Why does the ifelse() in R apply function when test is FALSE?-CodePudding

From a character column in a tibble, I'm trying to extract numbers and convert to numeric type using ifelse() to apply as.numeric() on values that do not contain letters (or " ").

While this example works, it returns a warning message:

library(dplyr)

tibble(a = c("0", "0.1", "abc")) %>% 
  mutate(b = ifelse(!grepl("[a-zA-Z\\ ] ", a), as.numeric(a), NA))

# A tibble: 3 × 2
  a         b
  <chr> <dbl>
1 0       0  
2 0.1     0.1
3 abc    NA  
Warning message:
Problem while computing `b = ifelse(grepl("[a-zA-Z\\ ] ", a), NA, as.numeric(a))`.
ℹ NAs introduced by coercion

Please help me understand why my script is still trying to convert the character value to numeric, and how I can fix this.

CodePudding user response：

As explained in this strongly related question: Does ifelse really calculate both of its vectors every time?, ifelse (generally) calculates both responses for every entry, and then puts the appropriate ones together into the result vector. So even though as.numeric("abc") isn't in your results, it still tries to calculate it.

Which means that there probably isn't much point in using ifelse() here. Might as well simplify to b = as.numeric(a), or if you want to avoid the warnings b = suppressWarnings(as.numeric(a)).