x_at_y.Rd

% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/x_at_y.R
\name{x_at_y}
\alias{x_at_y}
\title{Generate lookup vectors for composing nested indices}
\usage{
x_at_y(x, y, missing = NA)
}
\arguments{
\item{x}{Values in the resulting lookup vector. There should be only
one unique value of \code{x} for every corresponding value of \code{y}.}

\item{y}{Keys in the resulting lookup vector. Should be factors or integers.}

\item{missing}{Missing levels from \code{y} will be filled in with this value
in the resulting lookup vector. Default \code{NA}.}
}
\description{
Generates a lookup vector such that \code{x_at_y(x, y)[y] == x}. Particularly useful
for generating lookup tables for nested indices in conjunction with \code{\link[=compose_data]{compose_data()}}.
}
\details{
\code{x_at_y(x, y)} returns a vector \code{k} such that \code{k[y] == x}. It also
fills in missing values in \code{y}: if \code{y} is an integer, \code{k} will contain
entries for all values from \code{1} to \code{max(y)}; if \code{y} is a factor,
\code{k} will contain entries for all values from \code{1} to \code{nlevels(y)}.
Missing values are replaced with \code{missing} (default \code{NA}).
}
\examples{

library(magrittr)

df = data.frame(
  plot = factor(paste0("p", rep(1:8, times = 2))),
  site = factor(paste0("s", rep(1:4, each = 2, times = 2)))
)

# turns site into a nested index: site[p] gives the site for plot p
df \%>\%
  compose_data(site = x_at_y(site, plot))

}
\seealso{
\code{\link[=compose_data]{compose_data()}}.
}
\author{
Matthew Kay
}