create_combinations: Create Valid Combinations

In growR: Implementation of the Vegetation Model ModVege

Create Valid Combinations

Description

Generate a list which contains all possible combinations of the provided parameter values. This excludes combinations that are invalid because the sum criterion for functional groups w_FGA + w_FGB + w_FGC + w_FGD = 1 is not fulfilled.

Usage

create_combinations(param_values, eps = 0.02)

Arguments

param_values	A list giving all options for the parameter values which are to be combined. The format is list[[param_name]] = param_values where param_values is a vector with the values for the respective parameter. The parameter names for functional group weights (w_FGX with X in (A, B, C, D)) receive special treatment and therefore need to be spelled correctly.
eps	Float specifying the precision to which the sum criterion for functional group has to be satisfied. The criterion is considered satisfied, if “' abs(w_FGA + w_FGB + w_FGC + w_FGD) - 1) <= eps

Details

Assume for example the following list as argument param_values:

list(w_FGA = c(0, 0.5, 1), w_FGB = c(0, 0.5, 1), NI = c(0.5, 0.9))

This would generate the combinations

w_FGA	w_FGB	NI
0	1	0.5
0	1	0.9
0.5	0.5	0.5
0.5	0.5	0.9
1	0	0.5
1	0	0.9

One can see that the input param_values has to be set up carefully: one has to ensure that the given w_FGX values can actually add up to 1. The following would be a bad counterexample, where only one single valid combination is found, even though many values for w_FGA and w_FGB are provided:

list(w_FGA = seq(0.5, 1, 0.01), w_FGB = c(0.5, 1, 0.01))

Similarly, if the steps in the w_FGX don't match, we might not end up with many valid combinations, even though the ranges are reasonabl:

list(w_FGA = seq(0.5, 1, 0.1), w_FGB = c(0, 0.5, 0.25))

Here, no combination can be made with w_FGA in c(0.6, 0.7, 0.8, 0.9) or w_FGB = 0.25.

Value

combinations An unnamed list where every entry is a list containing the parameter values (named as in the input param_values) for a valid combination.

Examples

# Define the parameter steps you want to explore. This is a minimal example.
# A more realistic one follows below.
param_values = list(w_FGA = c(0, 0.5, 1),
                    w_FGB = c(0, 0.5, 1),
                    NI = c(0.5, 0.9)
)
# Create all valid combinations of the defined steps
create_combinations(param_values)

# More realistic example for an initial exploration of parameter space, 
# where we suspect that functional groups A and B should be more prevalent 
# than C and D. This produces 54 parameter combinations, which is a number 
# of model evaluations that can run within a reasonable timeframe 
# (depending on your system).
param_values = list(w_FGA = seq(0, 1, 0.33),
                    w_FGB = seq(0, 1, 0.33),
                    w_FGC = seq(0, 0.7, 0.33),
                    w_FGD = seq(0, 0.7, 0.33),
                    NI = seq(0.5, 1.0, 0.25)
)
length(create_combinations(param_values))

# The default value for *eps* made sure that combinations of 0.33 + 0.66 = 
# 0.99 etc. are considered "valid". If we make *eps* too small, no valid 
# combinations can be found:
length(create_combinations(param_values, eps = 1e-3))

growR documentation built on May 29, 2024, 9:12 a.m.