Allow mutate() to choose the position of new columns

[krlmlr]

These arguments would specify the position where the new columns are inserted, before or after some column given as index or name.

See also tidyverse/tibble#99.

[krlmlr]

@hadley: What should the interface look like? Perhaps separate verbs?

[hadley]

I have no idea. I doesn't seem worthwhile to have a whole separate verb just for this, so maybe a .where argument to mutate()?

We'll need to think through what happens when you insert variables with the same name as existing variables:

df <- tibble(x = 1:5, y = 1:5)
df %>% mutate(y = x * 2, .where = 0) # error?
df %>% mutate(y = x * 2, .where = 1) # overridden by existing y?
df %>% mutate(y = x * 2, .where = 2) # overrides existing y

[krlmlr]

That's why I suggested a new verb, which could also forbid mutating existing columns. If we call it poke(), I'll be gently reminded of my C64 past once in a while ;-)

[hadley]

I'd prefer to keep it as an argument. We can just say it applies to creating new variables, not replacing existing variable.

[krlmlr]

We could do nicer things with the new rlang syntax:

tibble(x = 1, y = 2) %>% mutate("z" %after% "x" := 3)

@lionel-: Would that be parsed correctly w.r.t. operator precedence?

[lionel-]

yes because := has a very low precedence (just between ~ and <-)

[hadley]

To me, that seems overly complicated. I think 90% of the time people just want variables to go in the front, instead of in the back.

[krlmlr]

df %>% mutate(y = x * 2, .to_front = TRUE)

would be a fair bit easier to implement than the generic solution.

[krlmlr]

Best after #2311, otherwise we'd need to change two places.

[romainfrancois]

What's wrong with using a select after the mutate ?
imo, this goes against the simplicity of the interface.

[krlmlr]

If we're only interested in supporting adding to the front, maybe we could use transmute()?

@lionel-: What would it take to make the following syntax work?

library(tidyverse)
tibble(a = 2, b = 3) %>% transmute(x = 1, everything())
#> Error in mutate_impl(.data, dots): Evaluation error: No tidyselect variables were registered.

Created on 2018-03-30 by the reprex package (v0.2.0).