For my masters project I studied hypergraphs and their applications in the analysis of biological systems. It was annoying that there was no package for creating hypergraph objects that could have many different analyses applied to them in R or Python. There are many options for traditional graphs (e.g. *igraph* or *NetworkX*) but when it comes to hypergraphs I found that each package could only perform one type of analysis.

I already had a lot of the code and algorithms for working on hypergraphs that I created for my project. So I defined a hypergraph object and modified my algorithms to work on them. So I ended up with this: **rhype**.

The underlying philosophy of rhype is to pass data into a function that will transform it into a hypergraph object that can then be passed into other functions to perform manipulations and calculations. This means that once the hypergraph object has been created, no further processing of the data is needed to carry out any operation. This hopefully allows for users to easily implement hypergraph based techniques into their workflows with minimal technical skill required.

However, for those users with a greater technical skillset, they are able to work directly with the R6 hypergraph object allowing for operations and manipulations that may be prohibited by the other functions. Such users are directed to the Hypergraph Objects section for a more in depth explanation.

# Installation

The `rhype`

package cane be downloaded from CRAN by

`install.packages("rhype")`

The development version of rhype is available from GitHub with:

```
#install.packages("devtools")
devtools::install_github("hwarden162/rhype")
```

# Getting Started

The first thing to do is to create a hypergraph object. Hypergraph objects can be created from your own data, but the `example_hype()`

function can be used to create many different types of hypergraphs that are useful for exploring rhype.

```
library(rhype)
hype1 <- example_hype()
hype1
```

```
## Hypergraph Object:
## Number of vertices: 4
## Number of hyperedges: 2
## Oriented: FALSE Directed: FALSE
## Real Coefficients: FALSE Weighted: FALSE
```

Just typing the name of a hypergraph will give you a short summary of the of its structure. To quickly get a more in depth overview of a hypergraph use the `hype_info()`

function

`hype_info(hype1)`

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 4 vertices
##
## These vertices are called:
## v1, v2, v3, v4
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2
##
## The hyperedges have the structure:
## $h1
## [1] 1 2 3
##
## $h2
## [1] 2 3 4
##
## ---------------WEIGHTING INFORMATION--------------------
##
## This hypergraph is not weighted
##
## --------------------Orientation Information--------------------
##
## This hypergraph is not oriented
##
## This hypergraph is not directed
##
## --------------------REAL COEFFICIENTS INFORMATION--------------------
##
## This hypergraph does not have real coefficients associating vertices to hyperedges
##
## There is no incidence matrix associating vertices to hyperedges with non-binary coefficients
```

This can generate rather long sections of text and so for bigger hypergraphs whole sections of output can be omitted by passing different values

`hype_info(hype1, weighted = FALSE, oriented = FALSE, real_coef = FALSE)`

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 4 vertices
##
## These vertices are called:
## v1, v2, v3, v4
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2
##
## The hyperedges have the structure:
## $h1
## [1] 1 2 3
##
## $h2
## [1] 2 3 4
```

However, if too many of these settings are being set to false it is recommended that the user just call out each section of the structure individually to investigate.

`hype1`

is an example of a very basic hypergraph. `rhype`

allows for three more ways to augment a hypergraph: weighting, orientation and real coefficients.

## Weighting

Weights can be applied to both vertices and hyperedges. `hype2`

is an example of a weighted hypergraph

```
hype2 <- example_hype(vertex_weighted = TRUE, edge_weighted = TRUE)
hype_info(hype2, oriented = FALSE, real_coef = FALSE)
```

