I have a multidimensional array - in this case with 4 dimensions of x,y,z
and time.  I'd like to take the time derivative of this matrix, i.e.
perform the diff operator along dimension number 4.

In Matlab, there is a simple option to specify which dimension the
difference is to be taken across., but I can't see this in R.

I realise I can use aperm to rotate my array so that time is in the 1st
dimension, then do the difference, but this seems a little messy.

Surely I can't be the first person wanting to do this, but I can't find any
answers after doing a (what I thought was thorough) search.

Thanks in advance,

Phil.

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