How can I add column titles into a Dataset?

Question

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]]

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

Answer 1

I think what you need is more like this:

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

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

Answer 2

One option could be

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

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.