R help - Sep 2012 - eval(parse(...)) only once in a function

Hi

I would like to have something like

str <- "df$JT == 12"

fun <- function(df) {

  b <- eval(parse(str))

  return(b)
}

but for performance "eval(parse(a))" should not be evaluated at each
function call, but should work as

fun <- function(df) {

  b <- df$JT == 12

  return(b)
}

Do you have an idea how I can implement this?

Thx
Christof

If you have a data frame "df" with a column "JT"

Try this one:
str <- "df$JT == 12"

fun<-function(str){b<-eval(parse(text=str))
return(b)}

fun(str)

On Mon, Sep 17, 2012 at 11:57 AM, Christof Kluß
<ckluss@email.uni-kiel.de>wrote:
> Hi
>
> I would like to have something like
>
> str <- "df$JT == 12"
>
> fun <- function(df) {
>
>   b <- eval(parse(str))
>
>   return(b)
> }
>
> but for performance "eval(parse(a))" should not be evaluated at
each
> function call, but should work as
>
> fun <- function(df) {
>
>   b <- df$JT == 12
>
>   return(b)
> }
>
> Do you have an idea how I can implement this?
>
> Thx
> Christof
>
> ______________________________________________
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On Sep 16, 2012, at 11:27 PM, Christof Klu? wrote:
> Hi
> 
> I would like to have something like
> 
> str <- "df$JT == 12"
> 
> fun <- function(df) {
> 
>  b <- eval(parse(str))
> 
>  return(b)
> }
> 
What are you trying to do?

str <- quote(df$JT == 12)
> but for performance "eval(parse(a))" should not be evaluated at
each
> function call, but should work as
> 
> fun <- function(df) {
> 
>  b <- df$JT == 12
> 
>  return(b)
> }
> 
> Do you have an idea how I can implement this?
Implement what?  And what issues regarding "performance" are relevant?
What does it mean to "not evaluate at each function call"?

str <- quote(df$JT == 12)
fun <- function(df) {
    b<-eval(str)
    return(b)
   }

df3 <- data.frame(JT=12)
fun(df3)
#[1] TRUE

-- 



David Winsemius, MD
Alameda, CA, USA

On Mon, Sep 17, 2012 at 6:27 PM, Christof Klu? <ckluss at
email.uni-kiel.de> wrote:
> Hi
>
> I would like to have something like
>
> str <- "df$JT == 12"
>
> fun <- function(df) {
>
>   b <- eval(parse(str))
>
>   return(b)
> }
>
> but for performance "eval(parse(a))" should not be evaluated at
each
> function call, but should work as
>
> fun <- function(df) {
>
>   b <- df$JT == 12
>
>   return(b)
> }
>
> Do you have an idea how I can implement this?
You can do it with bquote()
> e<-parse(text="df$str==12")[[1]]
> e
df$str == 12
> bquote(function(df) b<-.(e))
function(df) b <- df$str == 12
> eval(bquote(function(df) b<-.(e)))
function (df)
b <- df$str == 12

This saves more time than I expected, about 100ms per evaluation on my
computer.

  -thomas

-- 
Thomas Lumley
Professor of Biostatistics
University of Auckland