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 4 vertices
##
## These vertices are called:
## v1, v2, v3, v4
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2
##
## The hyperedges have the structure:
## $h1
## [1] 1 2 3
##
## $h2
## [1] 2 3 4
##
## ---------------WEIGHTING INFORMATION--------------------
##
## This hypergraph is weighted
##
## The hyperedges have weights:
## h1 h2
## 1 2
##
## The vertices have weights:
## v1 v2 v3 v4
## 1 2 3 4
```

Hyperedge weightings will change the way information is flows through the hypergraph and so will have an effect on almost every calculation done in `rhype`

. Vertex weightings are currently not used in any `rhype`

functions except as a store for hyperedge weightings in dual hypergraphs. However, **this is likely to change**, all efforts will be made such that changes should not affect previously written code but this may not be possible/feasible.

## Orientation

Orientations can be applied to hyperedges. This creates two groups within the hyperedge such that information can only travel across a hyperedge from a vertex in one group to a vertex in the other group. `hype3`

is an example of an oriented hypergraph

```
hype3 <- example_hype(oriented = TRUE)
hype_info(hype3, weighted = FALSE, real_coef = FALSE)
```

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 4 vertices
##
## These vertices are called:
## v1, v2, v3, v4
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2
##
## The hyperedges have the structure:
## $h1
## $h1[[1]]
## [1] 1 2
##
## $h1[[2]]
## [1] 3 4
##
##
## $h2
## $h2[[1]]
## [1] 2 3 4
##
## $h2[[2]]
## [1] 1 2
##
##
## --------------------Orientation Information--------------------
##
## This hypergraph is oriented
##
## This hypergraph is not directed
```

In the hyperedge information section, it can be seen that each hyperedge now has two groups, representing the two different ends of the hyperedge. These hyperedges can then be directed such that information can only flow across a hyperedge from one specific end to the other

```
hype4 <- example_hype(oriented = TRUE, directed = TRUE)
hype_info(hype4, weighted = FALSE, real_coef = FALSE)
```

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 4 vertices
##
## These vertices are called:
## v1, v2, v3, v4
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2
##
## The hyperedges have the structure:
## $h1
## $h1$from
## [1] 1 2
##
## $h1$to
## [1] 3 4
##
##
## $h2
## $h2$from
## [1] 2 3 4
##
## $h2$to
## [1] 1 2
##
##
## --------------------Orientation Information--------------------
##
## This hypergraph is oriented
##
## This hypergraph is directed
```

Now looking at the hyperedge information, it can be seen in the naming of the groups that each hyperedge has a dedicated set of vertices that information can flow from and a dedicated set of vertices that information can flow to. Vertices can be a part of both ends of an oriented/directed hyperedge, sometimes these are referred to as catalytic with resepect to the hyperedge.

A very important rule to remember is that **all directed hypergraphs must be oriented** but not all oriented hypergraphs are directed.

## Real Coefficients

Real coefficients can be seen as an extension of weightings. A real coefficient can be used to connect a vertex to a hyperedge with a certain weight. This is useful as sometimes you may want two vertices to both be a member of a hyperedge, but for one of those vertices to be twice as important within that hyperedge as the other. Therefore, the vertex can be connected specifically to that hyperedge with a coefficient that is twice as big as the other’s.

```
hype5 <- example_hype(real_coef = TRUE)
hype_info(hype5, weighted = FALSE, oriented = FALSE)
```

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 4 vertices
##
## These vertices are called:
## v1, v2, v3, v4
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2
##
## The hyperedges have the structure:
## $h1
## [1] 1 2 3
##
## $h2
## [1] 2 3 4
##
## --------------------REAL COEFFICIENTS INFORMATION--------------------
##
## This hypergraph has real coefficients associating vertices to hyperedges
##
## The incidence matrix associating vertices to hyperedges is given by:
## h1 h2
## v1 1 0
## v2 2 2
## v3 3 3
## v4 0 4
```

At the bottom of the printout for `hype5`

an incidence matrix can be seen, this can essentially be viewed as a table that shows how strongly connected each vertex (row) is connected to each hyperedge (column).

A vertex is considered to be a member of a hyperedge if the respective entry in the matrix is not 0. However, there is a soft assumption that these real coefficients are non-negative. Although it is possible to put negative values as real coefficients, and there are many occasions where one may want to, this should be done **with great care**. Negative coefficients can cause many problems, expecially when calculating hypergraph matrices and hyperpaths between vertices. This is because when calculating the adjacency coefficient between two vertices, negative numbers means this can sum to 0. This would mean that the vertices will be considered not adjacent (even though they are). rhype does not check for this so manual checking is necessary.

