r zoo包rollapply函数

Question

Beloe是我的测试数据和代码，通过计算正数值来汇总tbl表。然后使用rollapply和FUN sum添加5个连续的行计数。我在1,2 - 5,6,7,8 - 10,11行获得NA。

NA在5,6和10,11是由于缺少下一行而预期但我不明白为什么我在第1,2和7,8行获得NA。有些人可以看看代码并指出我的错误吗？

library(tidyverse)
library(zoo)

tbl<-tribble(
  ~z, ~x, ~y,
  "x", "a", 2,
  "x", "b", 1,
  "x", "b", 3,
  "y", "c", 3,
  "x", "c", 1,
  "x", "d", -1,
  "x", "q", 2,
  "x", "q", 2,
  "x", "a", 2,
  "x", "s", -1,
  "y", "q", -1,
  "y", "b", 3,
  "x", "c", 3,
  "y", "c", -1,
  "y", "q", 1,
  "y", "w", 2,
  "y", "w", -2,
  "y", "t", 2,
  "y", "t", 1
)

tbl %>%
  group_by(z, x) %>%
  summarise(xy = sum(y>0, na.rm = T))%>%
  mutate(zzz = rollapply(xy, width=5, sum, fill=NA))

输出：

# A tibble: 11 x 4
# Groups:   z [2]
       z     x    xy   zzz
   <chr> <chr> <int> <dbl>
 1     x     a     2    NA
 2     x     b     2    NA
 3     x     c     2     8
 4     x     d     0     6
 5     x     q     2    NA
 6     x     s     0    NA
 7     y     b     1    NA
 8     y     c     1    NA
 9     y     q     1     6
10     y     t     2    NA
11     y     w     1    NA