Parse and Evaluate Column of String Expressions in R?

Question

How can I parse and evaluate a column of string expressions in R as part of a pipeline?

In the example below, I produce my desired column, evaluated. But I know this isn't the right approach. I tried taking a tidyverse approach. But I'm just very confused.

library(tidyverse)
df <- tibble(name = LETTERS[1:3], 
             to_evaluate = c("1-1+1", "iter+iter", "4*iter-1"), 
             evaluated = NA)
iter = 1
for (i in 1:nrow(df)) {
  df[i,"evaluated"] <- eval(parse(text=df$to_evaluate[[i]]))
}
print(df)
# # A tibble: 3 x 3
# name  to_evaluate evaluated
# <chr> <chr>           <dbl>
# 1 A     1-1+1               1
# 2 B     iter+iter           2
# 3 C     4*iter-1            3

As part of a pipeline, I tried:

df %>% mutate(evaluated = eval(parse(text=to_evaluate)))
df %>% mutate(evaluated = !!parse_exprs(to_evaluate))
df %>% mutate(evaluated = parse_exprs(to_evaluate))
df %>% mutate(evaluated = eval(parse_expr(to_evaluate)))
df %>% mutate(evaluated = parse_exprs(to_evaluate))
df %>% mutate(evaluated = eval(parse_exprs(to_evaluate)))
df %>% mutate(evaluated = eval_tidy(parse_exprs(to_evaluate)))

None of these work.

Answer 1

You can try:

df %>%
 rowwise() %>%
 mutate(iter = 1,
        evaluated = eval(parse(text = to_evaluate))) %>%
 select(-iter)

  name  to_evaluate evaluated
  <chr> <chr>           <dbl>
1 A     1-1+1               1
2 B     iter+iter           2
3 C     4*iter-1            3

Following this logic, also other possibilities could work. Using rlang::parse_expr():

df %>%
 rowwise() %>%
 mutate(iter = 1,
        evaluated = eval(rlang::parse_expr(to_evaluate))) %>%
 select(-iter)

On the other hand, I think it is important to quote @Martin Mächler:

The (possibly) only connection is via parse(text = ....) and all good R programmers should know that this is rarely an efficient or safe means to construct expressions (or calls). Rather learn more about substitute(), quote(), and possibly the power of using do.call(substitute, ......).

Answer 2

Here's a slightly different way that does everything within mutate.

df %>% mutate(
    evaluated = pmap_dbl(., function(name, to_evaluate, evaluated) 
                         eval(parse(text=to_evaluate)))
)

# A tibble: 3 x 3
  name  to_evaluate evaluated
  <chr> <chr>           <dbl>
1 A     1-1+1               1
2 B     iter+iter           2
3 C     4*iter-1            3