In R: dcast in function, pass column names (again!)

Question

Given a df in semi-long format with id variables a and b and measured data in columns m1and m2. The type of data is specified by the variable v (values var1 and var2).

set.seed(8)

df_l <- 
  data.frame(
    a = rep(sample(LETTERS,5),2),
    b = rep(sample(letters,5),2),
    v = c(rep("var1",5),rep("var2",5)),
    m1 = sample(1:10,10,F),
    m2 = sample(20:40,10,F)) 

Looks as:

   a b    v m1 m2
1  W r var1  3 40
2  N l var1  6 32
3  R a var1  9 28
4  F g var1  5 21
5  E u var1  4 38
6  W r var2  1 35
7  N l var2  8 33
8  R a var2 10 29
9  F g var2  7 30
10 E u var2  2 23

If I want to make a wide format of values in m1 using id a as rows and values in v1as columns I do:

> reshape2::dcast(df_l, a~v, value.var="m1")
  a var1 var2
1 E    4    2
2 F    5    7
3 N    6    8
4 R    9   10
5 W    3    1

How do I write a function that does this were arguments to dcast (row, column and value.var) are supplied as arguments, something like:

fun <- function(df,row,col,val){
  require(reshape2)
  res <-
    dcast(df, row~col, value.var=val)
  return(res)
}

I checked SO here and here to try variations of match.call and eval(substitute()) in order to "get" the arguments inside the function, and also tried with the lazyeval package. No succes.

What am I doing wrong here ? How to get dcast to recognize variable names?

Answer 1

Formula argument also accepts character input.

foo <- function(df, id, measure, val) {
    dcast(df, paste(paste(id, collapse = " + "), "~", 
                    paste(measure, collapse = " + ")), 
          value.var = val)
}

require(reshape2)
foo(df_l, "a", "v", "m1")

---

Note that data.table's dcast (current development) can also cast multiple value.var columns directly. So, you can also do:

require(data.table) # v1.9.5
foo(setDT(df_l), "a", "v", c("m1", "m2"))
#    a m1_var1 m1_var2 m2_var1 m2_var2
# 1: F       1       6      28      21
# 2: H       9       2      38      29
# 3: M       5      10      24      35
# 4: O       8       3      23      26
# 5: T       4       7      31      39