CODEWITHSUNDEEP
rswitch-statement
Simon
Simon

Reputation: 2002

How to use the switch statement in R functions?

I would like to use for my function in R the statement switch() to trigger different computation according to the value of the function's argument.

For instance, in Matlab you can do that by writing 

switch(AA)        
case '1'   
...   
case '2'   
...   
case '3'  
...  
end

I found this post - switch() statement usage - that explain how to use switch, but not really helpful to me as I want to perform more sophisticated computation (matrix operations) and not a simple mean.

Upvotes: 106

Views: 189194

Answers (4)

Rolf L
Rolf L

Reputation: 19

This is a more general answer to the missing "Select cond1, stmt1, ... else stmtelse" connstruction in R. It's a bit gassy, but it works an resembles the switch statement present in C

while (TRUE) {
  if (is.na(val)) {
    val <- "NULL"
    break
  }
  if (inherits(val, "POSIXct") || inherits(val, "POSIXt")) {
    val <- paste0("#",  format(val, "%Y-%m-%d %H:%M:%S"), "#")
    break
  }
  if (inherits(val, "Date")) {
    val <- paste0("#",  format(val, "%Y-%m-%d"), "#")
    break
  }
  if (is.numeric(val)) break
  val <- paste0("'", gsub("'", "''", val), "'")
  break
}

Upvotes: 1

petermeissner
petermeissner

Reputation: 12890

those various ways of switch ... 

# by index
switch(1, "one", "two")
## [1] "one"


# by index with complex expressions
switch(2, {"one"}, {"two"})
## [1] "two"


# by index with complex named expression
switch(1, foo={"one"}, bar={"two"})
## [1] "one"


# by name with complex named expression
switch("bar", foo={"one"}, bar={"two"})
## [1] "two"

Upvotes: 63

Tyler Rinker
Tyler Rinker

Reputation: 109894

I hope this example helps. You ca use the curly braces to make sure you've got everything enclosed in the switcher changer guy (sorry don't know the technical term but the term that precedes the = sign that changes what happens). I think of switch as a more controlled bunch of if () {} else {} statements.

Each time the switch function is the same but the command we supply changes.

do.this <- "T1"

switch(do.this,
    T1={X <- t(mtcars)
        colSums(mtcars)%*%X
    },
    T2={X <- colMeans(mtcars)
        outer(X, X)
    },
    stop("Enter something that switches me!")
)
#########################################################
do.this <- "T2"

switch(do.this,
    T1={X <- t(mtcars)
        colSums(mtcars)%*%X
    },
    T2={X <- colMeans(mtcars)
        outer(X, X)
    },
    stop("Enter something that switches me!")
)
########################################################
do.this <- "T3"

switch(do.this,
    T1={X <- t(mtcars)
        colSums(mtcars)%*%X
    },
    T2={X <- colMeans(mtcars)
        outer(X, X)
    },
    stop("Enter something that switches me!")
)

Here it is inside a function:

FUN <- function(df, do.this){
    switch(do.this,
        T1={X <- t(df)
            P <- colSums(df)%*%X
        },
        T2={X <- colMeans(df)
            P <- outer(X, X)
        },
        stop("Enter something that switches me!")
    )
    return(P)
}

FUN(mtcars, "T1")
FUN(mtcars, "T2")
FUN(mtcars, "T3")

Upvotes: 52

Tommy
Tommy

Reputation: 40841

Well, switch probably wasn't really meant to work like this, but you can:

AA = 'foo'
switch(AA, 
foo={
  # case 'foo' here...
  print('foo')
},
bar={
  # case 'bar' here...
  print('bar')    
},
{
   print('default')
}
)

...each case is an expression - usually just a simple thing, but here I use a curly-block so that you can stuff whatever code you want in there...

Upvotes: 136

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