character vectors used for names (e.g. colnames for matrices or data.frames)
are often treated by base::make.names() or similar functions that
produce suitable names (e.g. by prepending "X" to numbers). Such names
cannot be directly converted to numeric.
extract_numbers(X)character with numbers hidden inside
numeric
extract_numbers() tries to extract numbers from X which may be
surrounded by such "protecting" characters.
tmp <- data.frame(flu[[, , 400 ~ 410]])
(wl <- colnames(tmp))
#> [1] "X405" "X405.5" "X406" "X406.5" "X407" "X407.5" "X408" "X408.5"
#> [9] "X409" "X409.5" "X410"
extract_numbers(wl)
#> [1] 405.0 405.5 406.0 406.5 407.0 407.5 408.0 408.5 409.0 409.5 410.0