arraydim() -- returns the dimension of the array, which is passed as an array reference

a=arraydim(b())

If you apply the arraydim()-function on a one-dimensional array (i.e. a vector) it will return 1, on a two-dimensional array (i.e. a matrix) it will return 2, and so on.
This is mostly used within subroutines, which expect an array among their parameters. Such subroutines tend to use the arraydim-funtion to check, if the array which has been passed, has the right dimension. E.g. a subroutine to multiply two matrices may want to check, if it really is invoked with two 2-dimensional arrays.

dim a(10,10),b(10) 
print arraydim(a()),arraydim(b()) 

This will print 2 1, which are the dimension of the arrays a() and b(). You may check out the function arraysize for a full-fledged example.