This function enables the smart exploration of the solution space of potential structural configurations of a funGp model, and the consequent selection of a high quality configuration. funGp currently relies on an ant colony based algorithm to perform this task. The algorithm defines the solution space based on the levels of each structural parameter currently available in the fgpm function, and performs as smart exploration of it. More details on the algorithm are provided in a dedicated technical report. funGp might evolve in the future to include improvements in the current algorithm or alternative solution methods.

fgpm_factory(
  sIn = NULL,
  fIn = NULL,
  sOut = NULL,
  ind.vl = NULL,
  ctraints = list(),
  setup = list(),
  time.lim = Inf,
  nugget = 1e-08,
  n.starts = 1,
  n.presample = 20,
  par.clust = NULL,
  pbars = TRUE
)

Arguments

sIn

an optional matrix of scalar input values to train the model. Each column must match an input variable and each row a training point. Either scalar input coordinates (sIn), functional input coordinates (fIn), or both must be provided.

fIn

an optional list of functional input values to train the model. Each element of the list must be a matrix containing to the set of curves corresponding to one functional input. Either scalar input coordinates (sIn), functional input coordinates (fIn), or both must be provided.

sOut

a vector (or 1-column matrix) containing the values of the scalar output at the specified input points.

ind.vl

an optional numerical matrix specifying which points in the three structures above should be used for training and which for validation. If provided, the optimization will be conducted in terms of the hold-out Q2, which comes from training the model with a subset of the points, and then estimate the prediction error in the remaining points. In that case, each column of ind.vl will be interpreted as one validation set, and the multiple columns will imply replicates. In the simplest case, ind.vl will be a one-column matrix or simply an array, meaning that a simple replicate should be used for each model configuration explored. If not provided, the optimization will be conducted in terms of the leave-one-out cross-validation Q2, which for a total number of n observations, comes from training the model n times, each using n-1 points for training and the remaining one for validation. This procedure is typically costly due to the large number of hyperparameters optimizations that should be conducted, nonetheless, fgpm_factory implements the virtual equations introduced by Dubrule (1983) for Gaussian processes, which require a single hyperparameters optimization. See the reference below for more details.

ctraints

an optional list specifying the constraints of the structural optimization problem. Valid entries for this list are:

*s_keepOn: a numerical array indicating the scalar inputs that should remain active in the model. It should contain the index of the columns of sIn corresponding to the inputs to keep active.

*f_keepOn: a numerical array indicating the functional inputs that should remain active in the model. It should contain the index of the elements of fIn corresponding to the inputs to keep active.

*f_disTypes: a list specifying the set of distances that should be tested for some functional inputs. The values should be taken from the possibilities offered by the fgpm function for the argument f_disType therein. Valid choices at this time are "L2_bygroup" and "L2_byindex". Each element of the list should receive as name the index of a functional input variable, and should contain an array of strings with the name of the distances allowed for this input. All the available distances will be tried for any functional input not included in the list.

*f_fixDims: a two-row matrix specifying a particular projection dimension for some functional inputs. For each input, the value should be a number between 0 and its original dimension, with 0 denoting no projection. The first row of the matrix should contain the index of each input, and the second row should contain the corresponding dimensions. All the possible dimensions will be tried for any functional input not included in the matrix (unless affected by the f_maxDims argument below).

*f_maxDims: a two-row matrix specifying the largest projection dimension for some functional inputs. For each input, the value should be a number between 1 and its original dimension. The first row of the matrix should contain the index of each input, and the second row should contain the corresponding largest dimensions. All the possible dimensions will be tried for any functional input not included in the matrix (unless affected by the f_fixDims argument above).

*f_basTypes: a list specifying the set of basis families that should be tested for some functional inputs. The values should be taken from the possibilities offered by the fgpm function for the argument f_basType therein. Valid choices at this time are "B-splines" and "PCA". Each element of the list should receive as name the index of a functional input variable, and should contain an array of strings with the name of the distances allowed for this input. All the available basis families will be tried for any functional input not included in the list.

*kerTypes: an array of strings specifying the kernel functions allowed to be tested. The values should be taken from the possibilities offered by the fgpm function for the argument kerType therein. Valid choices at this time are "gauss", "matern5_2" and "matern3_2". If not provided, all the available kernel functions will be tried.

setup

an optional list indicating the value for some parameters of the structural optimization algorithm. The ant colony optimization algorithm available at this time allows the following entries:

Initial pheromone load

*tao0: a number indicating the initial pheromone load on links pointing out to the selection of a distance type, a projection basis or a kernel type. Default is 0.1.

