# I need help with permutations.

조회 수: 2 (최근 30일)
Muhendisleksi 2017년 5월 5일
댓글: Andrei Bobrov 2017년 5월 5일
for example: There are matrix m and r:
m = [1.2000 2.4000 1.3000
2.3000 1.5000 1.0000];
r = [0.2000 0.4000 0.3000];
rr = triu(ones(3),1);
rr(rr > 0) = r;
mm = bsxfun(@times,permute(m,[2,3,1]),permute(m,[3,2,1]));
K = bsxfun(@times,mm,rr + rr.' + eye(3));
This code gives me the result I want.
The values of the matrix m can vary. for example:
m = [1.2000 2.4000 1.3000
2.3000 1.5000 1.0000
1.2000 2.4000 3.0000];
r = [0.2000 0.4000 0.3000 0.4444];
So how can I write code like "[row col] = size (m)"?
I mean;
rr = triu(ones(row),1);
rr(rr > 0) = r;
mm = bsxfun(@times,permute(m,[?,?,?]),permute(m,[?,?,?]));
K = bsxfun(@times,mm,rr + rr.' + eye(row));
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Muhendisleksi 2017년 5월 5일
Andrei Bobrov 2017년 5월 5일
" m = [1.2000 2.4000 1.3000
2.3000 1.5000 1.0000
1.2000 2.4000 3.0000];
r = [0.2000 0.4000 0.3000 0.4444]; "
What result do you expect, what is the expression for K ?

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### 답변 (1개)

dpb 2017년 5월 5일
편집: dpb 2017년 5월 5일
What's wrong with
[r,c]=size(m);
mm = bsxfun(@times,permute(m,[r,c,1]),permute(m,[c,r,1]));
Which, if you haven't recognized it, can be written as
permute(m,[size(m),1]),permute(m,[flip(size(m)),1])
ERRATUM
As Guillaume astutely points out, the permute indices are NOT size(m)-dependent, but reflect the dimensionality of m as 2D array. The above, while cute, is nonsensical for the purpose.
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Guillaume 2017년 5월 5일
No idea what the original purpose of the code is, and I'm not going to try to decrypt some obscure image with no associated explanation, but it's clear to me that in the original code, the dimensions to permute do not depend on the size of m. It's always going to be [2 3 1] and [3 2 1] respectively.
The only thing that would need to change in the original code if the size of m changes is the size of rr, with
rr = triu(ones(size(m, 2)));
You would of course need more elements in r to accodomate the change in size of rr.
dpb 2017년 5월 5일
Good catch, Guillaume! The permutations are not size-dependent but dimensional.
I didn't really try to read the image, either, and while the above will respond to OP's plea, it isn't useful. I'll scratch it altho I was kinda' proud of the flip(size()) thingie; I'd not thunk a' it before and can recall at least one instance could have used it to eliminate a temporary...

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