# Rubber indices

The constants .. and … (type \dots and hit the tab key) can be used in array indexation to left or right justify the other indices. For instance, assuming A is a 3×4×5×6 array, then all the following equalities hold:

A[..]          === A # the two are the same object
A[..,3]         == A[:,:,:,3]
A[2,..]         == A[2,:,:,:]
A[..,2:4,5]     == A[:,:,2:4,5]
A[:,2:3,..]     == A[:,2:3,:,:]
A[2:3,..,1,2:4] == A[2:3,:,1,2:4]

As you can see, the advantage of the rubber index .. is that it automatically expands as the number of colons needed to have the correct number of indices. The expressions are also more readable. The idea comes from the Yorick language by Dave Munro. Similar notation exists in NumPy.

The rubber index may also be used for setting values. For instance:

A[..] .= 1         # to fill A with ones
A[..,3] = A[..,2]  # to copy A[:,:,:,2] in A[:,:,:,3]
A[..,3] .= A[..,2] # idem but faster
A[2,..] = A[3,..]  # to copy A[3,:,:,:] in A[2,:,:,:]
A[..,2:4,5] .= 7   # to set all elements in A[:,:,2:4,5] to 7

Leading/trailing indices may be specified as Cartesian indices (of type CartesianIndex).

Technically, the constant .. is defined as RubberIndex() where RubberIndex is the singleron type that represents any number of indices.

Call colons(n) if you need a n-tuple of colons :. When n is known at compile time, it is faster to call colons(Val(n)).