# 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

1 |

### Arguments

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

`check.xor` |
character (either |

### 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 `<expression> <- <logical expression>`

or `<logical expression> -> <expression>`

. 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.

### 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.

### Examples

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 | ```
# 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)
``` |

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