14  Answering one way or another

when asks a question and gives the answer beside it, then asks the next one.

R

pupils <- data.frame(
  name = c("ann", "bob", "cat"),
  score = c(95, 75, 50),
  stringsAsFactors = FALSE
)

pupils |> add(band = when(score >= 90, "A", score >= 70, "B", otherwise = "C"))

name	score	band
ann	95	A
bob	75	B
cat	50	C

Python

pupils = pd.DataFrame({
    "name": ["ann", "bob", "cat"],
    "score": [95, 75, 50],
})

pupils >> add(band = when(col.score >= 90, "A", col.score >= 70, "B", otherwise = "C"))

name	score	band
ann	95	A
bob	75	B
cat	50	C

The arguments come in pairs: a question, then what it gives. The first question that is true wins, so the order is part of what the sentence means. Ask the same two the other way round and the answers change, which is not a trap but the thing you are choosing when you write them in an order:

R

pupils |> add(band = when(score >= 70, "B", score >= 90, "A", otherwise = "C"))

name	score	band
ann	95	B
bob	75	B
cat	50	C

Python

pupils >> add(band = when(col.score >= 70, "B", col.score >= 90, "A", otherwise = "C"))

name	score	band
ann	95	B
bob	75	B
cat	50	C

ann scored 95, met the first question that was asked, and got a B.

Leave out otherwise and a row that matched nothing is missing:

R

pupils |> add(top = when(score >= 90, "yes"))

name	score	top
ann	95	yes
bob	75	NA
cat	50	NA

Python

pupils >> add(top = when(col.score >= 90, "yes"))

name	score	top
ann	95	yes
bob	75	NaN
cat	50	NaN

Every answer has to be the same kind of thing, because they all end up in one column:

R

collect(pupils |> add(band = when(score >= 90, "A", otherwise = 0)))

Error:
! 
illegal: `when` gives one column, so all of its answers have to be the same kind of thing. One of them is text and this is a number
  |
2 |   then add [band] as when(([score] >= 90), "A", otherwise 0)
  |                                                           ^

Python

try:
    collect(pupils >> add(band = when(col.score >= 90, "A", otherwise = 0)))
except GodError as refusal:
    print(refusal)

illegal: `when` gives one column, so all of its answers have to be the same kind of thing. One of them is text and this is a number
  |
2 |   then add [band] as when(([score] >= 90), "A", otherwise 0)
  |                                                           ^

Python has a conditional of its own, and it is worth saying why god does not use it. Writing "A" if col.score >= 90 else "B" would decide the answer once, while the pipeline was being built, and throw the question away. Nothing would report a problem. R has no such expression at all. So the word is when in both, and it is the same word in the text form.