删除零行小标题的列表元素

Question

我有一个 tibbles 列表。我正在尝试过滤所有小标题共有的列，然后删除任何以零行结尾的小标题（但在技术上不是空的，因为它们有列）。 purrr:::compact() 似乎是为此目的而设计的，但我认为我说得不太对。有没有更好的解决方案？

require(tidyverse)
#> Loading required package: tidyverse
mylst <- lst(cars1 = cars %>% as.tibble(), cars2 = cars %>% as.tibble() %>% mutate(speed = speed + 100))

#This produces a list with zero-row tibble elements:
mylst %>% map(function(x) filter(x, speed == 125))
#> $cars1
#> # A tibble: 0 x 2
#> # ... with 2 variables: speed <dbl>, dist <dbl>
#> 
#> $cars2
#> # A tibble: 1 x 2
#>   speed  dist
#>   <dbl> <dbl>
#> 1  125.   85.

#This results in the same thing:
mylst %>% map(function(x) filter(x, speed == 125)) %>% compact()
#> $cars1
#> # A tibble: 0 x 2
#> # ... with 2 variables: speed <dbl>, dist <dbl>
#> 
#> $cars2
#> # A tibble: 1 x 2
#>   speed  dist
#>   <dbl> <dbl>
#> 1  125.   85.

#Putting compact inside the map function reduces $cars1 to 0x0, but it's still there:
mylst %>% map(function(x) filter(x, speed == 125) %>% compact())
#> $cars1
#> # A tibble: 0 x 0
#> 
#> $cars2
#> # A tibble: 1 x 2
#>   speed  dist
#>   <dbl> <dbl>
#> 1  125.   85.

#This finally drops the empty element, but seems clumsy. 
mylst %>% map(function(x) filter(x, speed == 125) %>% compact()) %>% compact()
#> $cars2
#> # A tibble: 1 x 2
#>   speed  dist
#>   <dbl> <dbl>
#> 1  125.   85.

由reprex package (v0.2.0) 于 2018 年 4 月 6 日创建。

Answer 1

您正在尝试使用compact，但这只会过滤掉NULL 元素。要过滤掉零行元素，可以使用discard：

mylst %>% 
    map(function(x) filter(x, speed == 125)) %>% 
    discard(function(x) nrow(x) == 0)
#$cars2
## A tibble: 1 x 2
#  speed  dist
#  <dbl> <dbl>
#1  125.   85.