[plyr] Question regarding ddply: use of .(as.name(varname)) and varname in ddply function

require(data.table)
var <- "g10"
df <- data.table(df)
str(df)

df[,list(min=min(d1), max = max(d1)), by = eval(var)]

Dear R-Helpers:

I am using trying to use *ddply* to extract min and max of a particular
column in a data.frame. I am using two different forms of the function:


## var_name_to_split is a string -- something like "var1" which is the name
of a column in data.frame

ddply( df, .(as.name(var_name_to_split)), function(x) c(min(x[ , 3] , max(x[
, 3]))) ## fails with an error - case 1
ddply( df, var_name_to_split , function(x) c(min(x[ , 3] , max(x[ , 3])))
? ? ? ? ? ? ? ## works fine - case 2

I can't understand why I get the error in case 1. Can someone help me
please?

Thank you in advance.

S.

----------

Here is the reproducible code:

https://gist.github.com/730069

Here is sample data:

structure(list(g10 = c(1L, 1L, 1L, 10L, 10L, 10L), l1 =
c(0.410077661080032,
0.607497980054711, 0.640488621149069, -1.47837849145189, -1.48199933642397,
-1.42815840788069), d1 = c(0.917769870675383, 0.959256755797054,
0.772928570498006, 0.473545787883884, 0.590580940273922, 0.0448629265021484
), l13 = c(0.0803696045647364, -0.291741079837731, -0.00191015929550312,
0.295889063381279, 0.615383505686296, 0.71991154637985), d13 =
c(-1.40821713632015,
-1.27501365601403, -1.41150703235157, 0.708943640186729, 0.276034890463749,
0.663383934998686)), .Names = c("g10", "l1", "d1", "l13", "d13"
), row.names = c(1L, 2L, 3L, 1758L, 1759L, 1760L), class = "data.frame")


-----------
If some one doesn't want to open github - here is the code

## Doesn't work

# grp -- name of a column of the the data.frame df
# function call is -- getMinMax1( df1 , grp = "var1")

getMinMax1 <-function(df, grp){
? ? ?dfret <- ddply( df , .(as.name(grp)), ## I am using
as.name(grp), source of error
? ? ? ? ? ?function(x){
? ? ? ? ? ? ? ?minmax <- c(mix(x[ , 3]), max(x[ ,3]))
? ? ? ? ? ? ? ?return(minmax)
? ? ? ? ? ?}
? ? ? ? ? ?)
? ? ?return(dfret)
?}

## Works fine
# grp -- name of a column of the the data.frame df
# function call is -- getMinMax2( df1 , grp = "var1")

getMinMax2 <-function(df, grp){
? ? ?dfret <- ddply( df , grp, ## using the quoted variable name
passed to grp when the fun is called
? ? ? ? ? ?function(x){
? ? ? ? ? ? ? ?minmax <- c(min(x[ , 3]), max(x[ ,3]))
? ? ? ? ? ? ? ?return(minmax)
? ? ? ? ? ?}
? ? ? ? ? ?)
? ? ?return(dfret)
?}

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