Dynamic formula not working with startsWith and colnames

Question

I'm working on making a function to create tables and I need to have some conditional rules involved for formatting. One will be based on a column name, however when I send it down using as.formula it seems to be over doing it. I've made an example here:

library(tidyverse) 
library(rlang)

a <- as_tibble(x =cbind( Year = c(2018, 2019, 2020), a = 1:3,
             b.1 = c("a", "b", "c"),
             b.2 = c("d", "e", "f"), 
             fac = c("This", "This","That")))

foo <- function(x, y, z, ...){
  y_var <- enquo(y)
  
  x %>%
    filter(Year %in% c(2018, 2019),
           ...) %>%
    mutate(!!quo_name(y_var) := factor(!!y_var,
                              levels = z,
                              ordered = TRUE)) %>%
    arrange(!!y_var)
    
}


to.table <- function(x, y, z, ...){
  y_var <- enquo(y)
  
  df.in <- foo(x=x,
               y=!!y_var,
               z= z)
  cond <- paste("~!is.na(", quo_name(y_var),")")
  cond.2 <- paste("~startsWith(colnames(", df.in, "),\"b\")")
  
  
  flextable(df.in) %>%
    bold(i = as.formula(cond),
         part = "body") %>%
    bg(i = as.formula(cond.2),
       bg = "Red3",
       j = as.formula(cond.2))

}

to.table(x=a,
         y=Year,
         z= c(2020,2018,2019),
         fac == "This")
Error in startsWith(colnames(2:3), "b") : non-character object(s) 

From the error I've been reviving it looks like solved the expression before it gets put through the as.formula as those two columns are the correct answer. Proof:

df.in <- foo(x=a,
          y=Year,
          z= c(2020,2018,2019),
          fac == "This")
startsWith(colnames(df.in), prefix = "b")
[1] FALSE FALSE  TRUE  TRUE FALSE

What am I missing here? If anyone has a solution, or suggestion on how to do things differently potentially using quosures or other tidyverse friendly methods I would much appreciate it.

Extension: To make this a bit more clear, I may need to elaborate on my intended use of this example. I'm trying to figure out how to take names generated dynamically in a function represented as foo that start with a specified value (generally 3 columns), and then check those columns for a specified value that I can then highlight in a specific Color. Additionally in the answer cond is used in both of the i= designation, the two separate conditions in will likely never overlap.

Answer 1

We could specify the j with the column names of the data created i.e. startsWith returns a logical vector from the column names based on the names that starts with 'b', use the logical vector to extract the column names with [ (nm1).

to.table <- function(x, y, z, ...){
  y_var <- enquo(y)
  
  df.in <- foo(x=x,
               y=!!y_var,
               z= z)
  
  cond <- as.formula(glue::glue('~ !is.na({quo_name(y_var)})'))
  nm1 <- names(df.in)[startsWith(names(df.in), prefix = "b")]
  
  flextable(df.in) %>%
      bold(i = cond,
           part = "body") %>%
      bg(i = cond,
        bg = "Red3",
         j = nm1)
  
 
}

-testing

to.table(x=a,
         y=Year,
         z= c(2020,2018,2019),
         fac == "This")

-output

---

In the OP's post formula created for 'cond' is fine although it is a bit more flexible by using glue whereas the second one i.e. 'cond.2' returns

paste("~startsWith(colnames(", df.in, "),\"b\")")
[1] "~startsWith(colnames( 2:3 ),\"b\")"                   "~startsWith(colnames( c(\"1\", \"2\") ),\"b\")"      
[3] "~startsWith(colnames( c(\"a\", \"b\") ),\"b\")"       "~startsWith(colnames( c(\"d\", \"e\") ),\"b\")"      
[5] "~startsWith(colnames( c(\"This\", \"This\") ),\"b\")"

It is because df.in is a data.frame on which we are trying to paste the startsWith(colnames( string. Each of the lines returned are the column values

---

If we want to get either 'a' or 'b' column names prefix with 'red' color, change the startsWith to grep which can take a regex as pattern

to.table <- function(x, y, z, ...){
  y_var <- enquo(y)
  
  df.in <- foo(x=x,
               y=!!y_var,
               z= z)
  
  cond <- as.formula(glue::glue('~ !is.na({quo_name(y_var)})'))
  nm1 <- grep("^(a|b)", names(df.in), value = TRUE)
  
  flextable(df.in) %>%
    bold(i = cond,
         part = "body") %>%
    bg(i = cond,
       bg = "Red3",
       j = nm1)
  
  
}
to.table(x=a,
         y=Year,
         z= c(2020,2018,2019),
         fac == "This")

-output

---

If we want to color based on the value of 'a'

to.table <- function(x, y, z, ...){
  y_var <- enquo(y)
  
  df.in <- foo(x=x,
               y=!!y_var,
               z= z)
  
  cond <- as.formula(glue::glue('~ !is.na({quo_name(y_var)})'))
  nm1 <- names(df.in)[startsWith(names(df.in), prefix = "b")]
  
  flextable(df.in) %>%
    bold(i = cond,
         part = "body") %>%
    bg(i = ~ a == 2,
       bg = "Red3",
       j = nm1)
  
}



to.table(x=a,
         y=Year,
         z= c(2020,2018,2019),
         fac == "This")

-output