R: Conditional Element Selection (original) (raw)
| ifelse {base} | R Documentation |
|---|
Description
ifelse returns a value with the same shape astest which is filled with elements selected from either yes or nodepending on whether the element of testis 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 testis 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 ofyes 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](../../base/help/oldClass.html)) 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) whenevertest is a simple true/false result, i.e., whenlength(test) == 1.
The srcref attribute of functions is handled specially: iftest 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 noargument). This functionality is only for backwards compatibility, the form if(test) yes else no should be used whenever yes andno are functions.
References
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)The New S Language. Wadsworth & Brooks/Cole.
See Also
[if](../../base/help/if.html).
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)
utils::str( ifelse(NA, yes, no) ) # logical, length 1
utils::str( ifelse(TRUE, yes, no) ) # integer, length 1
utils::str( ifelse(FALSE, yes, no) ) # double, length 1
[Package _base_ version 4.6.0 Index]