df <- mpg, col_name <- hwy, how to call mpg$hwy? df[col_name] doesn't work

Question

I want to call filter(df, col_name == value) such as filter(mpg, hwy == 27), but

df <- mpg
col_name <- 'hwy'
value <- 27
col <- df[col_name]
filter(df, col == value)

yields the error

Error: Problem with filter() input ..1. i Input ..1 is col == value. x object 'value' not found Run rlang::last_error() to see where the error occurred.

How can I correct col_name <- df[col_name] so that it doesn't produce this error? I've noticed that typeof() and class() produce different results for col (list; tbl_df, tbl, data.frame) and mpg$df (int;int). How do I change col (an object of class tibble and type list) to the same type as mpg$df (int)?

rlang::last_error() gave:

<error/dplyr_error> Problem with filter() input ..1.

i Input ..1 is .data[["hwy"]] == value.

x Column hwy not found in .data

Backtrace:

Run rlang::last_trace() to see the full context.

`df[[col_name]]` See [Difference between `[` and `[[`](https://stackoverflow.com/questions/1169456/the-difference-between-bracket-and-double-bracket-for-accessing-the-el) — Rui Barradas, Jun 10 '21 at 16:25
It works with me, the return value is a df with 14 rows and 11 columns. — Rui Barradas, Jun 10 '21 at 16:34
I assume from the error message that you're using `dplyr::filter`, but it's good to note where all your non-base R functions come from — camille, Jun 10 '21 at 16:56

akrun · Accepted Answer · 2021-06-10T16:25:54.393

We could use .data

library(dplyr)
filter(df,  .data[[col_name]] == value)

-output

# A tibble: 14 x 11
   manufacturer model        displ  year   cyl trans      drv     cty   hwy fl    class     
   <chr>        <chr>        <dbl> <int> <int> <chr>      <chr> <int> <int> <chr> <chr>     
 1 audi         a4             3.1  2008     6 auto(av)   f        18    27 p     compact   
 2 audi         a4 quattro     2    2008     4 auto(s6)   4        19    27 p     compact   
 3 chevrolet    malibu         2.4  1999     4 auto(l4)   f        19    27 r     midsize   
 4 hyundai      sonata         2.4  1999     4 manual(m5) f        18    27 r     midsize   
 5 hyundai      tiburon        2    2008     4 auto(l4)   f        20    27 r     subcompact
 6 nissan       altima         2.4  1999     4 auto(l4)   f        19    27 r     compact   
 7 nissan       altima         3.5  2008     6 manual(m6) f        19    27 p     midsize   
 8 pontiac      grand prix     3.8  1999     6 auto(l4)   f        17    27 r     midsize   
 9 subaru       forester awd   2.5  2008     4 manual(m5) 4        20    27 r     suv       
10 subaru       impreza awd    2.5  2008     4 auto(s4)   4        20    27 r     compact   
11 subaru       impreza awd    2.5  2008     4 manual(m5) 4        20    27 r     compact   
12 toyota       camry          2.2  1999     4 auto(l4)   f        21    27 r     midsize   
13 toyota       camry solara   2.2  1999     4 auto(l4)   f        21    27 r     compact   
14 toyota       camry solara   3.3  2008     6 auto(s5)   f        18    27 r     compact

or convert to symbol and evaluate (!!

filter(df,  !! rlang::sym(col_name) == value)

Note: Here we used

col_name <- "hwy"

@Mark Are you using `col_name <- "hwy"` or `col_name <- df[col_name]`. I assumed that it is the first case — akrun, Jun 10 '21 at 16:23
@Mark Please note that when you assign the same object to `df[col_name]` is now a data.frame with one column as opposed to the string you have first assigned — akrun, Jun 10 '21 at 16:24
@Mark Both of the soluitons I showed is working for me with `packageVersion('dplyr')# [1] ‘1.0.6’` — akrun, Jun 10 '21 at 16:26
when I ran yours I forgot to stor value <-27 . its working. thanks — Mark, Jun 10 '21 at 16:34

df <- mpg, col_name <- hwy, how to call mpg$hwy? df[col_name] doesn't work

1 Answers1