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### facility.R

# Copyright 2023, Gurobi Optimization, LLC
#
# Facility location: a company currently ships its product from 5 plants
# to 4 warehouses. It is considering closing some plants to reduce
# costs. What plant(s) should the company close, in order to minimize
# transportation and fixed costs?
#
# Note that this example uses lists instead of dictionaries.  Since
# it does not work with sparse data, lists are a reasonable option.
#
# Based on an example from Frontline Systems:
#   http://www.solver.com/disfacility.htm
# Used with permission.

library(Matrix)
library(gurobi)

# define primitive data
nPlants     <- 5
nWarehouses <- 4
# Warehouse demand in thousands of units
Demand      <- c(15, 18, 14, 20)
# Plant capacity in thousands of units
Capacity    <- c(20, 22, 17, 19, 18)
# Fixed costs for each plant
FixedCosts  <- c( 12000, 15000, 17000, 13000, 16000)
# Transportation costs per thousand units
TransCosts  <- c(4000, 2000, 3000, 2500, 4500,
2500, 2600, 3400, 3000, 4000,
1200, 1800, 2600, 4100, 3000,
2200, 2600, 3100, 3700, 3200 )

flowidx <- function(w, p) {nPlants * (w-1) + p}

# Build model
model <- list()
model$modelname <- 'facility' model$modelsense <- 'min'

# initialize data for variables
model$lb <- 0 model$ub       <- c(rep(1, nPlants),   rep(Inf, nPlants * nWarehouses))
model$vtype <- c(rep('B', nPlants), rep('C', nPlants * nWarehouses)) model$obj      <- c(FixedCosts, TransCosts)
model$varnames <- c(paste0(rep('Open',nPlants),1:nPlants), sprintf('Trans%d,%d', c(mapply(rep,1:nWarehouses,nPlants)), 1:nPlants)) # build production constraint matrix A1 <- spMatrix(nPlants, nPlants, i = c(1:nPlants), j = (1:nPlants), x = -Capacity) A2 <- spMatrix(nPlants, nPlants * nWarehouses, i = c(mapply(rep, 1:nPlants, nWarehouses)), j = mapply(flowidx,1:nWarehouses,c(mapply(rep,1:nPlants,nWarehouses))), x = rep(1, nWarehouses * nPlants)) A3 <- spMatrix(nWarehouses, nPlants) A4 <- spMatrix(nWarehouses, nPlants * nWarehouses, i = c(mapply(rep, 1:nWarehouses, nPlants)), j = mapply(flowidx,c(mapply(rep,1:nWarehouses,nPlants)),1:nPlants), x = rep(1, nPlants * nWarehouses)) model$A           <- rbind(cbind(A1, A2), cbind(A3, A4))
model$rhs <- c(rep(0, nPlants), Demand) model$sense       <- c(rep('<', nPlants), rep('=', nWarehouses))
model$constrnames <- c(sprintf('Capacity%d',1:nPlants), sprintf('Demand%d',1:nWarehouses)) # Save model gurobi_write(model,'facilityR.lp') # Guess at the starting point: close the plant with the highest fixed # costs; open all others first open all plants model$start <- c(rep(1,nPlants),rep(NA, nPlants * nWarehouses))

# find most expensive plant, and close it in mipstart
cat('Initial guess:\n')
worstidx <- which.max(FixedCosts)
model$start[worstidx] <- 0 cat('Closing plant',worstidx,'\n') # set parameters params <- list() params$method <- 2

# Optimize
res <- gurobi(model, params)

# Print solution
if (res$status == 'OPTIMAL') { cat('\nTotal Costs:',res$objval,'\nsolution:\n')
cat('Facilities:', model$varnames[which(res$x[1:nPlants]>1e-5)], '\n')
active <- nPlants + which(res$x[(nPlants+1):(nPlants*(nWarehouses+1))] > 1e-5) cat('Flows: ') cat(sprintf('%s=%g ',model$varnames[active], res\$x[active]), '\n')
rm(active)
} else {
cat('No solution\n')
}

# Clear space
rm(res, model, params, A1, A2, A3, A4)


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