Nim has OrderedTable how can I get the element by its index, and not by its key?
If it's possible - is this an efficient operation, like O(log n) or better?
import tables
let map = {"a": 1, "b": 2, "c": 3}.toOrderedTable
Something like
map.getByIndex(1) # No such method
P.S.
I'm currently using both seq and Table to provide both key and indexed access and wonder if it could be replaced by OrderedTable
type IndexedMap = ref object
list*: seq[float]
map*: Table[string, float]
There is no direct index access to ordered tables because of their internal structure. The typical way to access the elements in order is:
import tables
let map = {"a": 1, "b": 2, "c": 3}.toOrderedTable
for f in map.keys:
echo $f
Basically, accessing the keys iterator. If you click through the source link in the documentation, you reach the actual iterator code:
let L = len(t)
forAllOrderedPairs:
yield t.data[h].key
And if you follow the implementation of the forAllOrderedPairs template (it's recommended you are using an editor with jump to implementation capabilities to inspect such code easier):
if t.counter > 0:
var h = t.first
while h >= 0:
var nxt = t.data[h].next
if isFilled(t.data[h].hcode):
yieldStmt
h = nxt
No idea about performance there, but it won't be as fast as accessing a simple list/array, because the internal structure of OrderedTable contains a hidden data field with the actual keys and values, and it requires an extra conditional check to verify that the entry is actually being used. This implementation detail is probably a compromise to avoid reshuffling the whole list after a single item deletion.
If your accesses are infrequent, using the iterator to find the value might be enough. If benchmarking shows it's a bottleneck you could try freezing the keys/values iterator into a local list and use that instead, as long as you don't want to mutate further the OrderedTable.
Or return to your original idea of keeping a separate list.
