`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`

.

`ifelse(test, yes, no)`

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`

.

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`

.

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.

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.

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
*The New S Language*.
Wadsworth & Brooks/Cole.

`if`

.

# NOT RUN { 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 # }

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