How do I assign 3D variables when third dimension has size one?

Is this a bug? If not, how am I supposed to code this so dd1 has shape [2 3 1]?

>> sd1 = reshape(1:12,[4,3,1])
sd1 =
     1     5     9
     2     6    10
     3     7    11
     4     8    12
>> sd2 = reshape(1:24,[4,3,2])
sd2(:,:,1) =
     1     5     9
     2     6    10
     3     7    11
     4     8    12
sd2(:,:,2) =
    13    17    21
    14    18    22
    15    19    23
    16    20    24
>> for k = 1:2, dd1(k,:,:) = sd1(2*k,:,:); end
>> for k = 1:2, dd2(k,:,:) = sd2(2*k,:,:); end
>> size(dd1)
ans =
     2     1     3
>> size(dd2)
ans =
     2     3     2

[Matt J]

I'm assuming dd1 was not pre-allocated prior to your for-loop.

Answer 1

[Matt J]

dd1=sd1(2:2:end,:,:)

[Matt J]

Thomas commented:

that is certainly one way around the issue. Still not sure whether it is a bug.

[Matt J]

I doubt it's a bug. If dd1 and dd2 are not pre-defined, then note that your for-loop results are consistent with the "shiftdim rule" that I describe in your other thread.

The bottom line - it's just one more reason why its dangerous to define or modify the size/shape of an array through assignment.

[Thomas]

Yes, your answer there (I had forgotten about that) seems to fully answer both questions. Combining the two we get the interesting MATLAB koan

>> it113 = rand(1,1,3)

it113(:,:,1) =
    0.9572
it113(:,:,2) =
      0.4854
it113(:,:,3) =
    0.8003

>> vom113(1,:,:) = it

vom113 =
    0.2785    0.5469    0.9575

>> it131 = rand(1,3,1)

it131 =
    0.1419    0.4218    0.9157

>> vom131(1,:,:) = it131

vom131(:,:,1) =
    0.1419
vom131(:,:,2) =
    0.4218
vom131(:,:,3) =
    0.9157