Nested ifelse statement

Question

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 nationality: idnat (french, foreigner),

If idnat is french then:

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

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

• status: k (mainland, overseas, foreigner)

All these variables use "character type".

Results expected in column idnat2 :

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

---

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; 

---

Here is my attempt in R:

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

I receive this warning:

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

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") 

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?

Answer 1

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

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

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

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

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.

In spreadsheet:

  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) 

In R:

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

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>)              )        ) 

To calculate column idnat2 you can:

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

What is the condition has length > 1 and only the first element will be used? Let's see:

> # 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 

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) 

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