Lori Simpson
2009-May-14 17:16 UTC
[R] Function to read a string as the variables as opposed to taking the string name as the variable
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. OR To phrase this problem in a more generic manner, here is an example using a simpler function: first=4 second=6 third="first,second" 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. Lori Simpson 703.760.8575 [[alternative HTML version deleted]]
hadley wickham
2009-May-14 18:41 UTC
[R] Function to read a string as the variables as opposed to taking the string name as the variable
On Thu, May 14, 2009 at 12:16 PM, Lori Simpson <lori.simpson at dc-energy.com> 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"It's complex in general, but for cast you can just supply a string: cast(table, paste(string_with_columns, "~ column4")) Hadley -- http://had.co.nz/
Marc Schwartz
2009-May-14 19:08 UTC
[R] Function to read a string as the variables as opposed to taking the string name as the variable
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. > > > > > OR > > > > > To phrase this problem in a more generic manner, here is an example > using a simpler function: > > first=4 > second=6 > third="first,second" > 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 models). For example: string_with_columns <- paste("column", 1:3, sep = "", collapse = " + ") > string_with_columns [1] "column1 + column2 + column3" form <- paste(string_with_columns, "column4", sep = " ~ ") > form [1] "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 = ", ")) > vars [1] "first" "second" > sapply(vars, get) first second 4 6 > max(sapply(vars, get)) [1] 6 > sum(sapply(vars, get)) [1] 10 So, see ?as.formula and ?get. Also see ?paste and ?strsplit for manipulating the character vectors. HTH, Marc Schwartz
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