pivot_longer only returns a subset of data from cols

[tdhock]

I'm using pivot_longer with names_pattern, and the number of rows returned depends on the names_to/names_pattern arguments. I expected that the number of rows returns should only depend on cols.

For example consider two columns of the iris data. I expected that both of the following would result in 8 rows.

tidyr::pivot_longer(iris[c(1,51),], 1:4, names_to="part", names_pattern="(.*)[.].*")
tidyr::pivot_longer(iris[c(1,51),], 1:4, names_to=c("part", "dim"), names_pattern="(.*)[.](.*)")

However the first one only returns 4 rows (only the Width data, not the Length data).

> tidyr::pivot_longer(iris[c(1,51),], 1:4, names_to="part", names_pattern="(.*)[.].*")
# A tibble: 4 x 3
  Species    part  value
  <fct>      <chr> <dbl>
1 setosa     Sepal   3.5
2 setosa     Petal   0.2
3 versicolor Sepal   3.2
4 versicolor Petal   1.4
> tidyr::pivot_longer(iris[c(1,51),], 1:4, names_to=c("part", "dim"), names_pattern="(.*)[.](.*)")
# A tibble: 8 x 4
  Species    part  dim    value
  <fct>      <chr> <chr>  <dbl>
1 setosa     Sepal Length   5.1
2 setosa     Sepal Width    3.5
3 setosa     Petal Length   1.4
4 setosa     Petal Width    0.2
5 versicolor Sepal Length   7  
6 versicolor Sepal Width    3.2
7 versicolor Petal Length   4.7
8 versicolor Petal Width    1.4
> 

I would have expected a warning, something like "you asked to pivot columns 1:4 but we only are returning data from columns 2 and 4"

[hadley]

Slightly simpler reprex:

library(tidyr)
df <- tibble(g = 1:2, x1 = 1:2, y1 = 1:2, x2 = 1:2)

df %>% pivot_longer(-g)
#> # A tibble: 6 x 3
#>       g name  value
#>   <int> <chr> <int>
#> 1     1 x1        1
#> 2     1 y1        1
#> 3     1 x2        1
#> 4     2 x1        2
#> 5     2 y1        2
#> 6     2 x2        2
df %>% pivot_longer(-g, names_pattern = "(..)")
#> # A tibble: 6 x 3
#>       g name  value
#>   <int> <chr> <int>
#> 1     1 x1        1
#> 2     1 y1        1
#> 3     1 x2        1
#> 4     2 x1        2
#> 5     2 y1        2
#> 6     2 x2        2
df %>% pivot_longer(-g, names_pattern = "(.).")
#> # A tibble: 4 x 3
#>       g name  value
#>   <int> <chr> <int>
#> 1     1 x         1
#> 2     1 y         1
#> 3     2 x         2
#> 4     2 y         2

df %>% pivot_longer(-g, names_to = c("x", "num"), names_pattern = "(.)(.)")
#> # A tibble: 6 x 4
#>       g x     num   value
#>   <int> <chr> <chr> <int>
#> 1     1 x     1         1
#> 2     1 y     1         1
#> 3     1 x     2         1
#> 4     2 x     1         2
#> 5     2 y     1         2
#> 6     2 x     2         2

^{Created on 2019-11-27 by the reprex package (v0.3.0)}

[hadley]

I think it's a little easier to understand the problem by looking at the specs:

library(tidyr)
df <- tibble(g = 1:2, x1 = 1:2, y1 = 1:2, x2 = 1:2)

# OK: the additional columns uniquely identify each row
df %>% build_longer_spec(-g)
#> # A tibble: 3 x 3
#>   .name .value name 
#>   <chr> <chr>  <chr>
#> 1 x1    value  x1   
#> 2 y1    value  y1   
#> 3 x2    value  x2
df %>% build_longer_spec(-g, names_pattern = "(..)")
#> # A tibble: 3 x 3
#>   .name .value name 
#>   <chr> <chr>  <chr>
#> 1 x1    value  x1   
#> 2 y1    value  y1   
#> 3 x2    value  x2
df %>% build_longer_spec(-g, names_to = c("x", "num"), names_pattern = "(.)(.)")
#> # A tibble: 3 x 4
#>   .name .value x     num  
#>   <chr> <chr>  <chr> <chr>
#> 1 x1    value  x     1    
#> 2 y1    value  y     1    
#> 3 x2    value  x     2

# NOT OK: the additional columns don't uniquely identify each row
df %>% build_longer_spec(-g, names_pattern = "(.).")
#> # A tibble: 3 x 3
#>   .name .value name 
#>   <chr> <chr>  <chr>
#> 1 x1    value  x    
#> 2 y1    value  y    
#> 3 x2    value  x

^{Created on 2019-12-06 by the reprex package (v0.3.0)}

[hadley]

Just to circle back to the motivating example, this is the result with the dev version:

tidyr::pivot_longer(iris[c(1,51),], 1:4, names_to="part", names_pattern="(.*)[.].*")
#> # A tibble: 8 x 3
#>   Species    part  value
#>   <fct>      <chr> <dbl>
#> 1 setosa     Sepal   5.1
#> 2 setosa     Sepal   3.5
#> 3 setosa     Petal   1.4
#> 4 setosa     Petal   0.2
#> 5 versicolor Sepal   7  
#> 6 versicolor Sepal   3.2
#> 7 versicolor Petal   4.7
#> 8 versicolor Petal   1.4

^{Created on 2019-12-07 by the reprex package (v0.3.0)}