Marco, I believe that Michael is asking about a Version 10 Dataset

http://reference.wolfram.com/language/ref/Dataset.html

rather than a simple rectangular array of data.

Dear Mike,

let's generate a data set:

dataset = RandomReal[1, {10, 3}]

This is one way:

{#[[1]], #[[2]], #[[3]], #[[2]] + #[[3]]} & /@ dataset

This is another

Flatten[{#, #[[2]] + #[[3]]}] & /@ dataset

If you prefer procedural programming you can use

Table[Flatten[{dataset[[i]], dataset[[i, 2]] + dataset[[i, 3]]}], {i, 1, Length[dataset]}]

This one works, too

Transpose[Append[Transpose[dataset] , dataset[[All, 2]] + dataset[[All, 3]]]]

Cheers,

Marco

Thank you very much, David! when I see the solution it is really not too complicated - but I was not able to manage it by myself :-( . The documentation for Dataset in V10 is really not very instructive, so it is great to get a quick help here :-)

Greetings from Germany

Mike

Get a data set

In[16]:= Clear[daS1]
daS1 = Dataset[Table[<|"a" -> o, "b" -> RandomChoice[Characters["caitlin ramsey"]]|>, {o, 12}]]

and another one

In[24]:= Clear[daS2]
daS2 = Dataset[Table[<|"c" -> o, "d" -> Characters["caitlin"][[o]]|>, {o, StringLength["caitlin"]}]]

to create a third one using approaches mentioned in this discussion already

In[33]:= daS1 /. x_Association :> Join[x, <|{"Q" -> If[Intersection[{x[[2]]}, Normal[Query[All, "d"]@daS2]] != {}, True, False]}|>]

to point out a best way to do it one needs some criterion (what means good, what means bad) and at least two ways to get the job done to make a non-trivial decision ...

I think that will be quite sufficient, Udo, thank you. In this case, I was mainly interested in the syntax to incorporate the comparison test into the dataset modification. No requirement for optimality here -- I could have said "reasonably efficient" rather than "best" to express the problem -- and thank you for considering that detail!