[R] Difference across the Nth dimension of an array

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.,

[R] OPeNDAP access with R

I am in the process of converting from Matlab to R, but a major sticking point for me at the moment is accessing data via OPeNDAP. After several hours of searching, I found that other people have discussed this functionality in the past, but it does not appear to have yet been developed. One