R assign to an evaluated object-CodePudding

I want to be able to change the cell of a dataframe by referring to the object's name, rather than to the object itself, but when I attempt do do so it results in the warning could not find function "eval<-".

I can change a cell of a standard dataframe using the code below:

my_object = tibble(x = c("Hello", "Goodbye"), 
                   y = c(1,2))

object[2,1] <- "Bye"

But I am having trouble doing the same when using the object's name. I can evaluate the object using its name and extract the relevant cell:

object_name = "my_object"
eval(sym(object_name))[2, 1]

But I can't assign a new variable to the object (error: could not find function "eval<-"):

eval(sym(object_name))[2, 1] <- "Bye"

CodePudding user response：

You can use get() instead of eval(sym())to obtain an object by name. You can also use the [<- function to write a value to it without requiring an intermediate copy:

my_object = dplyr::tibble(x = c("Hello", "Goodbye"), 
                   y = c(1,2))

object_name = "my_object"

`[<-`(get(object_name), 2, 1, value ="Bye")
#> # A tibble: 2 x 2
#>   x         y
#>   <chr> <dbl>
#> 1 Hello     1
#> 2 Bye       2

^{Created on 2022-06-02 by the reprex package (v2.0.1)}

CodePudding user response：

If you want to define your command as a string, parse it as an expression, and then use eval:

eval(parse(text=paste0(object_name,"[2,1]<-'Bye'")))

> object
      x y
1 Hello 1
2   Bye 2