switch {base}

R Documentation

Select One of a List of Alternatives

Description

switch evaluates EXPR and accordingly chooses one of the further arguments (in ...).

Usage

switch(EXPR, ...)

Arguments

EXPR	an expression evaluating to a number or a character string.
...	the list of alternatives, given explicitly.

Details

If the value of EXPR is an integer between 1 and nargs()-1 then the corresponding element of ... is evaluated and the result returned.

If EXPR returns a character string then that string is used to match the names of the elements in .... If there is an exact match then that element is evaluated and returned if there is one, otherwise the next element is chosen, e.g., switch("cc", a=1, cc=, d=2) evaluates to 2.

In the case of no match, if there's a further argument in switch that one is returned, otherwise NULL.

Warning

Beware of partial matching: an alternative E = foo will match the first argument EXPR unless that is named. See the examples for good practice in naming the first argument.

References

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

Examples

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")
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 don't allow an 'otherwise':
for(i in c(-1:3,9))  print(switch(i, 1,2,3,4))

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