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What I am trying to do is to add columns titles into my data set that has unit information.
First I create a list of countries:

Countries = {"UnitedStates", "Mexico", "Canada", "Brazil","Argentina", "Peru", "Ecuador", "CostaRica", "ElSalvador"}

Then I compute for each country the population using CountryData.

populations = CountryData[#, "Population"] & /@ Countries

Now I proceed to create and association and then then data set.

dat = Dataset[AssociationThread[Countries -> populations]]

enter image description here

But when I try to add names to columns using dat[All, <|"1" -> "Country", "2" -> "Population"|>] I get this error. enter image description here

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I think what you need is more like this:

dat=Dataset[MapThread[<|"Country" -> #1,"Population" -> #2|> &, {Countries, populations}]]

enter image description here

The structure you are looking for is one association per row, with the keys equal to the column headings.

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  • $\begingroup$ Yeah it worked nice. $\endgroup$
    – Las Des
    May 2 '20 at 20:37
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    $\begingroup$ A more natural way to do this is to use AssociationThread[]: Dataset[AssociationThread[{"Country", "Population"}, #] & /@ Transpose[{Countries, populations}]] $\endgroup$
    – J. M.'s torpor
    May 3 '20 at 10:59
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One option could be

 PrependTo[dat, <|"Country" -> "Population"|>]

Mathematica graphics

I do not know how the semantics of your data set will change now by adding the title right into the data itself like this. I think normally column titles are added at the end, for display purposes and not part of the actual dataset itself.

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  • $\begingroup$ Yes that correct, I am using this as an example. $\endgroup$
    – Las Des
    May 2 '20 at 19:40

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