Running examples in ‘rPref-Ex.R’ failed
The error most likely occurred in:

> base::assign(".ptime", proc.time(), pos = "CheckExEnv")
> ### Name: psel
> ### Title: Preference Selection
> ### Aliases: peval psel psel.indices
> 
> ### ** Examples
> 
> 
> # Skyline and top-k/at-least skyline
> psel(mtcars, low(mpg) * low(hp))
                    mpg cyl  disp  hp drat    wt  qsec vs am gear carb
Datsun 710         22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1
Valiant            18.1   6 225.0 105 2.76 3.460 20.22  1  0    3    1
Merc 240D          24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2
Merc 280C          17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4
Cadillac Fleetwood 10.4   8 472.0 205 2.93 5.250 17.98  0  0    3    4
Honda Civic        30.4   4  75.7  52 4.93 1.615 18.52  1  1    4    2
Toyota Corona      21.5   4 120.1  97 3.70 2.465 20.01  1  0    3    1
AMC Javelin        15.2   8 304.0 150 3.15 3.435 17.30  0  0    3    2
> psel(mtcars, low(mpg) * low(hp), top = 5)
                    mpg cyl  disp  hp drat   wt  qsec vs am gear carb .level
Datsun 710         22.8   4 108.0  93 3.85 2.32 18.61  1  1    4    1      1
Valiant            18.1   6 225.0 105 2.76 3.46 20.22  1  0    3    1      1
Merc 240D          24.4   4 146.7  62 3.69 3.19 20.00  1  0    4    2      1
Merc 280C          17.8   6 167.6 123 3.92 3.44 18.90  1  0    4    4      1
Cadillac Fleetwood 10.4   8 472.0 205 2.93 5.25 17.98  0  0    3    4      1
> psel(mtcars, low(mpg) * low(hp), at_least = 5)
                    mpg cyl  disp  hp drat    wt  qsec vs am gear carb .level
Datsun 710         22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1      1
Valiant            18.1   6 225.0 105 2.76 3.460 20.22  1  0    3    1      1
Merc 240D          24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2      1
Merc 280C          17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4      1
Cadillac Fleetwood 10.4   8 472.0 205 2.93 5.250 17.98  0  0    3    4      1
Honda Civic        30.4   4  75.7  52 4.93 1.615 18.52  1  1    4    2      1
Toyota Corona      21.5   4 120.1  97 3.70 2.465 20.01  1  0    3    1      1
AMC Javelin        15.2   8 304.0 150 3.15 3.435 17.30  0  0    3    2      1
> 
> # preference with associated data frame and evaluation
> p <- low(mpg, df = mtcars) * (high(cyl) & high(gear))
> peval(p)
                     mpg cyl disp  hp drat    wt  qsec vs am gear carb
Cadillac Fleetwood  10.4   8  472 205 2.93 5.250 17.98  0  0    3    4
Lincoln Continental 10.4   8  460 215 3.00 5.424 17.82  0  0    3    4
Maserati Bora       15.0   8  301 335 3.54 3.570 14.60  0  1    5    8
> 
> # visualize the skyline in a plot
> sky1 <- psel(mtcars, high(mpg) * high(hp))
> plot(mtcars$mpg, mtcars$hp)
> points(sky1$mpg, sky1$hp, lwd=3)
> 
> # grouped preference with dplyr
> library(dplyr)

Attaching package: ‘dplyr’

The following object is masked from ‘package:rPref’:

    between

The following objects are masked from ‘package:stats’:

    filter, lag

The following objects are masked from ‘package:base’:

    intersect, setdiff, setequal, union

> psel(group_by(mtcars, cyl), low(mpg))
Error in psel.indices(df, pref, .dots = vars) : 
  Could not find grouping indices in grouped data.frame!
Calls: psel -> psel.indices
Execution halted