[datatable-help] Pivot part of a data table

[Stefano Grioni]

Thanks Arun, it worked!

Here is my code, in case someone has the same question:

names = c("John", "Steve", "Peter")
ages = c(24,16,12)
grade_spring_quarter = c("A","B+", "C")
grade_summer_quarter = c("A+","B", "B+")
gift_spring_quarter = c("A gift","B+ gift", "C gift")
gift_summer_quarter = c("A+ gift","B gift", "B+ gift")

db <-
data.table(names,ages,grade_spring_quarter,grade_summer_quarter,gift_spring_quarter,gift_summer_quarter)

db.M <- melt(db, measure = patterns("^grade","^gift"), variable.name="quarter",
value.name=c("grades","gifts"))

Cheers,

Stefano

On Sun, Nov 8, 2015 at 1:44 PM Arunkumar Srinivasan <aragorn168b at gmail.com>
wrote:

> Hi, have a look at `?melt` and the reshaping vignette:
> https://github.com/Rdatatable/data.table/wiki/Getting-started
>
>
> --
> Arun
>
> On 8 November 2015 at 20:42:05, Stefano Grioni (sgrioni at chicagobooth.edu)
> wrote:
>
> Hello,
>
> I am new to R and trying to understand how data manipulation works. I have
> a CSV file which allows me to build a data table that looks like the
> following (simplified example):
>
> >db
>    names ages grade_spring_quarter grade_summer_quarter
> 1:  John   24                    A                   A+
> 2: Steve   16                   B+                    B
> 3: Peter   12                    C                   B+
>
> Now, my goal is to denormalize the data table in order to transform the
> last 2 columns in rows, therefore obtaining the following:
>
> > db2
>    names ages quarter grades
> 1:  John   24  spring      A
> 2: Steve   16  spring     B+
> 3: Peter   12  spring      C
> 4:  John   24  summer     A+
> 5: Steve   16  summer      B
> 6: Peter   12  summer     B+
>
> I think that I would be able to do it using for loops, but I doubt that's
> the most efficient way. I have started looking into the Join functionality
> of data table but so far didn't manage to build db2 from db that way.
>
> Do you see how I could proceed?
>
> Many thanks
>
> Stefano
>
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