*dop.s: a number controlling how likely is to activate a scalar input. It operates on a relation of the type \(A = dop.s * I\), where A is the initial pheromone load of links pointing out to the activation of scalar inputs and I is the initial pheromone load of links pointing out to their inactivation. Default is 1.

*dop.f: analogous to dop.s for functional inputs. Default is 1.

*delta.f and dispr.f: two numbers used as shape parameters for the regularization function that determines the initial pheromone values on the links connecting the L2_byindex distance with the projection dimension. Default are 2 and 1.4, respectively.

Local pheromone update

*rho.l: a number specifying the pheromone evaporation rate. Default is 0.1

Global pheromone update

*u.gbest: a boolean indicating if at each iterations, the pheromone load on the links of the best ant of the whole trial should be reinforced. Default is FALSE.

*n.ibest: a number indicating how many top ants of each iteration should be used for pheromone reinforcement. Default is 1.

*rho.g: a number specifying the learning reinforcement rate. Default is 0.1.

Population factors

*n.iter: a number specifying the amount of iterations of the algorithm. Default is 15.

*n.pop: a number specifying the amount of ants per iteration; each ant corresponds to one structural configuration for the model. Default is 10.

Bias strength

*q0: ants use one of two rules to select their next node at each step. The first rule leads the ant through the link with higher pheromone load; the second rule works based on probabilities which are proportional to the pheromone load on the feasible links. The ants will randomly chose one of the two rules at each time. They will opt for rule 1 with probability q0. Default is 0.95.

time.lim

an optional number specifying a time limit in seconds to be used as stopping condition for the structural optimization.

nugget

an optional variance value standing for the homogeneous nugget effect. A tiny nugget might help to overcome numerical problems related to the ill-conditioning of the covariance matrix. Default is 1e-8.

n.starts

an optional integer indicating the number of initial points to use for the optimization of the hyperparameters. A parallel processing cluster can be exploited in order to speed up the evaluation of multiple initial points. More details in the description of the argument par.clust below. Default is 1.

n.presample

an optional integer indicating the number of points to be tested in order to select the n.starts initial points. The n.presample points will be randomly sampled from the hyper-rectangle defined by:

1e-10 \(\le\) ls_s.hyp[i] \(\le\) 2*max(sMs[[i]]), for i in 1 to the number of scalar inputs,
1e-10 \(\le\) ls_f.hyp[i] \(\le\) 2*max(fMs[[i]]), for i in 1 to the number of functional inputs,

with sMs and fMs the lists of distance matrices for the scalar and functional inputs, respectively. The value of n.starts will be assigned to n.presample if this last is smaller. Default is 20.

par.clust

an optional parallel processing cluster created with the makeCluster function of the parallel package. If not provided, structural configurations are evaluated in sequence.

pbars

an optional boolean indicating if progress bars should be displayed. Default is TRUE.

Value

An object of class Xfgpm containing the data structures linked to the structural optimization of a funGp model. It includes as the main component, an object of class fgpm corresponding to the optimized model. It is accessible through the @model slot of the Xfgpm object.

References

Betancourt, J., Bachoc, F., Klein, T., Idier, D., Pedreros, R., and Rohmer, J. (2020), "Gaussian process metamodeling of functional-input code for coastal flood hazard assessment". Reliability Engineering & System Safety, 198, 106870. [RESS] [HAL]

Betancourt, J., Bachoc, F., Klein, T., and Gamboa, F. (2020), Technical Report: "Ant Colony Based Model Selection for Functional-Input Gaussian Process Regression. Ref. D3.b (WP3.2)". RISCOPE project. [HAL]

Betancourt, J., Bachoc, F., and Klein, T. (2020), R Package Manual: "Gaussian Process Regression for Scalar and Functional Inputs with funGp - The in-depth tour". RISCOPE project. [HAL]

Dubrule, O. (1983), "Cross validation of kriging in a unique neighborhood". Journal of the International Association for Mathematical Geology, 15, 687-699. [MG]

See also

* plotX for diagnostic plots for a fgpm_factory output and selected model;

* plotEvol for a plot of the evolution of the model selection algorithm in fgpm_factory;

* get_active_in for post-processing of input data structures following a fgpm_factory call;

* predict for predictions based on a funGp model;

* simulate for simulations based on a funGp model;

* update for post-creation updates on a funGp model.

