# Custom functions for surrogate construction¶

This example discusses the modularity provided by the framework to change various optimizer and surrogate related methods. More specifically, we look at changing the following accessory functions:

Through this tutorial, we shall not solve any new problem. However, explanations will be provided to how can one add custom definitions to these standard methods. We shall use an external implementation of the standard methods for the user to easily compare with Getting Started. While the example shows the usage of all of these at once, the user is encouraged to understand the effect of changing each of these accessory functions separately:

## Initialization strategy¶

Several situations arise when the user has to specify his own initialization strategies. Some of these include:

• Constraints, such as equality, inequality and PDE based constraints. In such situations, all locations determined by the upper and lower bounds may not be feasible. PARyOpt requires all the initial evaluation points to be successful evaluations, hence a general latin hypercube sampling may not work in all cases.
• Biased sampling of search domain. There could be situations where the user knows strategic locations and thus can help PARyOpt to sample in those locations right from the beginning
• User defined sampling strategy. The user may want a completely different sampling strategy which has a better coverage of the domain for that specific optimization problem.

In order to specify custom initialization strategy, the user should define a function with the following signature:

def my_init_strategy(n_dim: int, n_init: int, l_bound: np.array, u_bound: np.array) -> List[np.array]


and then pass it during initialization to the parameter init_strategy=my_init_strategy.

## Kernel functions (Class)¶

Kernel function embeds most of the information about the continuity of the underlying function. Thus, it is one of the critical parameters to be selected by the user. For example, Matern class of kernel functions have only finite differentiability while the squared exponential kernel function is infinitely differentiable. Some other situations where the user may want to change the kernel function are :

• Underlying function continuity and differentiability, as discussed above.
• Prior information about periodicity of the underlying function. In such situations, periodicity can be embedded into the kernel function. This can drastically improve the number of function evaluations. One such example of a periodic kernel function is:
$k_{periodic}(x_1, x_2) = \theta_0 exp(-2 sin(\frac{\pi}{p} \frac{||x_1 - x_2||}{l})^2)$

where $$p$$ is the periodicity interval and $$l$$ is the length scale of the kernel.

• Anisotropic kernels may be used when the function behaviour is different across different optimization parameters. For example, if the function varies logarithmically in one parameter and linearly in another parameter

To specify custom kernel functions, the user has to derive the base kernel class. An example to specify the standard Matern 3/2 function by the user is as follows:

class MyKernelFunction(KernelFunction):
"""
user customized kernel function
"""

theta = 1.0

def eval(self, x1: np.array, x2: np.array) -> float:
"""
actual kernel function
:param x1:
:param x2:
:return:
"""
x1 = np.divide(x1, self.theta)
x2 = np.divide(x2, self.theta)
dist = utils.distance(x1, x2)
rval = np.sqrt(3.0) * dist
return self.theta0 * (1+rval) * np.exp(-rval)

def derivative(self, x1: np.array, x2: np.array) -> np.array:
"""
derivative of kernel function
currently not useful, so we will not implement anything here
:param x1:
:param x2:
:return:
"""
pass


This derived class requires defining the eval() and derivative() methods of the class. The parameter theta should contain all the hyper-parameters, for ex. length scale, related to the supplied kernel. This will be used by PARyOpt during hyper-parameter optimization. This kernel class can be passed to the constructor through the parameter kern_function=MyKernelFunction().

## Acquisition functions¶

Acquisition function define informative regions of the surrogate. Since the software is designed for minimization, areas of high information should have small acquisition values. In normal circumstances, the user should not need to change this. However, if the user wants implementing safety constraints and biased informativeness, it can be be done by passing a user defined function, with the following signature, to the constructor as acq_func=my_acquisition_function.

def my_acquisition_function(mean: float, variance: float, curr_best: float = 0., kappa: float = 1.) -> float:


## Acquisition optimizer¶

Bayesian optimization proceeds by evaluating locations with maximum information content. Hence, it requires finding the optimum (minimum in PARyOpt) of the acquisition function (cheap to evaluate). While the core software comes with a standard Powell algorithm, the user may quite often want to change this for a better global optimum. Some of the reasons include:

• Dimensionality of optimization could impose restrictions on the type of optimizer used
• Heuristic optimization can be an alternative for multi-modal functions. Since evaluating the acquisition is fast, these optimizers will also be efficient.
• Robust in-house optimizers may be available with research groups tailored for specific problems.

The process of adding user-defined acquisition optimizer is very similar to defining custom acquisition function. The function signature is :

def my_acq_optimizer(func: Callable[[np.array], float], x0: np.array, l_bound: np.array, u_bound: np.array) -> np.array:


which is passed to the constructor as acq_func_optimizer=my_acq_optimizer.