## Combining Extra Features

Hypergraph weighting, orientation and real coefficients can all be combined together the model very complicated systems

```
hype6 <- example_hype(
oriented = TRUE,
directed = TRUE,
vertex_weighted = TRUE,
edge_weighted = TRUE,
real_coef = TRUE
)
hype_info(hype6)
```

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 4 vertices
##
## These vertices are called:
## v1, v2, v3, v4
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2
##
## The hyperedges have the structure:
## $h1
## $h1$from
## [1] 1 2
##
## $h1$to
## [1] 3 4
##
##
## $h2
## $h2$from
## [1] 2 3 4
##
## $h2$to
## [1] 1 2
##
##
## ---------------WEIGHTING INFORMATION--------------------
##
## This hypergraph is weighted
##
## The hyperedges have weights:
## h1 h2
## 1 2
##
## The vertices have weights:
## v1 v2 v3 v4
## 1 2 3 4
##
## --------------------Orientation Information--------------------
##
## This hypergraph is oriented
##
## This hypergraph is directed
##
## --------------------REAL COEFFICIENTS INFORMATION--------------------
##
## This hypergraph has real coefficients associating vertices to hyperedges
##
## The incidence matrix associating vertices to hyperedges is given by:
## $from
## h1 h2
## v1 1 0
## v2 2 2
## v3 0 3
## v4 0 4
##
## $to
## h1 h2
## v1 0 1
## v2 0 2
## v3 3 0
## v4 4 0
```

All of these feature will interact together, with orientations restricting the movement of information across hyperedges, weightings changing how information moves globally through hyperedges and vertices and real coefficients changing how information moves locally from specific vertices to specific hyperedges.

All efforts have been made to make rhype as flexible as possible. However, although the modelling is extremely flexible not all functions are able to take all forms of hypergaphs, this will be stated in its documentation or as an error message if an execution is attempted. The reason for this is either because the computation is complex and has not been finished yet, or because the maths governing this calculation has not yet been finished.

If a function does not work for a particular use that you would like, please raise it on the GitHub Repo to move that job higher up the list of features to add.

# Creating Hypergraphs

There are currently two methods for creating hypergraphs in rhype, using a hyperedge list or using an incidence matrix.

## Hypergraphs From Hyperedge Lists

A hyperedge list is a list where each entry represents a hyperedge. For an unoriented hyperedge, this can just be represented as a vector of the vertices in the hyperedge. For an oriented hyperedge, this can be represented as a list of two vectors, with each vector containing the vertices in one end of the hyperedge. For an oriented hyperedge, the first vector represents all vertices in the tail of a hyperedge and the second all vertices in the head.

A vertex can be any variable that can be coerced into a string and then factored. It is recommended that vertices be represented as a string in the first place or an integer. Here is an example hyperedge list

```
l1 <- list(
h1 = c("a","b","c"),
h2 = c("c","d","e"),
h3 = c("a", "e")
)
```

This can be used to create a hypergraph with three hyperedges (named after the element names in the list).

```
hype1 <- hype_from_edge_list(l1)
hype_info(hype1, weighted = FALSE, oriented = FALSE, real_coef = FALSE)
```

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 5 vertices
##
## These vertices are called:
## a, b, c, d, e
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2, h3
##
## The hyperedges have the structure:
## $h1
## [1] "a" "b" "c"
##
## $h2
## [1] "c" "d" "e"
##
## $h3
## [1] "a" "e"
```

Here is an example of an oriented list

```
l2 <- list(
h1 = list(
c("a","b"),
c("b","c")
),
h2 = list(
c("b","c","d"),
c("e","f")
),
h3 = list(
"f",
"a"
)
)
```

This can be used to create an oriented hypergraph

```
hype2 <- hype_from_edge_list(l2)
hype_info(hype2, weighted = FALSE, real_coef = FALSE)
```

