Specify that the Gurobi software should be used to solve a conservation planning problem. This function can also be used to customize the behavior of the solver. It requires the gurobi package.

add_gurobi_solver(x, gap = 0.1, time_limit = .Machine\$integer.max,
presolve = 2, threads = 1, first_feasible = 0, verbose = TRUE)

## Arguments

x ConservationProblem-class object. numeric gap to optimality. This gap is relative when solving problems using gurobi, and will cause the optimizer to terminate when the difference between the upper and lower objective function bounds is less than the gap times the upper bound. For example, a value of 0.01 will result in the optimizer stopping when the difference between the bounds is 1 percent of the upper bound. numeric time limit in seconds to run the optimizer. The solver will return the current best solution when this time limit is exceeded. integer number indicating how intensively the solver should try to simplify the problem before solving it. The default value of 2 indicates to that the solver should be very aggressive in trying to simplify the problem. integer number of threads to use for the optimization algorithm. The default value of 1 will result in only one thread being used. logical should the first feasible solution be be returned? If first_feasible is set to TRUE, the solver will return the first solution it encounters that meets all the constraints, regardless of solution quality. Note that the first feasible solution is not an arbitrary solution, rather it is derived from the relaxed solution, and is therefore often reasonably close to optimality. Defaults to FALSE. logical should information be printed while solving optimization problems?

## Value

ConservationProblem-class object with the solver added to it.

## Details

Gurobi is a state-of-the-art commercial optimization software with an R package interface. It is by far the fastest of the solvers available in this package, however, it is also the only solver that is not freely available. That said, licenses are available to academics at no cost. The gurobi package is distributed with the Gurobi software suite. This solver uses the gurobi package to solve problems.

solvers.

## Examples

# load data
data(sim_pu_raster, sim_features)

# create problem
p <- problem(sim_pu_raster, sim_features) %>%
add_binary_decisions()# if the package is installed then add solver and generate solution
if (require("gurobi")) {
# specify solver and generate solution
s <- p %>% add_gurobi_solver(gap = 0.1, presolve = 2, time_limit = 5) %>%
solve()

# plot solutions
plot(stack(sim_pu_raster, s), main = c("planning units", "solution"),
axes = FALSE, box = FALSE)
}#> Loading required package: gurobi#> Loading required package: slam#> Optimize a model with 5 rows, 90 columns and 450 nonzeros
#> Variable types: 0 continuous, 90 integer (90 binary)
#> Coefficient statistics:
#>   Matrix range     [2e-01, 9e-01]
#>   Objective range  [2e+02, 2e+02]
#>   Bounds range     [1e+00, 1e+00]
#>   RHS range        [3e+00, 8e+00]
#> Found heuristic solution: objective 2337.9617505
#> Presolve time: 0.00s
#> Presolved: 5 rows, 90 columns, 450 nonzeros
#> Variable types: 0 continuous, 90 integer (90 binary)
#> Presolved: 5 rows, 90 columns, 450 nonzeros
#>
#>
#> Root relaxation: objective 1.931582e+03, 12 iterations, 0.00 seconds
#>
#>     Nodes    |    Current Node    |     Objective Bounds      |     Work
#>  Expl Unexpl |  Obj  Depth IntInf | Incumbent    BestBd   Gap | It/Node Time
#>
#>      0     0 1931.58191    0    4 2337.96175 1931.58191  17.4%     -    0s
#> H    0     0                    1985.6818841 1931.58191  2.72%     -    0s
#>
#> Explored 1 nodes (12 simplex iterations) in 0.00 seconds
#> Thread count was 1 (of 4 available processors)
#>
#> Solution count 2: 1985.68 2337.96
#>
#> Optimal solution found (tolerance 1.00e-01)
#> Best objective 1.985681884076e+03, best bound 1.931581908865e+03, gap 2.7245%