Why does only `case_when` give different results in R?

Question

I noticed this behavior below when I used dplyr::case_when instead of if in reference to this article. If the output of the second branch is an explicit string, it works as intended, but if the x itself is specified, the result changes.

Why does only case_when give different results?

x <- character(0)

dplyr::case_when(rlang::is_empty(x) ~ "Empty", !rlang::is_empty(x) ~ "Not empty")
#> [1] "Empty"
dplyr::case_when(rlang::is_empty(x) ~ "Empty", !rlang::is_empty(x) ~ x)
#> character(0)

if (rlang::is_empty(x)) "Empty" else if (!rlang::is_empty(x)) "Not empty"
#> [1] "Empty"
if (rlang::is_empty(x)) "Empty" else if (!rlang::is_empty(x)) x
#> [1] "Empty"

ifelse(rlang::is_empty(x), "Empty", "Not empty")
#> [1] "Empty"
ifelse(rlang::is_empty(x), "Empty", x)
#> [1] "Empty"

^{Created on 2022-08-16 by the reprex package (v2.0.1)}

Note: `dplyr`s `if_else`-function throws an error: `if_else(purrr::is_empty(x), "Empty", x)`. — Martin Gal, Aug 16 '22 at 08:34

score 6 · Accepted Answer · answered Aug 16 '22 at 08:37

This is likely a bug in case_when or its internal helper functions. We can put a browser in the source code of case_when to see what happens in both cases. Some internal functions must be called via :::.

f <- function (...) {
    browser()
    fs <- dplyr:::compact_null(rlang::list2(...))
    n <- length(fs)
    error_call <- rlang::current_env()
    if (n == 0) {
        abort("No cases provided.", call = error_call)
    }
    query <- vector("list", n)
    value <- vector("list", n)
    default_env <- rlang::caller_env()
    quos_pairs <- purrr::map2(fs, seq_along(fs), dplyr:::validate_formula, default_env = default_env, 
                                                        dots_env = rlang::current_env(), error_call = error_call)
    for (i in seq_len(n)) {
        pair <- quos_pairs[[i]]
        query[[i]] <- rlang::eval_tidy(pair$lhs, env = default_env)
        value[[i]] <- rlang::eval_tidy(pair$rhs, env = default_env)
        if (!is.logical(query[[i]])) {
            dplyr:::abort_case_when_logical(pair$lhs, i, query[[i]], 
                                                                            error_call = error_call)
        }
    }
    m <- dplyr:::validate_case_when_length(query, value, fs, error_call = error_call)
    out <- value[[1]][rep(NA_integer_, m)]
    replaced <- rep(FALSE, m)
    for (i in seq_len(n)) {
        out <- dplyr:::replace_with(out, query[[i]] & !replaced, value[[i]], 
                                                NULL, error_call = error_call)
        replaced <- replaced | (query[[i]] & !is.na(query[[i]]))
    }
    out
}

and the helper internal replace_with in dplyr,

replacer <- function (x, i, val, name, reason = NULL, error_call = rlang::caller_env()) {
    if (is.null(val)) {
        return(x)
    }
    dplyr:::check_length(val, x, name, reason, error_call = error_call)
    dplyr:::check_type(val, x, name, error_call = error_call)
    dplyr:::check_class(val, x, name, error_call = error_call)
    i[is.na(i)] <- FALSE
    if (length(val) == 1L) {
        x[i] <- val
    }
    else {
        x[i] <- val[i]
    }
    x
}

and then debug via

x <- character(0)
f(rlang::is_empty(x) ~ "Empty", !rlang::is_empty(x) ~ "x")
f(rlang::is_empty(x) ~ "Empty", !rlang::is_empty(x) ~ x)

the key is in value m, that in the working case results in 1L, in the faulty case it is 0L. out becomes character(0) instead of initializing to NA, length 1.

replaced should be a logical vector indicating whether a value has been replaced. In the faulty case rep(FALSE, 0L) is logical(0), which is queried later on via !replaced. FALSE & logical(0) gives logical(0).

When passed to replacer this gives a peculiar subsetting action character(0)[logical(0)], that gives character(0).

score 4 · Answer 2 · answered Aug 16 '22 at 13:49

This is because case_when tries to recycle all formulas to a common length. The documentation says:

Both LHS and RHS may have the same length of either 1 or n. The value of n must be consistent across all cases. The case of n == 0 is treated as a variant of n != 1.

