Description Usage Arguments Details See Also Examples

Collection of some useful functions which are applicable to all preference objects.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | ```
empty()
is.empty_pref(x)
## S4 method for signature 'preference'
length(x)
is.preference(x)
## S4 method for signature 'preference'
as.expression(x, ...)
## S4 method for signature 'preference'
assoc.df(x)
## S4 replacement method for signature 'preference'
assoc.df(x) <- value
``` |

`x` |
A preference, or, for |

`...` |
Optional arguments passed to |

`value` |
A data frame to associate with a preference object. |

The empty preference `empty()`

is a neutral element for the complex preference compositions `{*, &, +}`

.
It holds that `empty() * p`

and `empty() & p`

is equal to `p`

for all preferences `p`

.

The function `length(p)`

returns the term length of the preference term `p`

which is defined as the number of base preferences
in a complex preference term. The empty preference `empty()`

has length 0,
and all base preferences have length 1.

With `as.expression(p)`

for a preference `p`

the call to the preference is constructed.
This means, `eval(as.expression(p))`

returns the preference `p`

, evaluated in the current environment.

The function `is.empty_pref`

returns `TRUE`

if `x`

is the empty preference object
`empty()`

and `FALSE`

otherwise.

With `assoc.df`

the associated data frame of a preference can be retrieved or set.
Setting the associated data frame means that a partial evaluation based on this data frame is done.
See `show.pref`

for details on partial evaluation of preferences.
Next, the preference is linked to that data frame, such that `peval(p)`

can be used instead of `psel(df, p)`

.
It returns `NULL`

if no data frame is associated.
Use `set.assoc.df(NULL)`

to delete an associated data frame.

See `base_pref`

for the construction of base preferences,
and `complex_pref`

for the construction of complex preferences.
See `show.pref`

for string output and partial evaluation of preference terms.

1 2 3 4 5 6 7 8 | ```
# Same as low(a) * low(b)
p <- low(a) * low(b) * empty()
# returns 2, as empty() does not count
length(p)
# the preference expression (without empty())
as.expression(p)
``` |

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