show.pref: Partial Evaluation and String Output of Preferences

In rPref: Database Preferences and Skyline Computation

Partial Evaluation and String Output of Preferences

Description

Functions to substitute variables and functions in preferences which can be calculated before the preference is evaluated on a data frame and character output of preferences.

Usage

show.pref(p, df = NULL)

## S4 method for signature 'preference'
as.character(x, ...)

pref.str(p, df = NULL)

partial.eval.pref(p, df = NULL)

Arguments

p, x	The preference to be shown or partially evaluated.
df	(optional) A data frame on which the preference operates. Used for partial evaluation.
...	Optional arguments passed to as.character.

Details

The function pref.str (or as.character(p) for a preference p) returns the preference string while show.pref outputs it directly to the console, preceded by '[Preference]'. If df is specified, then a partial evaluation of the preference is done before converting it to a string.

The function partial.eval.pref (with given data frame df) partially evaluates the internal preference expression and returns again a preference object. All expressions in p are evaluated in the environment where p was defined, except the the column names in df (which are potential attributes in p) and except the special variable df__, which accesses the entire data set (see psel). The content of the data frame df does not matter; only names(df) is used to get the "free variables" in p.

If p has already an associated data frame (see assoc.df), then a partial evaluation was already done when the data frame was associated. In this case, the df parameter should not be used. The association will not be changed if one of these function are called with a given data frame on a preference object having an associated data frame.

Partial Evaluation Before String Output

The functions show.pref and pref.str have the optional parameter df. If this parameter is given, these functions call partial.eval.pref before they output or return the preference string. The following equalities hold:

• as.character(partial.eval.pref(p, df)) == pref.str(p, df)

• show(partial.eval.pref(p, df)) produces the same console output as show.pref(p, df)

See Also

See general_pref for more utility functions for preferences.

Examples

f <- function(x) 2*x
p <- true(cyl == f(1))

# prints 'true(cyl == f(1))'
p

# prints 'true(cyl == 2)'
show.pref(p, mtcars)
partial.eval.pref(p, mtcars)

rPref documentation built on Feb. 16, 2023, 6:09 p.m.