igraph-es-indexing {igraph}R Documentation

Indexing edge sequences

Description

Edge sequences can be indexed very much like a plain numeric R vector, with some extras.

Usage

## S3 method for class 'igraph.es'
x[...]

Arguments

x

An edge sequence

...

Indices, see details below.

Value

Another edge sequence, referring to the same graph.

Multiple indices

When using multiple indices within the bracket, all of them are evaluated independently, and then the results are concatenated using the c() function. E.g. E(g)[1, 2, .inc(1)] is equivalent to c(E(g)[1], E(g)[2], E(g)[.inc(1)]).

Index types

Edge sequences can be indexed with positive numeric vectors, negative numeric vectors, logical vectors, character vectors:

Edge attributes

When indexing edge sequences, edge attributes can be referred to simply by using their names. E.g. if a graph has a weight edge attribute, then E(G)[weight > 1] selects all edges with a weight larger than one. See more examples below. Note that attribute names mask the names of variables present in the calling environment; if you need to look up a variable and you do not want a similarly named edge attribute to mask it, use the .env pronoun to perform the name lookup in the calling environment. In other words, use E(g)[.env$weight > 1] to make sure that weight is looked up from the calling environment even if there is an edge attribute with the same name. Similarly, you can use .data to match attribute names only.

Special functions

There are some special igraph functions that can be used only in expressions indexing edge sequences:

.inc

takes a vertex sequence, and selects all edges that have at least one incident vertex in the vertex sequence.

.from

similar to .inc(), but only the tails of the edges are considered.

.to

is similar to .inc(), but only the heads of the edges are considered.

%--%

a special operator that can be used to select all edges between two sets of vertices. It ignores the edge directions in directed graphs.

%->%

similar to %--%, but edges from the left hand side argument, pointing to the right hand side argument, are selected, in directed graphs.

%<-%

similar to %--%, but edges to the left hand side argument, pointing from the right hand side argument, are selected, in directed graphs.

Note that multiple special functions can be used together, or with regular indices, and then their results are concatenated. See more examples below.

See Also

Other vertex and edge sequences: E(), V(), igraph-es-attributes, igraph-es-indexing2, igraph-vs-attributes, igraph-vs-indexing2, igraph-vs-indexing, print.igraph.es(), print.igraph.vs()

Other vertex and edge sequence operations: c.igraph.es(), c.igraph.vs(), difference.igraph.es(), difference.igraph.vs(), igraph-es-indexing2, igraph-vs-indexing2, igraph-vs-indexing, intersection.igraph.es(), intersection.igraph.vs(), rev.igraph.es(), rev.igraph.vs(), union.igraph.es(), union.igraph.vs(), unique.igraph.es(), unique.igraph.vs()

Examples

# -----------------------------------------------------------------
# Special operators for indexing based on graph structure
g <- sample_pa(100, power = 0.3)
E(g) [ 1:3 %--% 2:6 ]
E(g) [ 1:5 %->% 1:6 ]
E(g) [ 1:3 %<-% 2:6 ]

# -----------------------------------------------------------------
# The edges along the diameter
g <- sample_pa(100, directed = FALSE)
d <- get_diameter(g)
E(g, path = d)

# -----------------------------------------------------------------
# Select edges based on attributes
g <- sample_gnp(20, 3/20) %>%
  set_edge_attr("weight", value = rnorm(gsize(.)))
E(g)[[ weight < 0 ]]

# Indexing with a variable whose name matches the name of an attribute
# may fail; use .env to force the name lookup in the parent environment
E(g)$x <- E(g)$weight
x <- 2
E(g)[.env$x]


[Package igraph version 1.3.5 Index]