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  lt -  mainland    else if idbp   overseas  idbp   colony        idnat2  lt -  overseas    else       idnat2  lt -  foreigner      I receive this warning   Warning message  In if  idnat   french         the condition has length  gt  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  lt - ifelse  idnat   french    mainland           ifelse  idbp   overseas  idbp   colony    overseas                       else  idnat2  lt -  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

User · Answer

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.

User · Answer

Using the SQL CASE statement with the dplyr and sqldf packages   Data  df  lt -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   gt    mutate idnat2   case when   idbp     mainland     mainland                                idbp  in  c  colony    overseas      overseas                             TRUE    foreign      Output      idnat     idbp   idnat2 1  french mainland mainland 2  french   colony overseas 3  french overseas overseas 4 foreign  foreign  foreign

User · Answer

If the data set contains many rows it might be more efficient to join with a lookup table using data table instead of nested ifelse     Provided the lookup table below  lookup         idnat     idbp   idnat2 1   french mainland mainland 2   french   colony overseas 3   french overseas overseas 4  foreign  foreign  foreign    and a sample data set  library data table  n row  lt - 10L set seed 1L  DT  lt - 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    then we can do an update while joining   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

User · Answer

Try something like the following     some sample data idnat  lt - sample c  french   foreigner   100 TRUE  idbp  lt - rep NA 100  idbp idnat   french    lt - sample c  mainland   overseas   colony   sum idnat   french   TRUE     recoding out  lt - ifelse idnat   french   amp   idbp  in  c  overseas   colony     mainland                 ifelse idbp  in  c  overseas   colony    overseas                         foreigner    cbind idnat idbp out    check result   Your confusion comes from how SAS and R handle if-else constructions  In R  if and else are not vectorized  meaning they check whether a single condition is true  i e   if  french    french   works  and cannot handle multiple logicals  i e   if c  french   foreigner     french   doesn t work  and R gives you the warning you re receiving   By contrast  ifelse is vectorized  so it can take your vectors  aka input variables  and test the logical condition on each of their elements  like you re used to in SAS  An alternative way to wrap your head around this would be to build a loop using if and else statements  as you ve started to do here  but the vectorized ifelse approach will be more efficient and involve generally less code

User · Answer

Sorry for joining too late to the party  Here s an easy solution     building up your initial table idnat  lt - c 1 1 1 2   1 is french  2 is foreign  idbp  lt - c 1 2 3 4   1 is mainland  2 is colony  3 is overseas  4 is foreign  t  lt - cbind idnat  idbp    the last column will be a vector of row length   row length of your matrix idnat2  lt - vector        and we will populate that vector with a cursor  for i in 1 length idnat           check that we selected the cursor to for the length of one of the vectors          if  t i 1     2    this says  if idnat   foreign  then it s foreign                idnat2 i   lt - 3  3 is foreign           else if  t i 2     1    this says  if not foreign and idbp   mainland then it s mainland                idnat2 i   lt - 2   2 is mainland             else   this says  anything else will be classified as colony or overseas                idnat2 i   lt - 1   1 is colony or overseas              cbind t idnat2

User · Answer

You can create the vector idnat2 without if and ifelse   The function replace can be used to replace all occurrences of  colony  with  overseas    idnat2  lt - replace idbp  idbp     colony    overseas

User · Answer

The explanation with the examples was key to helping mine  but the issue that i came was when I copied it didn t work so I had to mess with it in several ways to get it to work right   I m super new at R  and had some issues with the third ifelse due to lack of knowledge    so for those who are super new to R running into issues        ifelse x  lt  -2  pretty negative   ifelse x  lt  1  close to zero   ifelse x  lt  3  in  1  3    large    all one line        normal tab      i used this in a function so it  ifelse     was tabbed over one  but the last     was completely to the left

User · Answer

Read in the data   idnat c  french   french   french   foreign   idbp c  mainland   colony   overseas   foreign      Initialize the new variable   idnat2 as character vector       Logically evaluate  idnat  and  idbp  for each case  assigning the appropriate level to  idnat2    for i in 1 length idnat       if idnat i      french   amp  idbp i      mainland         idnat2 i     mainland    else if  idnat i      french   amp   idbp i      colony    idbp i      overseas        idnat2 i     overseas    else     idnat2 i     foreign          Create a data frame with the two old variables and the new variable   data frame idnat idbp idnat2

User · Answer

With data table  the solutions is   DT   idnat2    ifelse idbp  in   foreign    foreign            ifelse idbp  in  c  colony    overseas     overseas    mainland        The ifelse is vectorized  The if-else is not  Here  DT is       idnat     idbp 1  french mainland 2  french   colony 3  french overseas 4 foreign  foreign   This gives      idnat     idbp   idnat2 1   french mainland mainland 2   french   colony overseas 3   french overseas overseas 4  foreign  foreign  foreign

[r] Nested ifelse statement

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