Nested ifelse statement
if-statement, nested, r, sas
Solution
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.
Problem
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?