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 6 vertices
##
## These vertices are called:
## a, b, c, d, e, f
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2, h3
##
## The hyperedges have the structure:
## $h1
## $h1[[1]]
## [1] "a" "b"
##
## $h1[[2]]
## [1] "b" "c"
##
##
## $h2
## $h2[[1]]
## [1] "b" "c" "d"
##
## $h2[[2]]
## [1] "e" "f"
##
##
## $h3
## $h3[[1]]
## [1] "f"
##
## $h3[[2]]
## [1] "a"
##
##
## --------------------Orientation Information--------------------
##
## This hypergraph is oriented
##
## This hypergraph is not directed
```

It can also be used to create a directed hypergraph, with the first element of each hyperedge representing the tail (where information can travel from) and the second representing the head (where information can travel to)

```
hype3 <- hype_from_edge_list(l2, directed = TRUE)
hype_info(hype3, weighted = FALSE, real_coef = FALSE)
```

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 6 vertices
##
## These vertices are called:
## a, b, c, d, e, f
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2, h3
##
## The hyperedges have the structure:
## $h1
## $h1$from
## [1] "a" "b"
##
## $h1$to
## [1] "b" "c"
##
##
## $h2
## $h2$from
## [1] "b" "c" "d"
##
## $h2$to
## [1] "e" "f"
##
##
## $h3
## $h3$from
## [1] "f"
##
## $h3$to
## [1] "a"
##
##
## --------------------Orientation Information--------------------
##
## This hypergraph is oriented
##
## This hypergraph is directed
```

## Hypergraphs From Incidence Matrices

An incidence matrix can be seen as a table where each row represents a vertex and each column represents a hyperedge. Each entry is a 1 if the vertex is a member of the hyperedge and a 0 if it is not. An example incidence matrix is given below

```
i1 <- matrix(
c(1,1,1,0,0,0,0,1,1,1,0,1,0,1,0),
nrow = 5,
ncol = 3,
dimnames = list(
paste0("v", 1:5),
paste0("h", 1:3)
)
)
i1
```

```
## h1 h2 h3
## v1 1 0 0
## v2 1 0 1
## v3 1 1 0
## v4 0 1 1
## v5 0 1 0
```

This can be made into a hypergraph using the `hype_from_inc_mat()`

function

```
hype1 <- hype_from_inc_mat(i1)
hype_info(hype1, weighted = FALSE, oriented = FALSE, real_coef = FALSE)
```

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 5 vertices
##
## These vertices are called:
## v1, v2, v3, v4, v5
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2, h3
##
## The hyperedges have the structure:
## $h1
## v1 v2 v3
## 1 2 3
##
## $h2
## v3 v4 v5
## 3 4 5
##
## $h3
## v2 v4
## 2 4
```

rhype represents oriented hypergraphs using a list of two incidence matrices. The first inidence matrix represents incidence to one end of the oriented hyperedge and the other represents incidence to the other end (it doesn’t matter which way round these are). An example is given below

```
i2 <- list(
matrix(
c(1,1,0,0,1,0,0,1,0,1,1,0),
nrow = 4,
ncol = 3,
dimnames = list(
paste0("v", 1:4),
paste0("h", 1:3)
)
),
matrix(
c(0,0,1,1,1,1,0,0,1,0,1,0),
nrow = 4,
ncol = 3,
dimnames = list(
paste0("v", 1:4),
paste0("h", 1:3)
)
)
)
i2
```

```
## [[1]]
## h1 h2 h3
## v1 1 1 0
## v2 1 0 1
## v3 0 0 1
## v4 0 1 0
##
## [[2]]
## h1 h2 h3
## v1 0 1 1
## v2 0 1 0
## v3 1 0 1
## v4 1 0 0
```

This can be turned into an oriented hypergraph by passing it to the same function

