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reval

Calculate formula from string saved in variable

Sorry, but I'm struggling with a quite simple problem: A formula/an expression like 1.28*10^2 is saved as a character in a data frame. Now I want to convert this string into a numeric value - this should result in "128", but it does not.

library(dplyr)

mydata <- data.frame(
  formula = c("5.89*10^3", "1.28*10^2", "4.11*10^5")
)

mydata <- mydata %>% 
  dplyr::mutate(eval = eval(parse(text = formula)))

The variable "eval" should contain the values 5890, 128 and 411000 in the end.

Can anyone help where my mistake is? Thank you!

Solution

  • Yet another solution, based on rlang::parse_expr:

    library(dplyr)
library(rlang)

mydata <- data.frame(
  formula = c("5.89*10^3", "1.28*10^2", "4.11*10^5")
)

mydata %>% 
  rowwise %>% 
  mutate(eval = eval(parse_expr(formula)))

#> # A tibble: 3 × 2
#> # Rowwise: 
#>   formula     eval
#>   <chr>      <dbl>
#> 1 5.89*10^3   5890
#> 2 1.28*10^2    128
#> 3 4.11*10^5 411000