How to accelerate the process of `find` in large for loop ?

3 次查看（过去 30 天）
wei zhang
wei zhang 2020-7-22
评论： wei zhang 2020-7-23
I have two arrays A,B. I want to find the index of every element of B in A. I'm sure that every element of B must appears in A twice. The code of mini case is as below.
A = [1 2 3 4 5 6 5 4 2 7];
B = [2 4 5];
result = zeros(length(B),2);
for i=1:length(B)
result(i,:) = find(A==B(i));
end
% result should be [2,9;4,8;5,7]
The A and B in practice is very large (8000000). It takes long time in this for loop way. Is there any way to improve the code?
I had tried to use ismember function. But it seems I could not bypass the find function.
[ia,ib] = ismember(A,B);
result = zeros(length(B),2);
for i=1:length(B)
result(i,:) = find(ib==i);
end
I know I could use parallel computing toolbox. I just want a more delicated coding way.

请先登录，再回答此问题。

采纳的回答

Stephen23
Stephen23 2020-7-22
>> [X,Y] = ismember(A,B);
>> [~,Z] = sort(Y(X));
>> V = find(X);
>> R = reshape(V(Z),2,[]).'
R =
2 9
4 8
5 7
  1 个评论
wei zhang
wei zhang 2020-7-23
How could you understand the appilication of these functions, sort ismember reshape unique, so well?! Your answer is so good! Even I know the meaning of the above functions, I still can't use them properly, for several years!

请先登录，再进行评论。

更多回答（1 个）

Mehmed Saad
Mehmed Saad 2020-7-22
Try the following code
A = [1 2 3 4 5 6 5 4 2 7];
B = [2 4 5];
[r,c] = find(A-B'==0);
[~,ic] = sort(A(c));
c(ic)
  1 个评论
wei zhang
wei zhang 2020-7-23
Because my size of A and B are very large, the matrix A-B' is out of memory very severely, unfortunately.

请先登录，再进行评论。

类别

在 Help Center 和 File Exchange 中查找有关 Loops and Conditional Statements 的更多信息

标签

产品


版本

R2019a

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by