passing parameters to R function used plyr

Question

I can'n solve one question. Want to write a function like this:

   f.describe <- function(.data, .var, .by)
   {
        require(plyr)

        res <- ddply(.data, .by, 
    summarize, 
    N = sum(!is.na(.var))
    `Mean (SD)`=sprintf("%5.2f (%5.2f)", 
                       mean(.var, na.rm=TRUE), sd=sd(.var, na.rm=TRUE)),
     Median = sprintf("%5.2f", median(.var))    
        )
    res
   }

But I can't find the way to pass a variable for processing (.var). Have this error:

( translated from other language, so it could be not verbatim for English users ) （ 从其他语言翻译，因此对于英语用户来说可能不是逐字的 ）

I tried eval, substitute but have no solution... Many thanks for help. Sometimes I do not understand the rules uses for evaluation. 用于评估的规则。

Answer 1

In this case it might be easier to pass a function to ddply instead of using summarize :

f.describe <- function(.data, .var, .by) {
    require(plyr)

    ddply(.data, .by, function(d) {
       c(N = sum(!is.na(d[[.var]])),
       `Mean (SD)`=sprintf("%5.2f (%5.2f)", 
           mean(d[[.var]], na.rm=TRUE), 
           sd=sd(d[[.var]], na.rm=TRUE)),
       Median = sprintf("%5.2f", median(d[[.var]])))    
    })
}