On Tuesday 10 May 2011 09:51:21, Øystein Kolsrud wrote:
Hi! What's the complexity of indexing arrays in Haskell? The Data.Array documentation doesn't seem to mention complexity of any of its functions, and it's not obvious to me how to implement an array data structure with indexing being O(1).
For the array types provided by the array package (Array, UArray, STArray, STUArray, IOArray, IOUArray), indexing is O(1) [for reasonable index types¹]. Implementing such a data structure with O(1) indexing of course requires to go low-level, for the payload, you need a contiguous chunk of memory where either the pointers to the data are stored (boxed arrays) or the raw data themselves (unboxed arrays). Then indexing is "read size bytes from address (start + index*size)", just like in C or similar. Of course there's bounds- checking and computing the effective index from the programmer-visible, but for reasonable index types, that is O(1) too. ¹ You could of course make an Ix instance for e.g. lazy Peano numbers, then indexing would become O(index).