Minimum cut in a graph

min_cut() calculates the minimum st-cut between two vertices in a graph (if the source and target arguments are given) or the minimum cut of the graph (if both source and target are NULL).

Usage

min_cut(
  graph,
  source = NULL,
  target = NULL,
  capacity = NULL,
  value.only = TRUE
)

Arguments

graph

The input graph.

source

The id of the source vertex.

target

The id of the target vertex (sometimes also called sink).

capacity

Vector giving the capacity of the edges. If this is NULL (the default) then the capacity edge attribute is used.

value.only

Logical scalar, if TRUE only the minimum cut value is returned, if FALSE the edges in the cut and a the two (or more) partitions are also returned.

Value

For min_cut() a nuieric constant, the value of the minimum cut, except if value.only = FALSE. In this case a named list with components:

value

Numeric scalar, the cut value.

cut

Numeric vector, the edges in the cut.

partition1

The vertices in the first partition after the cut edges are removed. Note that these vertices might be actually in different components (after the cut edges are removed), as the graph may fall apart into more than two components.

partition2

The vertices in the second partition after the cut edges are removed. Note that these vertices might be actually in different components (after the cut edges are removed), as the graph may fall apart into more than two components.

Details

The minimum st-cut between source and target is the minimum total weight of edges needed to remove to eliminate all paths from source to target.

The minimum cut of a graph is the minimum total weight of the edges needed to remove to separate the graph into (at least) two components. (Which is to make the graph not strongly connected in the directed case.)

The maximum flow between two vertices in a graph is the same as the minimum st-cut, so max_flow() and min_cut() essentially calculate the same quantity, the only difference is that min_cut() can be invoked without giving the source and target arguments and then minimum of all possible minimum cuts is calculated.

For undirected graphs the Stoer-Wagner algorithm (see reference below) is used to calculate the minimum cut.

References

M. Stoer and F. Wagner: A simple min-cut algorithm, Journal of the ACM, 44 585-591, 1997.

See also

Other flow: dominator_tree(), edge_connectivity(), is_min_separator(), is_separator(), max_flow(), min_separators(), min_st_separators(), st_cuts(), st_min_cuts(), vertex_connectivity()

Examples

g <- make_ring(100)
min_cut(g, capacity = rep(1, vcount(g)))
#> [1] 2
min_cut(g, value.only = FALSE, capacity = rep(1, vcount(g)))
#> $value
#> [1] 2
#> 
#> $cut
#> + 2/100 edges from 18b0947:
#> [1] 1--2 2--3
#> 
#> $partition1
#> + 1/100 vertex, from 18b0947:
#> [1] 2
#> 
#> $partition2
#> + 99/100 vertices, from 18b0947:
#>  [1]   1   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20
#> [20]  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39
#> [39]  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58
#> [58]  59  60  61  62  63  64  65  66  67  68  69  70  71  72  73  74  75  76  77
#> [77]  78  79  80  81  82  83  84  85  86  87  88  89  90  91  92  93  94  95  96
#> [96]  97  98  99 100
#> 

g2 <- make_graph(c(1, 2, 2, 3, 3, 4, 1, 6, 6, 5, 5, 4, 4, 1))
E(g2)$capacity <- c(3, 1, 2, 10, 1, 3, 2)
min_cut(g2, value.only = FALSE)
#> $value
#> [1] 1
#> 
#> $cut
#> + 1/7 edge from 2b3a7a6:
#> [1] 2->3
#> 
#> $partition1
#> + 1/6 vertex, from 2b3a7a6:
#> [1] 2
#> 
#> $partition2
#> + 5/6 vertices, from 2b3a7a6:
#> [1] 1 3 4 5 6
#>