make_diet_grid: Generate a regular grid of diet compositions
In qfasar: Quantitative Fatty Acid Signature Analysis in R

Description Usage Arguments Value Details References Examples

The function make_diet_grid generates a systematic grid of regularly-spaced diet compositions with a user-specified resolution.

1	make_diet_grid(uniq_types, inv_inc)

`uniq_types`	A factor of unique prey-type names.
`inv_inc`	The integer inverse of the resolution between consecutive diet compositions.

A list containing the following elements:

diet_grid: A numeric matrix of grid diet compositions, in column-major format.
err_code: An integer error code (0 if no error is detected).
err_message: A string contains a brief summary of the execution.

The function make_diet_grid generates a systematic grid of regularly-spaced diet compositions throughout the space of all possible diets for a given number of prey types. Such a diet composition grid may be useful in some simulation studies of estimator performance (e.g., Bromaghin et al. 2016). Given a diet composition, predator fatty acid signatures can be generated using the function make_pred_sigs. The diets of such simulated predators can then be estimated, and the resulting estimates can be compared to the known diet composition to evaluate bias, variance, and perhaps other properties.

The algorithm starts with a diet proportion of 1.0 assigned to the first prey type, and therefore 0.0 for the other prey types. The algorithm then begins an iterative loop in which an increment of diet proportion is repeatedly shifted to the other prey types, stopping when the last prey type has a diet proportion of 1.0. The user controls the resolution of the grid by specifying the integer inverse of the desired diet increment. For example, an inverse increment of 10 would produce diet compositions with proportions shifted by an increment of 0.1. See Bromaghin et al. (2016) for a small example with three prey types and a diet increment of 0.25. However, note that unlike Bromaghin et al. (2016), make_diet_grid retains diet compositions comprised of a single prey type.

It is critical that the prey-type names match those in the prey library. The easiest way to ensure this happens is to pass the object uniq_types returned a call to the function prep_sig as the uniq_types argument. Alternatively, and more risky, a vector of unique prey names can be created using the concatenate function and cast as a factor, i.e., uniq_types <- as.factor(c("Prey_1", "Prey_2", ..., "Prey_P)).

NOTE: The number of possible diets grows quickly as the number of prey types increases and the diet increment decreases, and may exceed memory limits.

Bromaghin, J.F., S.M. Budge, and G.W. Thiemann. 2016. Should fatty acid signature proportions sum to 1 for diet estimation? Ecological Research 31:597-606.

make_diet_grid(uniq_types = as.factor(c("Bearded",
                                        "Beluga",
                                        "Bowhead",
                                        "Ribbon",
                                        "Ringed",
                                        "Spotted",
                                        "Walrus")),
               inv_inc = 10)