Examples

# calling fgpm_factory with the default arguments__________________________________________
# generating input and output data
set.seed(100)
n.tr <- 32
sIn <- expand.grid(x1 = seq(0,1,length = n.tr^(1/5)), x2 = seq(0,1,length = n.tr^(1/5)),
                   x3 = seq(0,1,length = n.tr^(1/5)), x4 = seq(0,1,length = n.tr^(1/5)),
                   x5 = seq(0,1,length = n.tr^(1/5)))
fIn <- list(f1 = matrix(runif(n.tr*10), ncol = 10), f2 = matrix(runif(n.tr*22), ncol = 22))
sOut <- fgp_BB7(sIn, fIn, n.tr)
# \donttest{
# optimizing the model structure with fgpm_factory (~12 seconds)
xm <- fgpm_factory(sIn = sIn, fIn = fIn, sOut = sOut)
plotLOO(xm@model) # plotting the model

# building the model with the default fgpm arguments to compare
m1 <- fgpm(sIn = sIn, fIn = fIn, sOut = sOut)
plotLOO(m1) # plotting the model

# assessing the quality of the model
# in the absolute and also w.r.t. the other explored models
plotX(xm)

# checking the evolution of the algorithm
plotEvol(xm)
# }
# \donttest{
# improving performance with more iterations_______________________________________________
# generating input and output data
set.seed(100)
n.tr <- 32
sIn <- expand.grid(x1 = seq(0,1,length = n.tr^(1/5)), x2 = seq(0,1,length = n.tr^(1/5)),
                   x3 = seq(0,1,length = n.tr^(1/5)), x4 = seq(0,1,length = n.tr^(1/5)),
                   x5 = seq(0,1,length = n.tr^(1/5)))
fIn <- list(f1 = matrix(runif(n.tr*10), ncol = 10), f2 = matrix(runif(n.tr*22), ncol = 22))
sOut <- fgp_BB7(sIn, fIn, n.tr)

# default of 15 iterations (~12 seconds)
xm15 <- fgpm_factory(sIn = sIn, fIn = fIn, sOut = sOut)

# increasing to 25 iterations (~20 seconds)
xm25 <- fgpm_factory(sIn = sIn, fIn = fIn, sOut = sOut, setup = list(n.iter = 25))

# plotting both models
plotLOO(xm15@model)
plotLOO(xm25@model)
# }
# \donttest{
# custom solution space____________________________________________________________________
# generating input and output data
set.seed(100)
n.tr <- 32
sIn <- expand.grid(x1 = seq(0,1,length = n.tr^(1/5)), x2 = seq(0,1,length = n.tr^(1/5)),
                   x3 = seq(0,1,length = n.tr^(1/5)), x4 = seq(0,1,length = n.tr^(1/5)),
                   x5 = seq(0,1,length = n.tr^(1/5)))
fIn <- list(f1 = matrix(runif(n.tr*10), ncol = 10), f2 = matrix(runif(n.tr*22), ncol = 22))
sOut <- fgp_BB7(sIn, fIn, n.tr)

# setting up the constraints
myctr <- list(s_keepOn = c(1,2), # keep both scalar inputs always on
              f_keepOn = c(2), # keep f2 always active
              f_disTypes = list("2" = c("L2_byindex")), # only use L2_byindex distance for f2
              f_fixDims = matrix(c(2,4), ncol = 1), # f2 projected in dimension 4
              f_maxDims = matrix(c(1,5), ncol = 1), # f1 projected in dimension max 5
              f_basTypes = list("1" = c("B-splines")), # only use B-splines projection for f1
              kerTypes = c("matern5_2", "gauss")) # test only Matern 5/2 and Gaussian kernels

# calling the funGp factory with specific constraints (~17 seconds)
xm <- fgpm_factory(sIn = sIn, fIn = fIn, sOut = sOut, ctraints = myctr)

# verifying constraints with the log of some successfully built models
cbind(xm@log.success@sols, "Q2" = xm@log.success@fitness)
# }
# \donttest{
# custom heuristic parameters______________________________________________________________
# generating input and output data
set.seed(100)
n.tr <- 32
sIn <- expand.grid(x1 = seq(0,1,length = n.tr^(1/5)), x2 = seq(0,1,length = n.tr^(1/5)),
                   x3 = seq(0,1,length = n.tr^(1/5)), x4 = seq(0,1,length = n.tr^(1/5)),
                   x5 = seq(0,1,length = n.tr^(1/5)))
fIn <- list(f1 = matrix(runif(n.tr*10), ncol = 10), f2 = matrix(runif(n.tr*22), ncol = 22))
sOut <- fgp_BB7(sIn, fIn, n.tr)

