# gagern / random_pivot

Random binary matrices with given marginal sums

## Files

Failed to load latest commit information.
Type
Name
Commit time

# Random pivot R module

The goal of this code is to compute a random binary matrix satisfying given marginal sums. In other words, you specify the number of `TRUE` values in each row and column of the matrix, and get a matrix satisfying these counts.

## Algorithm

The algorithm is based on an initial incomplete fill using random pairings between rows and columns, followed by a search for augmenting paths in a network flow model of the matrix. The first step is performed in R code, the second in a C++ implementation called from R. As a third step, a trial swap may be used to approach equidistribution. Such a third step is not part of the code at hand.

The algorithm has been mentioned in Problems with bins: A critical reassessment of Gotelli and Ulrich's Bayes approach using bird data by von Gagern, von Gagern and Schmitz Ornés (Acta Oecologica 69 (2015) pp. 137–145). It has been discussed in more detail in Appendix A available as an online supplemental to that paper. In the long run, the description should become part of this repository as well.

## Installation

The code should eventually become available as a public R package, which can be downloaded from CRAN. Or perhaps it might be integrated into existing packages which already perform some kind of matrix randomization. But that requires some more work with portability testing.

So at the moment users are invited to install the package from this source repository, e.g. by invoking `R CMD INSTALL .` from the top directory of the working copy.

## Usage

The function `random.pivot` takes two arguments which are turned into factors using `as.factor`. They are interpreted as multisets, i.e. the order of elements is irrelevant. The number of occurrences of each level indicates the desired marginal sum for that level. So the rows and columns of the final matrix will correspond to the levels of the first and second argument.

The output will be a logical matrix. If you prefer the shorter binary notation (i.e. using `1`/`0` instead of `TRUE`/`FALSE`), simply multiply the result with `1`.

### Example

``````> random.pivot(c("r1", "r2", "r2", "r3"), c("c1", "c2", "c2", "c3"))*1
c1 c2 c3
r1  0  1  0
r2  1  1  0
r3  0  0  1
> random.pivot(c("r1", "r2", "r2", "r3"), c("c1", "c2", "c2", "c3"))*1
c1 c2 c3
r1  0  1  0
r2  0  1  1
r3  1  0  0
``````

## Naming

Turning a pair of factors into a pivot table (as Excel and therefore many other users call it) is a common operation (and can be achieved in many ways in R, including `xtabs`, `reshape` and functions from various packages). In this sense, the code here creates a pivot table after randomizing one of the vectors in a suitable way. Hence the name, `random.pivot`.

Random binary matrices with given marginal sums