感謝大大

用 dplyr 或 sqldf

xdata <- data.frame(name = c("A", "B", "C"), cost = c(7.5, 9, 6.5))
ydata <- data.frame(index = 1:9, name = c("A", "B", "C", "A", "A", "C", "B", "C", "A"), amount = c(10,14,3,4,15,9,12,6,7))

# use dplyr
install.packages('dplyr')
library(dplyr)
zdata=ydata%>%left_join(xdata)
zdata

# use sqldf
install.packages('sqldf')
library(sqldf)
zdata=sqldf('select a.*,b.* from ydata a left join xdata b on a.name=b.name')
zdata

聽過dplyr包處理資料速度較快, 這個要存記. 也感謝你的熱心分享