[R] Create a new dataframe from an existing dataframe

[jim holtman]

Does this do what you want?

> x <- read.table(textConnection("A    B   C   D
+ A1  B1 C1 D1
+ A1  B2 C1 D1
+ A1  B1 C1 D1
+ A1  B2 C2 D2
+ A2  B1 C1 D1
+ A1  B2 C2 D2
+ A1  B1 C2 D2
+ A2  B2 C1 D1
+ A1  B1 C2 D2
+ A2  B2  C1 D1"), header=TRUE)
> counts <- ave(seq(nrow(x)), x$B, x$C, x$D, FUN=length)  # get counts
> x.new <- x[, -1]  # delete "A"
> x.new$FreqD <- counts  # add new column
> # print out unique entries
> unique(x.new)
   B  C  D FreqD
1 B1 C1 D1     3
2 B2 C1 D1     3
4 B2 C2 D2     2
7 B1 C2 D2     2
>


On Jan 7, 2008 2:38 PM, Gang Chen <gangchen at mail.nih.gov> wrote:
> Yes, I misstated it when I said that I would keep B and C. I want to
> collapse column A, but count the frequency of D as a new column in
> the new dataframe DF2 while collapsing A. The rows of columns B, C,
> and D of course would be reduced because of A collapsing.
>
> For example, if dataframe DF is
>
> A    B   C   D
> A1  B1 C1 D1
> A1  B2 C1 D1
> A1  B1 C1 D1
> A1  B2 C2 D2
> A2  B1 C1 D1
> A1  B2 C2 D2
> A1  B1 C2 D2
> A2  B2 C1 D1
> A1  B1 C2 D2
> A2  B2  C1 D1
> ......
>
> I would like to have a new dataframe DF2
>
> B   C   D     FreqA
> B1 C1 D1     6
> B1 C1 D2     31
> B1 C2 D1     8
> B1 C2 D2    14
> B2 C1 D1    12
> B2 C1 D2    43
> B2 C2 D1    23
> B2 C2 D2    43
>
> Thanks,
> Gang
>
>
>
>
> On Jan 7, 2008, at 2:06 PM, Duncan Murdoch wrote:
>
> > On 1/7/2008 1:28 PM, Gang Chen wrote:
> >> I have a dataframe DF with 4 columns (variables) A, B, C, and D,
> >> and  want to create a new dataframe DF2 by keeping B and C in DF
> >> but  counting the frequency of D while collapsing A. I tried
> >> by(DF$D, list(DF$B, DF$C), FUN=summary)
> >> but this is not exactly what I want. What is a good way to do it?
> >
> > I think you can't do that.  If you want to keep B and C, then you
> > can't reduce the number of rows, but "collapsing A" sounds like you
> > want fewer rows.
> >
> > Perhaps if you posted a simple before and after example?
> >
> > Duncan Murdoch
>
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> and provide commented, minimal, self-contained, reproducible code.
>



-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

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