如何找到满足条件的组中的第一个元素 [英] How to find first element of a group that fulfill a condition

问题描述

structure(list(group = c(17L, 17L, 17L, 18L, 18L, 18L, 18L, 19L, 
19L, 19L, 20L, 20L, 20L, 21L, 21L, 22L, 23L, 24L, 25L, 25L, 25L, 
26L, 27L, 27L, 27L, 28L), var = c(74L, 49L, 1L, 74L, 1L, 49L, 
61L, 49L, 1L, 5L, 5L, 1L, 44L, 44L, 12L, 13L, 5L, 5L, 1L, 1L, 
4L, 4L, 1L, 1L, 1L, 49L), first = c(0, 0, 1, 0, 1, 0, 0, 0, 1, 
0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0)), .Names = c("group", 
"var", "first"), class = c("tbl_df", "tbl", "data.frame"), row.names = c(NA, 
-26L))

使用第一个数据两列我想创建第三列（称为 first ），其中仅当<$ c时 first == 1 $ c> var == 1 在组中第一次。换句话说，我想标记组中的第一个元素，其完全填充 var == 1 。如何在 dplyr 中做到这一点？当然应该使用 group_by ，但是下一步是什么？

With the data from the first two column I would like to create a third column (called first) where first == 1 only when var == 1 for the first time in a group. In other words I would like to mark first elements within group that fullfil var == 1. How can I do that in dplyr? Certainly group_by should be used but what next?

推荐答案

用于未分组的数据，一个解决方案是

For ungrouped data, one solution is

first_equal_to = function(x, value)
    (x == value) & (cumsum(x == value) == 1)

so

tbl %>% group_by(group) %>% mutate(first = first_equal_to(var, 1))

（将其保留为逻辑向量似乎很合适，因为这就是该列所代表的意思）。

(it seems appropriate to keep this as a logical vector, since that is what the column represents).

另一种实现方式是

first_equal_to2 = function(x, value) {
    result = logical(length(x))
    result[match(value, x)] = TRUE
    result
}

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