Behaviour of `df[condition, "colname"] <- value` differs for tibbles and data frames

[hongooi73]

When the condition in a subsetting assignment is entirely FALSE, this expression results in a column of NAs for a data frame, and no additional column for a tibble.

For a data frame:

df <- data.frame(a=1:3)
df[df$a < 0, "b"] <- 1

  a  b
1 1 NA
2 2 NA
3 3 NA

For a tibble:

tbl <- tibble(a=1:3)
tbl[tbl$a < 0, "b"] <- 1

# A tibble: 3 x 1
      a
  <int>
1     1
2     2
3     3

Is this intended behaviour?

[krlmlr]

Indeed, this is weird:

library(tibble)

tbl <- tibble(a = 1:3)
tbl[tbl$a < 0, "b"] <- 1
tbl
#> # A tibble: 3 x 1
#>       a
#>   <int>
#> 1     1
#> 2     2
#> 3     3
tbl[tbl$a > 0, "c"] <- 2
tbl
#> # A tibble: 3 x 2
#>       a     c
#>   <int> <dbl>
#> 1     1     2
#> 2     2     2
#> 3     3     2
tbl[tbl$a > 2, "d"] <- 3
tbl
#> # A tibble: 3 x 3
#>       a     c     d
#>   <int> <dbl> <dbl>
#> 1     1     2    NA
#> 2     2     2    NA
#> 3     3     2     3

^{Created on 2021-01-30 by the reprex package (v0.3.0)}

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