如何在data.frame中找到元素的第一个和最后一个出现位置？

I have searched exhaustively for a direct R translation for the FIRST. and LAST. pointers in SAS DATA steps but can't seem to find one. For those not familiar with SAS, FIRST. is a boolean that identifies the first appearance of a given element in a table and LAST. is a boolean that identifies the last appearance. For instance, consider the following sorted table:

我已经详尽地搜索了FIRST的直接翻译。最后。 SAS DATA步骤中的指针但似乎无法找到一个。对于那些不熟悉SAS的人，FIRST。是一个布尔值，用于标识表和LAST中给定元素的第一个外观。是一个标识最后一个外观的布尔值。例如，考虑以下排序表：

V1    V2    V3
1     1     1
1     1     2
1     2     3
1     2     4
2     3     5
2     3     6
2     4     7
2     4     8
3     5     9
3     5     10
3     6     11
3     6     12

Because SAS DATA steps read tables line by line, I can use a statement like:

因为SAS DATA逐行读取表格，所以我可以使用如下语句：

IF FIRST.V1 THEN DO ...

FIRST.V1 will return TRUE if and only if this is the first time the observation has been encountered in V1. In other words, it will return true for V1[1] (the first appearance of '1'), V1[5] (the first appearance of '2'), and V1[9] (the first appearance of '3'). The LAST. pointer functions in analogous fashion, but with the final appearance of that element.

当且仅当这是第一次在V1中遇到观察时，FIRST.V1将返回TRUE。换句话说，对于V1 [1]（第一次出现'1'），V1 [5]（第一次出现'2'）和V1 [9]（第一次出现'3'）它将返回true ）。最后。指针以类似的方式起作用，但具有该元素的最终外观。

Is there anything in R that emulates this?

R中有什么模仿这个吗？

1 个解决方案

#1

You can do this with duplicated and rev (for LAST):

您可以使用duplicated和rev（对于LAST）执行此操作：

> v1=c(1,1,1,2,2,3,3,3,3,4,4,5)

> data.frame(v1,FIRST=!duplicated(v1),LAST=rev(!duplicated(rev(v1))))
   v1 FIRST  LAST
1   1  TRUE FALSE
2   1 FALSE FALSE
3   1 FALSE  TRUE
4   2  TRUE FALSE
5   2 FALSE  TRUE
6   3  TRUE FALSE
7   3 FALSE FALSE
8   3 FALSE FALSE
9   3 FALSE  TRUE
10  4  TRUE FALSE
11  4 FALSE  TRUE
12  5  TRUE  TRUE