Spreading NA keys makes NA column names

[gregmacfarlane]

Trying to create a cross tabulation using tally() combined with spread() works nicely, unless there are missing values in the key columns.

mtcars %>% 
  group_by(cyl, gear) %>%
  tally() %>%
  spread(gear, n, fill = 0)
Source: local data frame [3 x 4]

  cyl  3 4 5
1   4  1 8 2
2   6  2 4 1
3   8 12 0 2

mtcars %>%
  mutate(
    cyl = ifelse(cyl > 6, NA, cyl),
    gear = ifelse(gear > 4, NA, gear)
  ) %>%
  group_by(cyl, gear) %>%
  tally()
Source: local data frame [8 x 3]
Groups: cyl

  cyl gear  n
1   4    3  1
2   4    4  8
3   4   NA  2
4   6    3  2
5   6    4  4
6   6   NA  1
7  NA    3 12
8  NA   NA  2

# DITTO # %>%
  spread(gear, n)
Error in if (any(names2(x) == "")) { : 
  missing value where TRUE/FALSE needed

It seems like a good solution would be to either treat NA values as another key or to drop them from the resulting table, controlled by an option switch. Like useNA = "always" in the base table() function, but with better syntax.

[zross]

Thanks for adding this issue. I second it. At a minimum a warning "There cannot be NA values in the key variable" would be useful. As always, thanks for this very useful package Hadley.

[hadley]

Simpler example:

data.frame(x = c(1, 2, NA), y = 1, z = c(1, 2, 3)) %>%
  tbl_df() %>%
  spread(x, z)

Note that it's actually a dplyr problem since this works:

data.frame(x = c(1, 2, NA), y = 1, z = c(1, 2, 3)) %>%
  spread(x, z)

Should fix by turning NA column names into something valid <NA> ?

[zross]

I think that makes sense. Or an argument that allows the user to decide along the lines of useNA=c(TRUE, FALSE) and if TRUE then turn NA to something valid.

[jennybc]

I just tripped on this same thing. Before I found this issue, I had made my own example, so will include here anyway. As for what to name the columns, it seems like an option to prepend the key name would be useful here and more generally.

jtxt <- "
name, grp,    sex, age
 joe,   1,   male,  30
 jan,   1,     NA,  43
 jen,   1, female,  44
 pat,   2,     NA,  24
 pam,   2, female,  63
 peg,   2, female,  50
"
jdat <- read.csv(text = jtxt, stringsAsFactors = FALSE, strip.white = TRUE)

Cross-tabulating and spreading with (sex, grp) works:

> jdat %>%
+ count(sex, grp) %>%
+ spread(grp, n, fill = 0)
Source: local data frame [3 x 3]

     sex 1 2
1 female 1 2
2   male 1 0
3     NA 1 1

But going the other way round, (grp, sex), does not:

> jdat %>%
+ count(grp, sex) %>%
+ spread(sex, n, fill = 0)
Error in if (any(names2(x) == "")) { : 
  missing value where TRUE/FALSE needed

To illustrate my proposal re column names, here's possible output:

grp sex_female sex_male sex_NA
  1          1        1      1
  2          2        0      1

The prepending would fix up the value of NA as a column name and would also help when the values are cryptic, e.g. naked integers like cyl and gear from mtcars.

[gregmacfarlane]

@jennybc I like the idea of prepending. It will resolve the problem where kable(spread()) needs some in-text explanation as to what it is showing.

grp          1        2      3
  1          1        1      1
  2          2        0      1

[hadley]

@jennybc Seems like most of the time you'll want the key name prepended if the value is non-character, but most of the time you won't want it prepended if it is a character. So maybe something like include_key = !is.character(value) ?

[jennybc]

Yes, @hadley, I think you're right about when prepending the key is most desirable. That, together with the ability to have a variable/column named <NA>, would represent a nice gain in functionality for spread().

[hadley]

This no longer fails for me, presumably because dplyr does a better job of handling columns names that are missing.

I'm a bit leery of automatically changing the column names, because traditionally that's been done in a separate step (which is long but makes things more explicit, and sticks to the principle of each function doing one thing).

[hadley]

Implemented with sep argument. If anyone strongly dislikes this API, please speak up ASAP!

data_frame(x = c(1, NA), y = 1:2) %>% 
  spread(x, y) %>% 
  names()
#> [1] "1"    "<NA>"

data_frame(x = c(1, NA), y = 1:2) %>% 
  spread(x, y, sep = "_") %>% 
  names()
#> [1] "x_1"  "x_NA"