[R] mapply on multiple data frames

[arun]

Hi,
It would be better to provide a reproducible example.
set.seed(25)
all_dfs<- list(df1=data.frame(col1=sample(1:40,20,replace=TRUE),col2=sample(20:40,20,replace=TRUE),col3= sample(1:3,20,replace=TRUE)),df2=data.frame(col1=sample(30:60,20,replace=TRUE),col2=sample(35:65,20,replace=TRUE),col3=sample(1:3,20,replace=TRUE)),df3=data.frame(col1=sample(40:80,20,replace=TRUE),col2=sample(25:45,20,replace=TRUE),col3=sample(1:3,20,replace=TRUE)))

all_nums<-as.list(1:3)

f<- function(x,y){
out<-t.test(x$col1[x$col3==y],x$col2[x$col3==y],paired=T,alternative="greater")$p.value
out
}
mapply(f,all_dfs,all_nums)
#        df1         df2         df3 
#0.926400219 0.713943295 0.004510331 


##Just for validation:
df1New<-all_dfs[[1]]
df2New<-all_dfs[[2]]
df3New<- all_dfs[[3]]
t.test(df1New$col1[df1New$col3==1],df1New$col2[df1New$col3==1],paired=T,alternative="greater")$p.value
#[1] 0.9264002
t.test(df2New$col1[df2New$col3==2],df2New$col2[df2New$col3==2],paired=T,alternative="greater")$p.value
#[1] 0.7139433
 t.test(df3New$col1[df3New$col3==3],df3New$col2[df3New$col3==3],paired=T,alternative="greater")$p.value
#[1] 0.004510331
A.K.



----- Original Message -----
From: "Webb,Elizabeth E" <webbe at ufl.edu>
To: "r-help at R-project.org" <r-help at r-project.org>
Cc: 
Sent: Tuesday, June 11, 2013 2:07 AM
Subject: [R] mapply on multiple data frames

Hi all-

I am wondering about using the mapply function to multiple data frames.  Specifically, I would like to do a t-test on a subset of multiple data frames.  All data frames have the same structure.
Here is my code so far:

f<-function(x,y) {
test<-t.test(x$col1[x$col3=="num",],v$col2[x$col3=="num",],paired=T,alternative="greater")
out<-test$p.value
return(out) 
}

all_nums<-list(num1,num2,num3,num4)
all_dfs<-list(df1,df2,df3,df4)

mapply(f,all_dfs,all_nums)


This tells me that $ operator is invalid for atomic vectors.  I have tried shifting to notation using [  ,] to denote columns, but that gives me this error: incorrect number of dimensions.  

Thank you in advance,
Elizabeth
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