Sign up ×
Mathematica Stack Exchange is a question and answer site for users of Mathematica. It's 100% free, no registration required.


It was a bug in the documentation of V10.0, this functionality was not implemented yet, and V10.1 changed the documentation, removing it. It's a pity, because it's a very useful operations, common in other languages like R. I miss data.frame like notation in Mathematica.

Mathematica graphics

In the new guide Computation With Structured Datasets we can find this part, on how to change a Dataset

enter image description here

But if we create a Dataset like:


And then make:

ds[[1, 1]] = 2

Or, closer to my real case test:

ds[[All, "a"]] = Accumulate@Normal@ds[[All, "a"]]

We get an error:

"Part specification ds[[1,1]] is longer than depth of object"

"Part specification ds[[All,1]] is longer than depth of object. "

Is this a Bug?

Setting is not working on Dataset as stated by documentation.

This post on Wolfram Community

share|improve this question
Unfortunately not in V10.0.1 yet... – Murta Sep 17 '14 at 1:42
StringReplace[%,"V10.0.1"-> "V10.0.2"] – Murta Dec 11 '14 at 1:35
StringReplace[%%,"V10.0.1"-> "V10.1.0"] – Murta Mar 30 at 21:01
This is no longer documented to work as of 10.1.0. As Tali mentions in his answer below, the inclusion of this comment in the original documentation was erroneous. – Stefan R Jun 8 at 15:07
@StefanR I know about that. But this would be a nice way to handler data, and should be considered in the future. In R, it's a very natural way to do Data Frame manipulations. – Murta Jun 8 at 18:41

3 Answers 3

up vote 25 down vote accepted

I'm the developer of Dataset.

Yes, this is a gross documentation oversight. We planned this functionality but had to push it back to a point release. Somehow no-one caught this piece of legacy documentation.

I'm filed a bug on the documentation problem right now, it's easy to fix.

As for when L-value assignment will be available, I'm hoping 10.0.1 or 10.0.2, which are in the next month or two. It gets complicated, because you might well want to write things like:

dataset[ Select[#age > 30&] , "salary"] *= 2

That's certainly a powerful kind of operation, but also hard to implement. Even part-like assignments can get complicated when you are assigning multidimensional datasets to each other.

Thanks for trying the functionality, though!

share|improve this answer
Tks for your clarification. I'll wait for it, it's a very useful operation and I'm happy that I won't need to wait for V11. Using this opportunity, have you saw this post in Wolfram Community about Dataset memory consumption? There are plans to efficient Tabular Data in V10? – Murta Jul 10 '14 at 19:31
@Murta Yes, moving to column-oriented will make things much better. But before I could do that I had to lay the groundwork in the form of a type system that could represent the "logical shape", even if the "physical layout" is different. And of course Leonid is working on making this whole process scale to out-of-core computation against data that lives on disk. – Taliesin Beynon Jul 10 '14 at 20:26
@TaliesinBeynon your example is quite funny ! – faysou Nov 17 '14 at 8:46

In lieu of Set, the Query syntax offers various ways to update selective elements of a dataset. For example, we can change the value of the field a in the first row like this:

ds[{1 -> (<| #, "a" -> 999|> &)}]

dataset screenshot

or like this:

ds[{1 -> Query[{"a" -> (999 &)}]}]

dataset screenshot

Multiple fields can be updated simultaneously:

ds[{1 -> (<| #, "a" -> 999, "b" -> "ZZZ" |> &)}]

dataset screenshot

We can update selective rows, in this case field "b" in rows with even a:

ds[All, If[EvenQ[#a], <| #, "b" -> "!!!!"|>, #] &]

dataset screenshot

The accumulation use case can be accomplished like this:

With[{a = ds[Accumulate, "a"]}
, ds @ MapIndexed[<| #, "a" -> a[[First@#2]] |> &]

dataset screenshot

or like this:

Module[{acc = 0}, ds[All, {"a" -> (acc += # &)}]]

dataset screenshot

Note that none of these operations involve destructively altering the dataset, so they should all read ds = ds[...] if desired. Presumably Set will eventually perform destructive updates in those restricted circumstances that Mathematica tolerates mutation.

share|improve this answer
Examples such as these are sure to increase the fun factor for the WRI employees working to compile the Query language into SQL ;) – WReach Jul 27 '14 at 0:11
Nice examples. +1. – Murta Jul 27 '14 at 0:15
@WReach, are WRI employees working to compile the Query language into SQL? Will this be brought into DatabaseLink? – ArgentoSapiens Nov 6 '14 at 16:24
@ArgentoSapiens I have no current information about this. My glib comment was based upon the fact that pre-release versions of the V10 documentation contained extensive references to such capability. Those references were withdrawn very late, just before the official V10 release. I speculate that the functionality under discussion in the question would (or did) prove to be challenging to support across multiple back-end technologies. – WReach Nov 6 '14 at 16:55

Though I don't know what is the efficiency impact of it, a workaround could be converting the Dataset to Association by Normal, making the update on the Association, then converting it back to Dataset.

ds = Dataset[{<|"a" -> 1, "b" -> "x"|>, <|"a" -> 2, "b" -> "y"|>, <|"a" -> 6, "b" -> "z"|>}]

ds = Module[{temp = Normal[ds]},
            temp[[All, "a"]] = Accumulate[temp[[All, "a"]]];
            temp // Dataset]

Dataset updating

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.