Lookup in an association when the keys are lists?

Questions
1

Hi everyone,

I have an Association in which the keys are lists. The first key might be

{20, a, b}

and the second key might be

{47, e, g}

and so on. How can I lookup or extract all values for which the first element in each key is, say, 20? I guess I’m looking for the Association equivalent of

Select[myTable,  #[[1]] == 20 &]

or

GatherBy[myTable, #[[1]]&]

Regards,

Gregory

2

You might be able to accomplish this with the new KeyValuePattern object in Mathematica 10.4. I’ve tried, though, and can’t seem to figure out what function should then be used to actually apply the pattern to select matching key->value pairs from an association. I crafted a test association that is similar to yours, with keys that are lists, and was able to get MatchQ working with a KeyValuePattern such as:

MatchQ[testAssoc, KeyValuePattern[{{20, _, _} -> _}]]

That returned True, as it should, but I couldn’t figure out how to actually return an association of key->value pairs that match the pattern.

3

One simple approach might be to do key-selection outside of the Association framework like so:

testAssoc /@ Cases[Keys[testAssoc], {20, _, _}]

or likewise:

testAssoc[[Key /@ Cases[Keys[testAssoc], {20, _, _}]]]

Regards – Henrik

4

Hi Christopher and Henrik,
Thanks for your tips. They were a big help.
Gregory