dplyr - Substitute LHS of = in R -

i replace lhs of "=" in expression in r. in personal case, need make sure following creates variable not exist in data frame

df %>% mutate(v = mean(w)) 

i tried eval(substitute()) lhs not substituted

eval(substitute(df %>% mutate(v = mean(w)), list(v = as.name("id"))))  #similarly in list eval(substitute(l <- list(v=1:10),list(v=as.name("id")))) l $v [1]  1  2  3  4  5  6  7  8  9 10 

why can't v substituted throught eval/substitute? what's best way work around it?

1) eval/parse create cmd string, parse , evaluate it:

f2 <- function(df, x, env = parent.frame()){   cmd <- sprintf("mutate(%s, %s = mean(v1))", deparse(substitute(df)), x)   eval(parse(text = cmd), env) }  f2(df, "v1_name") 

giving

  v1 v1_mean 1  1       2 2  2       2 3  3       2 ... etc ... 

2) eval/as.call way construct list, convert call , evaluate it. (this approach mutate_each_q in dplyr takes.)

f3 <- function(df, x, env = parent.frame()) {     l <- list(quote(mutate), .data = substitute(df), quote(mean(v1)))     names(l)[3] <- x     eval(as.call(l), env) }  f3(df, "v1_name") 

3) do.call form list equal last 2 components of list in prior solution , use do.call :

f3 <- function(df, x, env = parent.frame()) {     l <- list(.data = substitute(df), quote(mean(v1)))     names(l)[2] <- x     do.call(mutate, l) }  f3(df, "v1_name") 

upodate added additional solutions.