik 2020-2-5
This was a fun puzzle. I believe the code below is one of the many possible solutions. It does use implicit expansion, so for pre-R2016b you will have to use bsxfun.
A=[11 12 13 14 15;
21 22 23 24 25;
31 32 33 34 35;
41 42 43 44 45];
B=[3 0 2 4 1];
a=-(1:size(A,1))';
L= a+B(:)' >=0; %the (:)' is to ensure B is a row vector
C=A;C(L)=0
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Rik
Rik 2020-2-6
Thank you. While I like the one-linerness of the compact version, I think it looks like voodoo witchcraft, so I decided to post this as the answer
Adam Danz
Adam Danz 2020-2-6
Ha! I know what you mean....

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