Generating a position lookup function for an arbitrary list of integers

In my code, I generate a list of integers called numlist. Here is an example numlist:

numlist = Sort[Table[RandomInteger[{1, 100}], {3}]]

which gives, for example, the output:

{17, 74, 96}

I would like to write a function generateFun that, using numlist as input, generates the following function myfun:

myfun[int_Integer] := Which[
int == 17, 1,
int == 74, 2,
int == 96, 3
]

In other words, myfun takes an integer and returns the position of that integer in numlist. numlist is created at the beginning of the code and does not change during a given execution; however, a new execution will in general create a new list of integers numlist. The length of numlist varies (it is not guaranteed to be 3), although this fact is not present in the minimal working example that I gave above.

I could bypass generateFun and simply use this function myfun2:

myfun2[int_Integer, numlist_] := First[First[Position[numlist, int]]]

which returns the position of int_ in numlist_. However, since I will need to call the function (myfun or myfun2) many times (~1,000,000 times), I think that calling Position every time may be quite expensive -- since numlist, once generated, does not change during the execution.

Do you have any advice on how I can generate the Which expression automatically from numlist?

You really need a hash-table. Your method with Which statement won't scale to a large number of elements in a list, since Which does a linear look-up. Here are some possibilities.

DownValues and SubValues

Create one via DownValues, like @belisarius demonstrated in his answer.

Dispatch-ed replace rules

Another way (a little less stateful, since rules won't be attached to a symbol) is to use Dispatch-ed rules, like so:

lst = {2, 0, 11, 9, 20, 1, 16}

rules = Dispatch@Thread[# -> Range[Length[#]]] &[lst]

Then, for example,

11 /. rules

(*  3  *)

SystemUtilitiesHashTable

Use SystemUtilitiesHashTable, like so:

h = SystemUtilitiesHashTable[];
MapThread[SystemUtilitiesHashTableAdd[h, ##] &, {lst, Range[Length[lst]]}];

where now you can extract the position for a given element as

SystemUtilitiesHashTableGet[h,11]

(* 3 *)

More details on the use of this hash table can be found in this excellent answer by Oleksandr.

Using (sparse) arrays

Sometimes you may simply use arrays, or possibly sparse arrays, as a hash table. This is particularly useful for integer lists where integer values are not too large (although sparse arrays will handle also large values). I will demonstrate this by defining a new data type storage, like so:

Clear[makeStorage, storage];
makeStorage[lst_, $threshold_: 10^5] := With[{len = Max[lst] - Min[lst] + 1}, Module[{arr = If[len <$threshold,
Table[0, {len}],
SparseArray[{0}, {len}, 0]
]},
arr[[lst - Min[lst] + 1]] = Range[Length[lst]];
storage[arr, Min[lst]]]];

storage /: getPosition[storage[arr_, min_], elem_] := arr[[elem - min + 1]];

Here is how you use it:

st = makeStorage[lst];

getPosition[st,11]

(* 3 *)

The advantage of this method is that when plain (non-sparse) arrays can be used,this will be the fastest one. The disadvantage is that using plain arrays is very memory-wasteful, so you trade memory for speed. You may wish to also define a bulk element extractor method

storage /: getPositions[storage[arr_, min_], elems_List] :=
arr[[elems - min + 1]];

since this will be very efficient. You can see this approach put to practical use in this answer, where you can also see how much speed-up one can get by using it. I also put it (in a slightly different form) to use in this answer, where it was inside Compile.

I think the most straightforward way is:

MapIndexed[(f[#1] = #2[]) &, numlist]

{#, f@#} & /@ numlist // TableForm
(*
7   1
81  2
96  3
*)

Edit

If you need to do this for an unknown number of lists and functions in your program, you could go:

defFun[g_Symbol, l_List]:= MapIndexed[(g[#1] = #2[]) &, l]

Usage

ClearAll@h;
defFun[h, k = Table[Fibonacci@i, {i, 2, 10}]]
{#, h@#} & /@ k // TableForm
(*
1   1
2   2
3   3
5   4
8   5
13  6
21  7
34  8
55  9
*)
• Just a warning. If your list contains duplicates, only the second one will get assigned. Example: if your list is {1,2,1}, then f == 3 – Dr. belisarius Sep 14 '12 at 17:46

Nowadays, one can just build an Association[]. Using Leonid's example:

assoc = Association[MapIndexed[# -> #2[] &, {2, 0, 11, 9, 20, 1, 16}]];

or as Carl notes, you can use PositionIndex[] to build the required association:

assoc = First /@ PositionIndex[{2, 0, 11, 9, 20, 1, 16}];

assoc
3

If you need to get the values for two or more keys, use Lookup[]:

Lookup[assoc, {11, 9}]
{3, 4}
• You can use PositionIndex to create the association. – Carl Woll Mar 25 '18 at 1:40
• Thanks @Carl, I've edited. – J. M. will be back soon Mar 25 '18 at 1:50

You can also use Nearest or Pick:

nf = Nearest[lst -> "Index"];
nf [ {11, 9, 3} , {1,0}]

{{3}, {4}, {}}

To get both the index and the element:

nf2 = Nearest[lst -> {"Element", "Index"}];
nf2[ {11, 9, 3} , {1,0}]

{{{11, 3}}, {{9, 4}}, {}}

Pick[Range @ Length @ lst, lst, #] & /@ {11,9,3}

{{3}, {4}, {}}