这是我的数据：
ITEM <- c("A","A","A","B","B","B","B","C","C","D","D","E","E","F","G","G","G")
LOCATION <- c("aaa","bbb","ccc","bbb","fff","ggg","zzz","zzz","eee","hhh","iii","kkk","jjj","iii","iii","yyy","xxx")
df <- as.data.frame(cbind(ITEM,LOCATION))
Long Form:
ITEM LOCATION
1 A aaa
2 A bbb
3 A ccc
4 B bbb
5 B fff
6 B ggg
7 B zzz
8 C zzz
9 C eee
10 D hhh
11 D iii
12 E kkk
13 E jjj
14 F iii
15 G iii
16 G yyy
17 G xxx
宽格式（更易于阅读）：
ITEM LOCATION.1 LOCATION.2 LOCATION.3 LOCATION.4
A aaa bbb ccc <NA>
B bbb fff ggg …