嵌套的ifelse语句 [英] Nested ifelse statement

问题描述

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：

I'm still learning how to translate a SAS code into R and I get warnings. I need to understand where I'm making mistakes. What I want to do is create a variable which summarizes and differentiates 3 status of a population: mainland, overseas, foreigner. I have a database with 2 variables:

• id国籍： idnat （法国人，外国人），

• id nationality: idnat (french, foreigner),

如果 idnat 是法国人那么：

• id birthplace： idbp （大陆，殖民地，海外）

• id birthplace: idbp (mainland, colony, overseas)

我想总结来自 idnat 和 idbp 进入名为的新变量idnat2 ：

I want to summarize the info from idnat and idbp into a new variable called idnat2:

• 状态：k（大陆，海外，外国人）

所有这些变量都使用字符类型。

All these variables use "character type".

预计列idnat2中的结果：

Results expected in column idnat2 :

   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：

---

Here is my SAS code I want to translate in R:

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：

---

Here is my attempt in R:

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

我收到此警告：

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 而不是因为它的容易性而是获得更多警告：

I was advised to use a "nested ifelse" instead for its easiness but get more warnings:

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

根据警告信息，长度大于1，所以只有在将考虑第一个括号。对不起，但我不明白这个长度与这里有什么关系？有人知道我哪里错了吗？

According to the Warning message, the length is greater than 1 so only what's between the first brackets will be taken into account. Sorry but I don't understand what this length has to do with here? Anybody know where I'm wrong?

推荐答案

如果您使用的是任何电子表格应用程序，那么基本功能 if（）语法：

If you are using any spreadsheet application there is a basic function if() with syntax:

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

ifelse（）<的语法完全相同/ code>在R中：

Syntax is exactly the same for ifelse() in R:

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

与的唯一区别if（） in电子表格应用程序是R ifelse（）被向量化（将向量作为输入并在输出时返回向量）。考虑以下电子表格应用程序和R中的公式比较，我们希望比较a> b，如果是则返回1，如果不是则返回0。

The only difference to if() in spreadsheet application is that R ifelse() is vectorized (takes vectors as input and return vector on output). Consider the following comparison of formulas in spreadsheet application and in R for an example where we would like to compare if a > b and return 1 if yes and 0 if not.

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中：

> 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() can be nested in many ways:

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 Documentation

什么是条件有长度> 1，只使用第一个元素？让我们看看：

> # 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

我还能使用如果（）？是的，你可以，但语法不是很酷：）

Can I still use if()? Yes, you can, but the syntax is not so cool :)

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，还可以使用 CASE sqldf 中的-syntax>声明 包。

If you are familiar with SQL, you can also use CASE statement in sqldf package.

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