如何在R中使用if…else语句？

我正在尝试使用if语句创建一个变量。我想检查变量“st”是否存在于数据帧“dflist”列表中的数据帧中，如果它不存在，我想创建变量“st”。我试着这样做（但是，它不起作用）：

                    #making list of dataframes, and reading them into r
    mylist = list.files(pattern="*.dta")
    dflist <- lapply(mylist, read.dta13)

# if "st" exists in every dataframe in dflist, return "yes", else if it doesn't exist in a particular dataframe, create variable "st" in those dataframes

if(exists(st, dflist)){
    "yes"
  } else{
    st <- c("total")
    dflist$st <- st
  }

#制作数据帧列表，并将其读入r
mylist=list.files（pattern=“*.dta”）
dflist我们可以使用
lappy
在
列表上循环，如果
'st'不存在，则在'data.frame'
中创建一列

                    #making list of dataframes, and reading them into r
    mylist = list.files(pattern="*.dta")
    dflist <- lapply(mylist, read.dta13)

# if "st" exists in every dataframe in dflist, return "yes", else if it doesn't exist in a particular dataframe, create variable "st" in those dataframes

if(exists(st, dflist)){
    "yes"
  } else{
    st <- c("total")
    dflist$st <- st
  }

dflist1 <- lapply(dflist, function(x) if(!exists("st", x))
                      transform(x, st = "total") else x)

dflist1