What does permute actually produce?

Question

David H 2014-11-20

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https://ww2.mathworks.cn/matlabcentral/answers/163567-what-does-permute-actually-produce

编辑： David H 2014-11-21

Trying some calculations earlier, I have realised permute does not do what I thought it did. To clarify I did this

    % code
test = rand(4,4,4,4) + rand(4,4,4,4)*1i;
test2a = zeros(4,4,4,4); test2b = zeros(4,4,4,4); 
for a = 1:4
    for b = 1:4
        for c = 1:4
            for d = 1:4
test2a(a,b,c,d) = test(a,b,d,c);
test2b(a,b,c,d) = test(b,a,c,d); 
            end
        end
    end
end
test3a = permute(test,[1,2,4,3]);
test3b = permute(test,[2,1,3,4]);
m1=max(test2a(:)-test3a(:))
m2=max(test2b(:)-test3b(:))
test2a = zeros(4,4,4,4); test2b = zeros(4,4,4,4); 
for a = 1:4
    for b = 1:4
        for c = 1:4
            for d = 1:4
test2a(a,b,c,d) = test(d,b,a,c);
test2b(a,b,c,d) = test(c,a,b,d); 
            end
        end
    end
end
test3a = permute(test,[4,2,1,3]); 
test3b = permute(test,[3,1,2,4]);
m3=max(test2a(:)-test3a(:))
m4=max(test2b(:)-test3b(:))
m1 =
     0

m2 =

0

m3 =

0.9525 + 0.5183i

m4 =

0.9293 - 0.7266i

clearly the first is the same but the latter is different. Is there some way to create the second variables (of form test2(a,b,c,d) == test(d,b,a,c) which involve two permutations ) just using permute on test?

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David H 2014-11-20

I realise reading this again I haven't really stated what I want to do very clearly.

If I want some variable satisfying var2(a,b,c,d) == var1(d,b,a,c) for any a b c d (up to the size of the variable) how do I produce this variable efficiently from var1?

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