R在發生狀態轉換的數據框中查找行？

Question

我有以下的數據幀，

id, date, state 
1 2012-01-01 a 
1 2012-01-02 a 
1 2012-01-03 a 
1 2012-01-04 b 
1 2012-01-05 b 
2 2013-01-01 a 
2 2013-01-02 a 
2 2013-01-03 b 
2 2013-01-04 b 

每個ID，我想當狀態從a到b以下，我想它插入作爲該ID的列改變到發現的日期。所以上面的例子會收益

id, date, state, changedate 
1 2012-01-01 a 2012-01-03 
1 2012-01-02 a 2012-01-03 
1 2012-01-03 a 2012-01-03 
1 2012-01-04 b 2012-01-03 
1 2012-01-05 b 2012-01-03 
2 2013-01-01 a 2013-01-02 
2 2013-01-02 a 2013-01-02 
2 2013-01-03 b 2013-01-02 
2 2013-01-04 b 2013-01-02 

有沒有辦法通過plyr函數或甚至在基礎R優雅地做到這一點？ 在此先感謝。

Answer 1

編輯：正如塞巴斯蒂安提到的那樣，我認爲data.frame是按列date排序的。

許多解決方案之一。可能棘手的一點是找到過渡期。這可以在rle的幫助下完成。

rle.df <- rle(df$state) 
# get indices of a-to-b transition -> 3,7 
idx <- cumsum(rle.df$lengths)[c(TRUE, FALSE)] 
# get indices of b-to-a transition -> 5,9 
idx2 <- cumsum(rle.df$lengths)[c(FALSE, TRUE)] 
# construct appropriate lengths -> 5,4 
idx2 <- c(idx2[1], diff(idx2)) 
# do a rep with idx2 fro times and df$date[idx] for value 
df$changedate <- unlist(lapply(1:length(idx2), function(vv) { 
    rep(df$date[idx[vv]], idx2[vv]) 
})) 

> df 
    id.  date. state changedate 
1 1 2012-01-01  a 2012-01-03 
2 1 2012-01-02  a 2012-01-03 
3 1 2012-01-03  a 2012-01-03 
4 1 2012-01-04  b 2012-01-03 
5 1 2012-01-05  b 2012-01-03 
6 2 2013-01-01  a 2013-01-02 
7 2 2013-01-02  a 2013-01-02 
8 2 2013-01-03  b 2013-01-02 
9 2 2013-01-04  b 2013-01-02 

使用data.table（我只注意到你也有.id.列，使我們可以拆分，並通過rle發現過渡指標申請之日起）的替代解決方案。

require(data.table) 
rle.df <- rle(df$state) 
idx <- cumsum(rle.df$lengths)[c(TRUE, FALSE)] 
idx2 <- cumsum(rle.df$lengths)[c(FALSE, TRUE)] 
idx <- c(idx[1], tail(idx, -1) - head(idx2, -1)) 

dt <- data.table(df, key="id.") 
out <- dt[, `:=`(changedate=date.[idx[id.]]), by=id.] 

> out 
    id.  date. state changedate 
1: 1 2012-01-01  a 2012-01-03 
2: 1 2012-01-02  a 2012-01-03 
3: 1 2012-01-03  a 2012-01-03 
4: 1 2012-01-04  b 2012-01-03 
5: 1 2012-01-05  b 2012-01-03 
6: 2 2013-01-01  a 2013-01-02 
7: 2 2013-01-02  a 2013-01-02 
8: 2 2013-01-03  b 2013-01-02 
9: 2 2013-01-04  b 2013-01-02