```
hype2 <- hype_from_inc_mat(i2)
hype_info(hype2, weighted = FALSE, real_coef = FALSE)
```

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 4 vertices
##
## These vertices are called:
## v1, v2, v3, v4
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2, h3
##
## The hyperedges have the structure:
## $h1
## $h1[[1]]
## [1] "v1" "v2"
##
## $h1[[2]]
## [1] "v3" "v4"
##
##
## $h2
## $h2[[1]]
## [1] "v1" "v4"
##
## $h2[[2]]
## [1] "v1" "v2"
##
##
## $h3
## $h3[[1]]
## [1] "v2" "v3"
##
## $h3[[2]]
## [1] "v1" "v3"
##
##
## --------------------Orientation Information--------------------
##
## This hypergraph is oriented
##
## This hypergraph is not directed
```

It can also be turned into a directed hypergraph using the directing option

```
hype3 <- hype_from_inc_mat(i2, directed = TRUE)
hype_info(hype3, weighted = FALSE, real_coef = FALSE)
```

```
## ====================HYPERGRAPH INFORMATION====================
##
## --------------------VERTEX INFORMATION--------------------
##
## This hypergraph has 4 vertices
##
## These vertices are called:
## v1, v2, v3, v4
##
## --------------------HYPEREDGE INFORMATION--------------------
##
## The hyperedges are called:
## h1, h2, h3
##
## The hyperedges have the structure:
## $h1
## $h1$from
## [1] "v1" "v2"
##
## $h1$to
## [1] "v3" "v4"
##
##
## $h2
## $h2$from
## [1] "v1" "v4"
##
## $h2$to
## [1] "v1" "v2"
##
##
## $h3
## $h3$from
## [1] "v2" "v3"
##
## $h3$to
## [1] "v1" "v3"
##
##
## --------------------Orientation Information--------------------
##
## This hypergraph is oriented
##
## This hypergraph is directed
```

# Hypergraph Objects

A hypergraph object is essentially a collection of eleven properties. Together, these properties fully describe many different forms of hypergraphs. These properties are listed and explained more in depth below:

## Number of Vertices - `numv`

This is simply a postive integer value representing the number of vertices the hypergraph has.

## Hyperedge List - `elist`

This property is a list where every entry represents a hyperedge.

For an unoriented hypergraph, a hyperedge is just a vector of the vertices contained within the hyperedge. Each vertex is represented as a string.

For an oriented hypergraph, each hyperedge is itself a list of two vectors. Each of these vectors contains strings representing the vertices contained in one end of the hyperedge.

For a directed hypergraph, each hyperedge is also a list of two vectors. In the directed case, the first vector represents the vertices contained in the tail of the hyperedge and the second the vertices contained in the head. These two entries are also named `from`

and `to`

.

## Vertex Names - `vnames`

This a vector of strings representing the name of each vertex. On a technical point, this is used as a reference for levels when factoring multiple groups of vertices.

## Vertex Weights - `vweights`

This is a vector of weights associated with each vertex. This has little practical use in this rhype implementation except to keep track of hyperedge weights when transforming a hypergraph into its dual.

## Hyperedge Names - `enames`

This a vector of strings representing the name of each hyperedge On a technical point, this is used as a reference for levels when factoring multiple groups of hyperedges.

## Hyperedge Weights - `eweights`

This is a vector of weights associated with each hyperedge.

## Weighted - `weighted`

This is a logical value indicating whether there are weights associated with the vertices or hyperedges.

## Oriented - `oriented`

This is a logical value indicating whether the hyperedges are oriented.

## Directed - `directed`

This is a logical value indicating whether the hyperedges are directed.

## Real Coefficients - `real_coef`

This is a logical vector indicating whether there are real coefficients associating the vertices to the hyperedges.

## Incidence Matrix - `inc_mat`

This is only used for hypergraphs with real coefficients.

For unoriented hypergraphs this is a matrix with a row for every vertex and a column for every hyperedge, where each entry represents the value with which the vertex and hyperedge are associated.

For oriented hypergraphs, `inc_mat`

is a list of two incidence matrices, with one representing incidence to one side of the hyepredge and the other representing incidence to the other.

For directed hypergraphs, `inc_mat`

is a list of two incidence matrices, with the first representing incidence to the tail of the hyperedge and the second representing incidence to the head of the hyperedge.