嵌套的ifelse语句

Question

我还在学习如何将SAS代码翻译成R,然后收到警告.我需要了解我犯错误的地方.我想要做的是创建一个变量来总结和区分人口的3种状态:大陆,海外,外国人.我有一个包含2个变量的数据库:

• 国籍:( idnat法国人,外国人),

如果idnat是法国人那么:

• id出生地:( idbp大陆,殖民地,海外)

我想从汇总信息idnat,并idbp进入一个所谓的新变量idnat2:

• 状态:k(大陆,海外,外国人)

所有这些变量都使用"字符类型".

列idnat2中预期的结果:

   idnat     idbp   idnat2
1  french mainland mainland
2  french   colony overseas
3  french overseas overseas
4 foreign  foreign  foreign

---

这是我要在R中翻译的SAS代码:

if idnat = "french" then do;
   if idbp in ("overseas","colony") then idnat2 = "overseas";
   else idnat2 = "mainland";
end;
else idnat2 = "foreigner";
run;

---

这是我在R中的尝试:

if(idnat=="french"){
    idnat2 <- "mainland"
} else if(idbp=="overseas"|idbp=="colony"){
    idnat2 <- "overseas"
} else {
    idnat2 <- "foreigner"
}

我收到这个警告:

Warning message:
In if (idnat=="french") { :
  the condition has length > 1 and only the first element will be used

我被建议使用"嵌套ifelse"代替它的容易性,但获得更多警告:

idnat2 <- ifelse (idnat=="french", "mainland",
        ifelse (idbp=="overseas"|idbp=="colony", "overseas")
      )
            else (idnat2 <- "foreigner")

根据警告消息,长度大于1,因此只考虑第一个括号之间的长度.对不起,但我不明白这个长度与这里有什么关系？谁知道我哪里错了？

Answer 1

如果您使用的是任何电子表格应用程序,则有一个基本的if()语法函数:

if(<condition>, <yes>, <no>)

ifelse()R中的语法完全相同:

ifelse(<condition>, <yes>, <no>)

if()电子表格应用程序的唯一区别是R ifelse()是矢量化的(将向量作为输入并在输出上作为返回向量).考虑以下电子表格应用程序和R中公式的比较,我们希望比较a> b,如果是,则返回1,否则返回0.

在电子表格中:

  A  B C
1 3  1 =if(A1 > B1, 1, 0)
2 2  2 =if(A2 > B2, 1, 0)
3 1  3 =if(A3 > B3, 1, 0)

在R:

> a <- 3:1; b <- 1:3
> ifelse(a > b, 1, 0)
[1] 1 0 0

ifelse() 可以通过多种方式嵌套:

ifelse(<condition>, <yes>, ifelse(<condition>, <yes>, <no>))

ifelse(<condition>, ifelse(<condition>, <yes>, <no>), <no>)

ifelse(<condition>, 
       ifelse(<condition>, <yes>, <no>), 
       ifelse(<condition>, <yes>, <no>)
      )

ifelse(<condition>, <yes>, 
       ifelse(<condition>, <yes>, 
              ifelse(<condition>, <yes>, <no>)
             )
       )

要计算列,idnat2您可以:

df <- read.table(header=TRUE, text="
idnat idbp idnat2
french mainland mainland
french colony overseas
french overseas overseas
foreign foreign foreign"
)

with(df, 
     ifelse(idnat=="french",
       ifelse(idbp %in% c("overseas","colony"),"overseas","mainland"),"foreign")
     )

R文档

什么是the condition has length > 1 and only the first element will be used？让我们来看看:

> # What is first condition really testing?
> with(df, idnat=="french")
[1]  TRUE  TRUE  TRUE FALSE
> # This is result of vectorized function - equality of all elements in idnat and 
> # string "french" is tested.
> # Vector of logical values is returned (has the same length as idnat)
> df$idnat2 <- with(df,
+   if(idnat=="french"){
+   idnat2 <- "xxx"
+   }
+   )
Warning message:
In if (idnat == "french") { :
  the condition has length > 1 and only the first element will be used
> # Note that the first element of comparison is TRUE and that's whay we get:
> df
    idnat     idbp idnat2
1  french mainland    xxx
2  french   colony    xxx
3  french overseas    xxx
4 foreign  foreign    xxx
> # There is really logic in it, you have to get used to it

我还能用if()吗？是的,你可以,但语法不是那么酷:)

test <- function(x) {
  if(x=="french") {
    "french"
  } else{
    "not really french"
  }
}

apply(array(df[["idnat"]]),MARGIN=1, FUN=test)

如果您熟悉SQL,您还可以使用CASE 声明的sqldf 包.

