R help - Apr 2000 - Using 'by()' in a function

I have a list of dataframes, and want to apply a function to subsets of the rows
of each dataframe.  It seemed natural to write a function that takes a dataframe
as an argument, and uses 'by() within it to apply the function to the
dataframe
subsets.  However, I cannot get it to work.  The problem seems to be passing the
data argument of by() as a function argument.  Is this bug, or am I missing
something (or both)?
> ### Generate some test data
> Test <- vector("list",2)
> Test[[1]] <-
data.frame(Dose=rep(c(0,1),c(10,10)),Resp1=rnorm(20),Resp2=rnorm(20))
> ### The summary function
> sumfun <- function(z)
+ {
+   by(data=z,
+      INDICES=list(factor(z[,"Dose"])),
+      FUN=function(y)
+      {
+        apply(as.matrix(y[,c("Resp1","Resp2")]),2,
+              function(x)c(Mean=mean(x),SD=sqrt(var(x))))
+      }
+      )
+ }
> ### Using by works by itself
> by(data=Test[[1]],
+    INDICES=list(factor(Test[[1]][,"Dose"])),
+    FUN=function(y)
+    {
+      apply(as.matrix(y[,c("Resp1","Resp2")]),2,
+            function(x)c(Mean=mean(x),SD=sqrt(var(x))))
+    }
+    )
: 0
          Resp1      Resp2
Mean -0.2426571 -0.1024979
SD    0.9203455  0.9988352
------------------------------------------------------------
: 1
          Resp1     Resp2
Mean -0.1632326 0.1079938
SD    1.4124645 0.8793081
> ### But not in a function
> sumfun(Test[[1]])
[1] "data.frame"
Error in nrow(z) : Object "z" not found
>
R. Woodrow Setzer, Jr.                                                    Phone:
(919) 541-0128
Biostatistics and
Fax:  (919) 541-4002
Research Support Staff
NHEERL MD-55; US EPA; RTP, NC 27711


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Setzer.Woodrow at epamail.epa.gov writes:
> I have a list of dataframes, and want to apply a function to subsets of the
rows
> of each dataframe.  It seemed natural to write a function that takes a
dataframe
> as an argument, and uses 'by() within it to apply the function to the
dataframe
> subsets.  However, I cannot get it to work.  The problem seems to be
passing the
> data argument of by() as a function argument.  Is this bug, or am I missing
> something (or both)?
It's a bug: This looks like the culprit

    ans <- eval(substitute(tapply(1:nrow(data), IND, FUNx)), 
        data)

That will substitute the unevaluated data argument into the
"tapply(1:nrow(data),...)" where it will not make sense except at top
level. 

I think we want 

    NN<-seq(len=nrow(data))
    ans <- eval(substitute(tapply(NN, IND, FUNx)), data)

(but I didn't test that)

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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