R help - Dec 2004 - evaluate expression on several dataframe columns

Hi R-users,

I have a collection of dataframes and know how to build
a string that refers to it, in this example, name_infra_alg_inc.
Then, I have a character string yval, which the user can select
from a drop down list. It contains the column names of the
dataframes.

assign(paste(name_infra_alg_inc, "ci", sep="."),
  ci(get(name_infra_alg_inc)[[yval]], confidence=0.95))

My problem is that I sometimes want to combine columns.
For example, if there are columns A, B and C.
Would it be possible that yval has the value "A+B*C" and
then call some sort of evaluate function?
Maybe I could attach the dataframe and then call some function,
I don't know how to figure this out, so hopefully someone can help me.

Thanks in advance

R user wrote:
> Hi R-users,
> 
> I have a collection of dataframes and know how to build
> a string that refers to it, in this example, name_infra_alg_inc.
> Then, I have a character string yval, which the user can select
> from a drop down list. It contains the column names of the
> dataframes.
> 
> assign(paste(name_infra_alg_inc, "ci", sep="."),
>   ci(get(name_infra_alg_inc)[[yval]], confidence=0.95))
> 
> My problem is that I sometimes want to combine columns.
> For example, if there are columns A, B and C.
> Would it be possible that yval has the value "A+B*C" and
> then call some sort of evaluate function?
> Maybe I could attach the dataframe and then call some function,
> I don't know how to figure this out, so hopefully someone can help me.
> 
> Thanks in advance

Anonymous R user,

I really don't know why are you going to do so, but you can do:

A <- 1
B <- 2
C <- 3
x <- expression(A+B*C)
eval(x)

Uwe Ligges

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