How do I do a bi-directional look up in Chapel for dictionary-like data?

Question

In Python, if I have a list I can find the index. This allows me to keep running IDs as I add things.

> things = []
> things.append("spinach")
> things.append("carrots")
> things.index("carrots")
1

So give a vegetable (or tuber) I can find an ID for it. Given an ID, I can find a vegetable (or tuber) to match.

What is the equivalent pattern in Chapel for an unknown number of objects and being able to reference from the name or the id?

If `things` will be large, you really should use a dictionary to do the reverse mapping. — kindall, Oct 06 '17 at 23:07

score 2 · Accepted Answer · answered Oct 08 '17 at 04:02

You can use push_back and find with 1D rectangular arrays:

var A : [1..0] string;
A.push_back("spinach");
A.push_back("carrots");
const (found, idx) = A.find("carrots");
if found then writeln("Found at: ", idx);
// Found at: 2

Note that find does a linear search, so as @kindall mentioned a dictionary is probably the better choice. In Chapel, that means an associative domain/array:

var thingsDom : domain(string);
var things : [thingsDom] int;
var idxToThing : [1..0] string;
// ...
// add a thing
idxToThing.push_back(something);
const newIdx = idxToThing.domain.last;
thingsDom.add(something);
things[something] = newIdx;

assert(idxToThing[things[something]] == something);

Two associative arrays would be better if the indices are not in a dense range.

How do I do a bi-directional look up in Chapel for dictionary-like data?

1 Answers1