Indexed Thread on dataset and list

Question

I currently have a dataset, and I would like to add a column based on another list.

For example:

ds = Dataset[{<|"name" -> "bob"|>, <|"name" -> "joe"|>}];
ages = {1,2};

I want to get

Dataset[{<|"name" -> "bob", "age"->1|>, <|"name" -> "joe", "age"->2|>}];

I was thinking of using MapIndex, but I don't really know how to make reform the results into a dataset again, and it seems wasteful.

How can I do this?

alancalvitti · Accepted Answer · 2015-02-05 18:07:27Z

6

ds[MapIndexed[Append[#1, "age" -> ages[[First@#2]]] &]]

{<|"name" -> "bob", "age" -> 1|>, <|"name" -> "joe", "age" -> 2|>}

Note 10.0.2 throws a warning - who knows what's going on in that private type system:

 First::normal: Nonatomic expression expected at position 1 in \
First[TypeSystem`ZSignatures`PackagePrivate`i]

answered Feb 5, 2015 at 4:55

alancalvitti

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$\begingroup$ Do you mind explaining how this works? What is the First function doing (I understand the rest) $\endgroup$
– soandos
Commented Feb 5, 2015 at 4:58
$\begingroup$ MapIndexed works on multidimensional arrays. MapIndexed[f, ages] gives {f[1, {1}], f[2, {2}]}. If ages were, say, a 2-dim array, then the indexes would be {i,j}. In this problem, First picks out the rows of ages. $\endgroup$
– alancalvitti
Commented Feb 5, 2015 at 5:03
$\begingroup$ ah, got it. Thank you. $\endgroup$
– soandos
Commented Feb 5, 2015 at 5:04
4

$\begingroup$ +1 We can dodge the warning message in 10.0.2 using something like ds[MapThread[Append[#, "age" -> #2 ] &, {#, ages}] &]. Messages like this are due to some serious evaluation leaks in the 10.0.2 type system code :( $\endgroup$
– WReach
Commented Feb 5, 2015 at 5:24
2

$\begingroup$ Are you a beta tester for 10.0.3 or is that a typo? $\endgroup$
– Mr.Wizard
Commented Feb 5, 2015 at 8:18

| Show 4 more comments

Alexei Boulbitch · Accepted Answer · 2015-02-05 08:35:52Z

2

Try this:

    Dataset@MapThread[
  Append, {Normal[ds], Association["age" -> #] & /@ ages}]

yielding the following:

enter image description here

Have fun!

answered Feb 5, 2015 at 8:35

Alexei Boulbitch

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