memisc (version 0.97)

cases: Distinguish between Cases Specified by Logical Conditions

Description

cases allows to simultaneously several cases determined by logical conditions. It can be used to code these case into a factor or as a multi-condition generalization of ifelse

Usage

cases(...,check.xor=FALSE)

Arguments

...

A sequence of logical expressions or assignment expressions containing logical expressions as "right hand side".

check.xor

logical; if true, cases checks, whether the case conditions are mutually exclusive and exhaustive. If this is not satisfied and check.xor an error exception is raised.

Value

• If it is called with logical expressions as ...arguments, cases returns a factor, if it is called with assignment expressions the function returns a vector with the same mode as the results of the "assigned" expressions and with the same length as the logical conditions.

Details

There are two distinct ways to use this function. Either the function can be used to construct a factor that represents several logical cases or it can be used to conditionally evaluate an expression in a manner similar to ifelse.

For the first use, the ... arguments have to be a series of logical expressions. cases then returns a factor with as many levels as logical expressions given as ... arguments. The resulting factor will attain its first level if the first condition is TRUE, otherwise it will attain its second level if the second condition is TRUE, etc. The levels will be named after the conditions or, if name tags are attached to the logical expressions, after the tags of the expressions. Not that the logical expressions all need to evaluate to logical vectors of the same length, otherwise an error condition is raised.

For the second use, the ... arguments have to be a series of assignment expression of the type <- or -> . For cases in which the first logical expression is TRUE, the result of first expression that appears on the other side of the assignment operator become elements of the vector returned by cases, for cases in which the second logical expression is TRUE, the result of the second expression that appears on the other side of the assignment operator become elements of the vector returned by cases, etc. Note that the logical expressions also here all need to evaluate to logical vectors of the same length. The expressions on the other side of the assignment operator should also be either vectors of the same length and mode or should scalars of the same mode, otherwise unpredictable results may occur.

Examples

# Examples of the first kind of usage of the function
#
df <- data.frame(x = rnorm(n=20), y = rnorm(n=20))
df <- df[do.call(order,df),]
(df <- within(df,{
  x1=cases(x>0,x<=0)
  y1=cases(y>0,y<=0)
  z1=cases(
    "Condition 1"=x<0,
    "Condition 2"=y<0,# only applies if x >= 0
    "Condition 3"=TRUE
    )
  z2=cases(x<0,(x>=0 & y <0), (x>=0 & y >=0))
  }))
xtabs(~x1+y1,data=df)
dd <- with(df,
  try(cases(x<0,
            x>=0,
            x>1,
            check.xor=TRUE)# let's be fussy
            )
  )
dd <- with(df,
  try(cases(x<0,x>=0,x>1))
  )
genTable(range(x)~dd,data=df)

# An example of the second kind of usage of the function:
# A construction of a non-smooth function
#
fun <- function(x)
  cases(
    x==0      -> 1,
    abs(x)> 1 -> abs(x),
    abs(x)<=1 -> x^2
  )
x <- seq(from=-2,to=2,length=101)
plot(fun(x)~x)