| switch {base} | R Documentation |
switch evaluates EXPR and accordingly chooses one of the
further arguments (in ...).
switch(EXPR, ...)
EXPR |
an expression evaluating to a number or a character string. |
... |
the list of alternatives. If it is intended that
|
switch works in two distinct ways depending whether the first
argument evaluates to a character string or a number.
If the value of EXPR is not a character string it is coerced to
integer. If this is between 1 and nargs()-1 then the
corresponding element of ... is evaluated and the result
returned: thus if the first argument is 3 then the fourth
argument is evaluated and returned.
If EXPR evaluates to a character string then that string is
matched (exactly)to the names of the elements in .... If
there is a match then that element is evaluated unless it is missing,
in which case the next non-missing element is evaluated, so for
example switch("cc", a=1, cc=, cd=, d=2) evaluates to 2.
If there is more than one match, the first matching element is used.
In the case of no match, if there is a unnamed element of ...
its value is returned. (If there is more than one such argument
an error is returned. Before R 2.13.0 the first one would have
been used.)
The first argument is always taken to be EXPR: if it is named
its name must (partially) match.
This is implemented as a primitive function that only evaluates its first argument and one other if one is selected.
The value of one of the elements of ..., or NULL,
invisibly (whenever no element is selected).
The result has the visibility (see invisible) of the
element evaluated.
Before R 2.11.0 it was necessary to avoid partial matching: an
alternative E = foo matched the first argument EXPR
unless that was named.
It is possible to write calls to switch that can be confusing
and may not work in the same way in earlier versions of R. For
compatibility (and clarity), always have EXPR as the first
argument, naming it if partial matching is a possibility. For the
character-string form, have a single unnamed argument as the default
after the named values.
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
require(stats)
centre <- function(x, type) {
switch(type,
mean = mean(x),
median = median(x),
trimmed = mean(x, trim = .1))
}
x <- rcauchy(10)
centre(x, "mean")
centre(x, "median")
centre(x, "trimmed")
ccc <- c("b","QQ","a","A","bb")
# note: cat() produces no output for NULL
for(ch in ccc)
cat(ch,":", switch(EXPR = ch, a=1, b=2:3), "\n")
for(ch in ccc)
cat(ch,":", switch(EXPR = ch, a=, A=1, b=2:3, "Otherwise: last"),"\n")
## Numeric EXPR does not allow a default value to be specified
## -- it is always NULL
for(i in c(-1:3,9)) print(switch(i, 1,2,3,4))
## visibility
switch(1, invisible(pi), pi)
switch(2, invisible(pi), pi)