Lookup between two lists

Question

I have several lists that I would like to match and operate values among them. For simplicity assume I have these two lists:

l1 = Transpose[{Range[1980, 2010, 1], Range[31]}]
l2 = {{1990, 1,100}, {2000, 2,200}, {2010, 3,300}}

I would like to divide the third column of l2 by the second column in l1 matching the year in both lists.

I should obtain:

l3 = {{1990, 1, 100/11}, {2000, 2, 200/21}, {2010, 3, 300/31}}

How can I set a function to do that?

Henrik Schumacher · Accepted Answer · 2018-06-07 18:05:32Z

5

Lookup says it all... ;)

lookuptable = AssociationThread[l1[[All, 1]], l1[[All, 2]]];
l3 = l2;
l3[[All, 3]] /= Lookup[lookuptable, l2[[All, 1]]];
l3

{{1990, 1, 100/11}, {2000, 2, 200/21}, {2010, 3, 300/31}}

answered Jun 7 '18 at 18:05

Henrik Schumacher

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$\begingroup$ Thanks Henrik. Gracias Tocayo! worked fine. $\endgroup$ – Enrique Vargas Jun 7 '18 at 18:21
$\begingroup$ I'm always glad to be of help! You're welcome! $\endgroup$ – Henrik Schumacher Jun 7 '18 at 19:14
2

$\begingroup$ For brevity, likely at the cost of some performance, you can eliminate AssociationThread and use simply Lookup[Rule @@@ l1, l2[[All, 1]]] $\endgroup$ – Mr.Wizard♦ Jun 8 '18 at 6:44

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user1066 · Accepted Answer · 2018-06-08 16:31:16Z

1

Cases[GatherBy[Join[l1, l2], First], {x_, y_} :> {x[[1]], y[[2]], y[[3]]/x[[2]]}]

{{1990, 1, 100/11}, {2000, 2, 200/21}, {2010, 3, 300/31}}

answered Jun 8 '18 at 16:31

user1066

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2 Answers 2