Indexing multidimensional matrices using logical arrays

2013 12 17
3 个回答
回答已采纳
更新时间：2013 12 17
3 次查看（30 天）

0 个投票

I want to efficiently find number in a multidimensional array that fall into
some category and do something to them,like in the code below:
nc=4;
mc=1000;
r1=normrnd(0,1,[nc,mc,n-1]);
for j=2:n
indexNeg(:,:)=r1(:,:,j)<0.5;
r1(:,:,j)=-r1(indexNeg,j);
end
here, I create a multidimensional array of random numbers and change sign of any number smaller than 0.5
I am getting this error Subscript indices must either be real positive integers or logicals.
How to properly use the logical index?

请先登录，再回答此问题。

 采纳的回答

Jos (10584)
Jos (10584) 2013-12-17

0 个投票

A statement like TF = A < 0.5 produces a logical array TF that has the same size as A. You can directly use TF to index into A. An example
A = rand(3,2,4)
TF = A < 0.5 ;
size(A)
size(TF)
A(TF) = -A(TF)
The number of dimensions of A is not affecting this at all.

更多回答（2 个）

Andrei Bobrov
Andrei Bobrov 2013-12-17
编辑：Andrei Bobrov 2013-12-17

0 个投票

l = r1 < .5;
r1(l) = -r1(l);
On comment by Marco
r1 = abs(r1);

2 个评论
显示 无 隐藏 无

Marco Pereira
Marco Pereira 2013-12-17
编辑：Marco Pereira 2013-12-17
In analogy to your answer
nc=4;
mc=1000;
n=100;
r1=normrnd(0,1,[nc,mc,n-1]);
l=r1(:,:,2)<0;
If I print r1(l) here, not all numbers are negative so the next line wouldn't do what I want done.
r1(l)=-r1(l);

请先登录，再进行评论。

Marco Pereira
Marco Pereira 2013-12-17

0 个投票

the problem is that I am looping through one of the dimensions and making the comparison.
l=r1(:,:,2)<0;
In this line I am comparing only the column where k=2 (in an r1(i,j,k) array). I could always to a loop but that would be inefficient.
This makes that the logical array to be one dimension smaller than the original array. The logical array needs to be concatenated with the integer corresponding to the missing dimension.
I am trying to learn best practices (the vectorized solution to a multidimensional array comparison problem) where the comparison is done is a section of the array at a time.

类别

在 帮助中心 和 File Exchange 中查找有关 Matrices and Arrays 的更多信息

标签

Community Treasure Hunt

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

Start Hunting!

Translated by