[R] Function to read a string as the variables as opposed to taking the string name as the variable
marc_schwartz at me.com
Thu May 14 21:08:02 CEST 2009
On May 14, 2009, at 12:16 PM, Lori Simpson wrote:
> I am writing a custom function that uses an R-function from the
> reshape package: cast. However, my question could be applicable to
> any R function.
> Normally one writes the arguments directly into a function, e.g.:
> result=cast(table1, column1 + column2 + column3 ~ column4,
> mean) (1)
> I need to be able to write this statement as follows:
> result=cast(table1, string_with_columns ~ column4, mean) (2)
> string_with_columns = group of functions that ultimately outputs:
> "column1 + column2 + column3"
> Statement 1 outputs the correct results because I have manually typed
> in the column names I want to use. However, statement 2 thinks that
> 'string' is the name of a column rather than knowing to paste the
> string in string.
> To phrase this problem in a more generic manner, here is an example
> using a simpler function:
> max(first,second) //correctly outputs 6
> max(third) //outputs "first,second" because it doesn't know to paste
> in the variables first and second, how do I get R to do this?
> Any help is appreciated.
Your two examples actually require different approaches.
In the first example, you want to create a character vector and coerce
it to a 'formula' object, which can be used here and with other
functions where a formula is one of the arguments (eg. regression
string_with_columns <- paste("column", 1:3, sep = "", collapse = " + ")
 "column1 + column2 + column3"
form <- paste(string_with_columns, "column4", sep = " ~ ")
 "column1 + column2 + column3 ~ column4"
You would then use something like:
cast(table1, as.formula(form), mean)
For your second example, you don't need a formula object, you just
need to 'get' the values in the objects that are named:
first <- 4
second <- 6
third <- "first, second"
vars <- unlist(strsplit(third, split = ", "))
 "first" "second"
> sapply(vars, get)
> max(sapply(vars, get))
> sum(sapply(vars, get))
So, see ?as.formula and ?get. Also see ?paste and ?strsplit for
manipulating the character vectors.
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