2个方法都得到 vector .用df[,'var1']这种方法 对dataframe 选一个变量的时候就会得到vector. 选2个或以上就会变成dataframe。
建议用
library(dplyr)
classroom <- read.csv("http://www-personal.umich.edu/~bwest/classroom.csv")
schoolLookupVec <- unique(classroom[c("classid","schoolid")])%>%select("schoolid")

###############################################################
classroom <- read.csv("http://www-personal.umich.edu/~bwest/classroom.csv")
glimpse(classroom)  
schoolLookupVec <- unique(classroom2[c("classid","schoolid")])[,"schoolid"]
glimpse(schoolLookupVec)


classroom2 <- read.csv("classroom.csv")
glimpse(classroom2)  
schoolLookupVec2 <- unique(classroom2[c("classid","schoolid")])[,"schoolid"]
glimpse(schoolLookupVec2)

classroom2 <- read.csv("classroom.csv")
glimpse(classroom2)  
schoolLookupVec2 <- unique(classroom2[c("classid","schoolid")])[,"schoolid"]
glimpse(schoolLookupVec2)

我把csv换成excel后，用以上code还是得出的是dataframe，请问这是为什么？
谢谢！