
ifelse Conditional Element Selection Description ifelse returns a value with the same shape as test which is filled with elements selected from either yes or no depending on whether the element of test is TRUE or FALSE.  Usage 
ifelse(test, yes, no)
 Arguments  
test  an object which can be coerced to logical mode. 
 
yes  return values for true elements of test. 
 
no  return values for false elements of test. 
  Details If yes or no are too short, their elements are recycled. yes will be evaluated if and only if any element of test is true, and analogously for no.  Missing values in test give missing values in the result.  Value A vector of the same length and attributes (including dimensions and "class") as test and data values from the values of yes or no. The mode of the answer will be coerced from logical to accommodate first any values taken from yes and then any values taken from no.  Warning The mode of the result may depend on the value of test (see the examples), and the class attribute (see oldClass) of the result is taken from test and may be inappropriate for the values selected from yes and no.  Sometimes it is better to use a construction such as    (tmp <- yes; tmp[!test] <- no[!test]; tmp)

, possibly extended to handle missing values in test.  Further note that if(test) yes else no is much more efficient and often much preferable to ifelse(test, yes, no) whenever test is a simple true/false result, i.e., when length(test) == 1.  The srcref attribute of functions is handled specially: if test is a simple true result and yes evaluates to a function with srcref attribute, ifelse returns yes including its attribute (the same applies to a false test and no argument). This functionality is only for backwards compatibility, the form if(test) yes else no should be used whenever yes and no are functions.  References Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.  See Also if.  Examples 
x <- c(6:-4)
sqrt(x)  #- gives warning
sqrt(ifelse(x >= 0, x, NA))  # no warning

## Note: the following also gives the warning !
ifelse(x >= 0, sqrt(x), NA)


## ifelse() strips attributes
## This is important when working with Dates and factors
x <- seq(as.Date("2000-02-29"), as.Date("2004-10-04"), by = "1 month")
## has many "yyyy-mm-29", but a few "yyyy-03-01" in the non-leap years
y <- ifelse(as.POSIXlt(x)$mday == 29, x, NA)
head(y) # not what you expected ... ==> need restore the class attribute:
class(y) <- class(x)
y
## This is a (not atypical) case where it is better *not* to use ifelse(),
## but rather the more efficient and still clear:
y2 <- x
y2[as.POSIXlt(x)$mday != 29] <- NA
## which gives the same as ifelse()+class() hack:
stopifnot(identical(y2, y))


## example of different return modes (and 'test' alone determining length):
yes <- 1:3
no  <- pi^(1:4)
utils8b7c:f320:99b9:690f:4595:cd17:293a:c069str( ifelse(NA,    yes, no) ) # logical, length 1
utils8b7c:f320:99b9:690f:4595:cd17:293a:c069str( ifelse(TRUE,  yes, no) ) # integer, length 1
utils8b7c:f320:99b9:690f:4595:cd17:293a:c069str( ifelse(FALSE, yes, no) ) # double,  length 1


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