# defining the heuristic parameters
mysup <- list(n.iter = 30, n.pop = 12, tao0 = .15, dop.s = 1.2, dop.f = 1.3, delta.f = 4,
              dispr.f = 1.1, q0 = .85, rho.l = .2, u.gbest = TRUE, n.ibest = 2, rho.g = .08)

# calling the funGp factory with a custom heuristic setup (~17 seconds)
xm <- fgpm_factory(sIn = sIn, fIn = fIn, sOut = sOut, setup = mysup)

# verifying heuristic setup through the details of the Xfgpm object
unlist(xm@details$param)
# }
# \donttest{
# stopping condition based on time_________________________________________________________
# generating input and output data
set.seed(100)
n.tr <- 32
sIn <- expand.grid(x1 = seq(0,1,length = n.tr^(1/5)), x2 = seq(0,1,length = n.tr^(1/5)),
                   x3 = seq(0,1,length = n.tr^(1/5)), x4 = seq(0,1,length = n.tr^(1/5)),
                   x5 = seq(0,1,length = n.tr^(1/5)))
fIn <- list(f1 = matrix(runif(n.tr*10), ncol = 10), f2 = matrix(runif(n.tr*22), ncol = 22))
sOut <- fgp_BB7(sIn, fIn, n.tr)

# setting up a sufficiently large number of iterations
mysup <- list(n.iter = 2000)

# defining time budget
mytlim <- 60

# calling the funGp factory with time limit (~60 seconds)
xm <- fgpm_factory(sIn = sIn, fIn = fIn, sOut = sOut, setup = mysup, time.lim = mytlim)
# }
# \donttest{
# passing fgpm arguments through fgpm_factory______________________________________________
# generating input and output data
set.seed(100)
n.tr <- 32
sIn <- expand.grid(x1 = seq(0,1,length = n.tr^(1/5)), x2 = seq(0,1,length = n.tr^(1/5)),
                   x3 = seq(0,1,length = n.tr^(1/5)), x4 = seq(0,1,length = n.tr^(1/5)),
                   x5 = seq(0,1,length = n.tr^(1/5)))
fIn <- list(f1 = matrix(runif(n.tr*10), ncol = 10), f2 = matrix(runif(n.tr*22), ncol = 22))
sOut <- fgp_BB7(sIn, fIn, n.tr)

# calling the funGp factory with custom fgpm parameters (~25 seconds)
xm <- fgpm_factory(sIn = sIn, fIn = fIn, sOut = sOut,
                   nugget = 0, n.starts = 3, n.presample = 12)

# NOTE: in the run above, some models crash. This happens because we set the nugget to 0
#       and some input points become duplicates when some variables are removed from
#       the model. We strongly recommend to always run fgpm_factory with at least a
#       small nugget in order to prevent loss of configurations. By default fgpm_factory
#       runs with 1e-8, which is enough in most cases.
xm@log.crashes
# }
# \donttest{
# parallelization in the model factory_____________________________________________________
# generating input and output data
set.seed(100)
n.tr <- 243
sIn <- expand.grid(x1 = seq(0,1,length = n.tr^(1/5)), x2 = seq(0,1,length = n.tr^(1/5)),
                   x3 = seq(0,1,length = n.tr^(1/5)), x4 = seq(0,1,length = n.tr^(1/5)),
                   x5 = seq(0,1,length = n.tr^(1/5)))
fIn <- list(f1 = matrix(runif(n.tr*10), ncol = 10), f2 = matrix(runif(n.tr*22), ncol = 22))
sOut <- fgp_BB7(sIn, fIn, n.tr)

# calling fgpm_factory in parallel
cl <- parallel::makeCluster(2)
xm.par <- fgpm_factory(sIn = sIn, fIn = fIn, sOut = sOut, par.clust = cl) #  (~260 seconds)
parallel::stopCluster(cl)

# NOTE: in order to provide progress bars for the monitoring of time consuming processes
#       ran in parallel, funGp relies on the doFuture and future packages. Parallel processes
#       suddenly interrupted by the user tend to leave corrupt connections. This problem is
#       originated outside funGp, which limits our control over it. On section 4.1 of the
#       of funGp, we provide a temporary solution to the issue and we remain attentive in
#       case it appears a more elegant way to handle it or a manner to suppress it.
#
#       funGp manual: https://hal.archives-ouvertes.fr/hal-02536624
# }