突變和ifelse（）時的列的邏輯檢查尾部頭值

Question

我有一個數據幀和失敗我想由條件檢查另一column.If的tail和head值條件爲匹配來添加新的列ex，插入ex1或ex2添加到新列。 這裏是我的意思

x = rep(c(seq(1,3),seq(2,3)),2) 
grp=gl(2,5) 
df <- data.frame(x,grp) 


#  x grp 
# 1 1 1 
# 2 2 1 
# 3 3 1 
# 4 2 1 
# 5 3 1 
# 6 1 2 
# 7 2 2 
# 8 3 2 
# 9 2 2 
# 10 3 2 

我試圖

庫（dplyr）

dff <- df%>% 
group_by(grp)%>% 
mutate(ex=ifelse(tail(x,2)==c(2,3),tail(ex,2)="ex2"&head(ex,3)="ex1",NA)) 

>Error: unexpected '=' in: 
"group_by(grp)%>% 
mutate(ex=ifelse(tail(x,2)==c(2,3),tail(ex,2)=" 

預期輸出

#   x grp ex 
    # 1 1 1 ex1 
    # 2 2 1 ex1 
    # 3 3 1 ex1 
    # 4 2 1 ex2 
    # 5 3 1 ex2 
    # 6 1 2 ex1 
    # 7 2 2 ex1 
    # 8 3 2 ex1 
    # 9 2 2 ex2 
    # 10 3 2 ex2 

我怎麼能這樣做？

Answer 1

我們可以data.table

library(data.table) 
i1 <- setDT(df)[, .I[tail(seq_len(.N),2)] , grp]$V1 
df[, ex:= "ex1"] 
df[df[i1, .I[x==c(2,3)] , grp]$V1, ex := "ex2"] 
df 
# x grp ex 
#1: 1 1 ex1 
#2: 2 1 ex1 
#3: 3 1 ex1 
#4: 2 1 ex2 
#5: 3 1 ex2 
#6: 1 2 ex1 
#7: 2 2 ex1 
#8: 3 2 ex1 
#9: 2 2 ex2 
#10:3 2 ex2 

---

或者使用dplyr

library(dplyr) 
df %>% 
    group_by(grp) %>% 
    mutate(x1 = rep(c(0, 2,3), c(n()-2, 1, 1)), 
      ex = if_else(x1==x & row_number() %in% tail(row_number(), 2), "ex2", "ex1")) %>% 
    select(-x1) 
#  x grp ex 
# <int> <fctr> <chr> 
#1  1  1 ex1 
#2  2  1 ex1 
#3  3  1 ex1 
#4  2  1 ex2 
#5  3  1 ex2 
#6  1  2 ex1 
#7  2  2 ex1 
#8  3  2 ex1 
#9  2  2 ex2 
#10  3  2 ex2