How to set value in a matrix

52 次查看（过去 30 天）

显示更早的评论

Shannon 2020-2-5

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/503907-how-to-set-value-in-a-matrix

评论： Adam Danz 2020-2-6

采纳的回答： Adam Danz

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

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Adam Danz 2020-2-5

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/503907-how-to-set-value-in-a-matrix#answer_414017

在 MATLAB Online 中打开

for i = 1:numel(B)
    A(1:B(i),i) = 0; 
end

4 个评论
显示 2更早的评论隐藏 2更早的评论

Rik 2020-2-5

You're welcome

Adam Danz 2020-2-5

编辑：Adam Danz 2020-2-5

在 MATLAB Online 中打开

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

请先登录，再进行评论。

更多回答（1 个）

Rik 2020-2-5

3
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/503907-how-to-set-value-in-a-matrix#answer_414019

在 MATLAB Online 中打开

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