mutate with possibility to select rows

gistfile1.txt

mutate2 = function (.data,.where, ...) {
  UseMethod("mutate2")
}

mutate2.data.frame =function (.data,.where, ...) 
{
    if (!missing(.where)) {
      .where = substitute(.where)
      dt = as.data.table(df)
      df = as.data.frame(mutate_where.data.table(.data=dt,.where.call=.where,...,inplace=TRUE))
      return(df)
    }
    tbl <- tbl_df(.data)
    res <- mutate.tbl_df(tbl, ...)
    as.data.frame(res)
}

mutate2.data.table <- function (.data,.where, ..., inplace = FALSE) 
{
    if (!inplace) 
        .data <- copy(.data)
    
    if (!missing(.where)) {
      .where = substitute(.where)
      dt = mutate_where.data.table(.data=dt,.where.call=.where,..., inplace=TRUE)
      return(dt)
    }
    
    env <- new.env(parent = parent.frame(), size = 1L)
    env$data <- .data
    cols <- named_dots(...)
    for (i in seq_along(cols)) {
      call <- substitute(data[, `:=`(lhs, rhs)], list(lhs = as.name(names(cols)[[i]]), 
          rhs = cols[[i]]))
      eval(call, env)
    }
    .data
}


mutate_where.data.table <- function (.data,.where.call, ..., inplace = FALSE) 
{
    if (!inplace) 
        .data <- copy(.data)
    env <- new.env(parent = parent.frame(), size = 1L)
    env$data <- .data
    cols <- named_dots(...)

    for (i in seq_along(cols)) {
      call <- substitute(data[.where.call, `:=`(lhs, rhs)], list(lhs = as.name(names(cols)[[i]]), rhs = cols[[i]], .where.call =.where.call))
      eval(call, env)
    }    
    .data
}



examples = function() {
  library(microbenchmark)
  #library(modify)
  library(dplyr)
  library(pryr)
  library(data.table)
  
  # Benckmark compared to directly using data.table or dplyr 
  n = 1e6
  df = data.frame(a= sample(1:5,n,replace=TRUE),
                   b= sample(1:100,n,replace=TRUE),
                   x=rnorm(n))
  dt = as.data.table(df)
  res1 = mutate2(df,a==3,x=x+100)
  res2 = mutate2(dt,a==3,x=x+100)
  
  
  microbenchmark(times = 5L,
    #modify(dt,a==2, x = x+100),
    mutate(dt, x=ifelse(a==2,x+100,x)),
    mutate2(dt, a==2, x=x+100),
    mutate(df, x=ifelse(a==2,x+100,x)),
    mutate2(df, a==2, x=x+100)
  )
}