Answer 2

尝试以下内容:

# some sample data
idnat <- sample(c("french","foreigner"),100,TRUE)
idbp <- rep(NA,100)
idbp[idnat=="french"] <- sample(c("mainland","overseas","colony"),sum(idnat=="french"),TRUE)

# recoding
out <- ifelse(idnat=="french" & !idbp %in% c("overseas","colony"), "mainland",
              ifelse(idbp %in% c("overseas","colony"),"overseas",
                     "foreigner"))
cbind(idnat,idbp,out) # check result

您的困惑来自于SAS和R如何处理if-else结构.在R中,if并且else没有矢量化,这意味着它们检查单个条件是否为真(即if("french"=="french")工作)并且不能处理多个逻辑(即,if(c("french","foreigner")=="french")不起作用)并且R为您提供您正在接收的警告.

相比之下,它ifelse是矢量化的,所以它可以采用你的向量(也就是输入变量)并测试每个元素的逻辑条件,就像你在SAS中习惯的那样.围绕这个问题的另一种方法是使用if和else语句构建一个循环(正如你在这里开始做的那样),但是矢量化ifelse方法将更有效并且通常涉及更少的代码.

Answer 3

如果数据集包含许多行,则使用查找表data.table而不是嵌套来加入查找表可能更有效ifelse().

提供下面的查找表

lookup

     idnat     idbp   idnat2
1:  french mainland mainland
2:  french   colony overseas
3:  french overseas overseas
4: foreign  foreign  foreign

和样本数据集

library(data.table)
n_row <- 10L
set.seed(1L)
DT <- data.table(idnat = "french",
                 idbp = sample(c("mainland", "colony", "overseas", "foreign"), n_row, replace = TRUE))
DT[idbp == "foreign", idnat := "foreign"][]

      idnat     idbp
 1:  french   colony
 2:  french   colony
 3:  french overseas
 4: foreign  foreign
 5:  french mainland
 6: foreign  foreign
 7: foreign  foreign
 8:  french overseas
 9:  french overseas
10:  french mainland

然后我们可以在加入时进行更新:

DT[lookup, on = .(idnat, idbp), idnat2 := i.idnat2][]

      idnat     idbp   idnat2
 1:  french   colony overseas
 2:  french   colony overseas
 3:  french overseas overseas
 4: foreign  foreign  foreign
 5:  french mainland mainland
 6: foreign  foreign  foreign
 7: foreign  foreign  foreign
 8:  french overseas overseas
 9:  french overseas overseas
10:  french mainland mainland

Answer 4

你可以idnat2不用if和创建矢量ifelse.

该功能replace可用于替代所有出现的"colony"有"overseas":

idnat2 <- replace(idbp, idbp == "colony", "overseas")

Answer 5

将 SQL CASE 语句与 dplyr 和 sqldf 包一起使用：

数据

df <-structure(list(idnat = structure(c(2L, 2L, 2L, 1L), .Label = c("foreign", 
"french"), class = "factor"), idbp = structure(c(3L, 1L, 4L, 
2L), .Label = c("colony", "foreign", "mainland", "overseas"), class = "factor")), .Names = c("idnat", 
"idbp"), class = "data.frame", row.names = c(NA, -4L))

sqldf

library(sqldf)
sqldf("SELECT idnat, idbp,
        CASE 
          WHEN idbp IN ('colony', 'overseas') THEN 'overseas' 
          ELSE idbp 
        END AS idnat2
       FROM df")

dplyr

library(dplyr)
df %>% 
mutate(idnat2 = case_when(idbp == 'mainland' ~ "mainland", 
                          idbp %in% c("colony", "overseas") ~ "overseas", 
                         TRUE ~ "foreign"))

输出

    idnat     idbp   idnat2
1  french mainland mainland
2  french   colony overseas
3  french overseas overseas
4 foreign  foreign  foreign