In your second example you've supplied a 0-length vector as the RHS of one of the formulas and this is the reason that a 0-length vector is returned; it's just a coincidence that the value returned is the same as the RHS of false case.

This behaviour might seem less surprising when considering these examples:

library(dplyr, warn.conflicts = FALSE)

case_when(TRUE ~ 1, FALSE ~ numeric(0))
#> numeric(0)

case_when(TRUE ~ 1, FALSE ~ numeric(1))
#> [1] 1

case_when(TRUE ~ 1, FALSE ~ numeric(2))
#> [1] 1 1

case_when(TRUE ~ 1, FALSE ~ numeric(3))
#> [1] 1 1 1

gaut · Answer 3 · 2022-08-16T08:30:41.270

One big difference between case_when and ifelse is that case_when coerces the right-hand side arguments into the same type. From ?case_when:

The RHS does not need to be logical, but all RHSs must evaluate to the same type of vector.

It might also throw errors.

dplyr::case_when(rlang::is_empty(x) ~ 1, !rlang::is_empty(x) ~ "2")
# Error in names(message) <- `*vtmp*` : 
#   'names' attribute [1] must be the same length as the vector [0]
dplyr::case_when(rlang::is_empty(x) ~ "1", !rlang::is_empty(x) ~ "2")
[1] "1"

These messages could for sure be more specific.

As mentioned, this does not explain what I think is a bug within the case_when source code, specifically validate_case_when_length:

m <- dplyr:::validate_case_when_length(query, value, fs, error_call = error_call)

which erroneously returns 0 in this case.

mycase_when <- function (...) 
{
  fs <- dplyr:::compact_null(rlang:::list2(...))
  n <- length(fs)
  error_call <- current_env()
  if (n == 0) {
    abort("No cases provided.", call = error_call)
  }
  query <- vector("list", n)
  value <- vector("list", n)
  default_env <- caller_env()
  quos_pairs <- dplyr:::map2(fs, seq_along(fs), dplyr:::validate_formula, default_env = default_env, 
                     dots_env = current_env(), error_call = error_call)
  for (i in seq_len(n)) {
    pair <- quos_pairs[[i]]
    query[[i]] <- eval_tidy(pair$lhs, env = default_env)
    value[[i]] <- eval_tidy(pair$rhs, env = default_env)
    if (!is.logical(query[[i]])) {
      abort_case_when_logical(pair$lhs, i, query[[i]], 
                              error_call = error_call)
    }
  }
  m <- dplyr:::validate_case_when_length(query, value, fs, error_call = error_call)
  
  print(paste('m',m))
  
  print(paste('value', value))
  print(paste('value[[1]][1]', value[[1]][1]))
  out <- value[[1]][rep(NA_integer_, m)]
  replaced <- rep(FALSE, m)
  print(paste('out 1',out))
  for (i in seq_len(n)) {
    out <- dplyr:::replace_with(out, query[[i]] & !replaced, value[[i]], 
                        NULL, error_call = error_call)
    replaced <- replaced | (query[[i]] & !is.na(query[[i]]))
  }
#  print(paste('out 2',out))
  out
}

mycase_when(rlang::is_empty(x) ~ "Empty", !rlang::is_empty(x) ~ "Not empty")
mycase_when(rlang::is_empty(x) ~ "Empty", !rlang::is_empty(x) ~ x)
mycase_when(rlang::is_empty(x) ~ "Empty", !rlang::is_empty(x) ~ "Not empty")
[1] "m 1"
[1] "value Empty"     "value Not empty"
[1] "value[[1]][1] Empty"
[1] "out 1 NA"
[1] "Empty"
mycase_when(rlang::is_empty(x) ~ "Empty", !rlang::is_empty(x) ~ x)
[1] "m 0"
[1] "value Empty"        "value character(0)"
[1] "value[[1]][1] Empty"
[1] "out 1 "
character(0)

But then that does not explain the unexpected result of `dplyr::case_when(rlang::is_empty(x) ~ "Empty", TRUE ~ x)`, does it? — Maël, Aug 16 '22 at 08:05

Why does only `case_when` give different results in R?

3 Answers3