# How visitor design pattern is useful for numerical programming?

## Introduction

The visitor design pattern separates operations from data storage. When the efficiency and memory consumption is not of concern this design pattern is not common. However, in numerical programming it can be a game-changer.

## Numerical Example

In most of the numerical programs, we face a huge amount of data stored in arrays. For example, on a Cartesian grid we have 10⁶ points where each point contains below properties:

``````class Point{
public:
double Density;
double VelocityX;
double VelocityY;
}
``````

and we consume it in a vector

``````vector<Point>(1e6)
``````

so far so good. However, this data is supposed to be modified through some solvers or equations like

``````Update Density => {
Density = Alpha *(VelocityX+VelocityY)/2;
}
``````

or an initializer

``````Initialize Density ==> {
Density = Beta*(x+y); // dependent to location
}
``````

We cannot simply make this modifiers class members for three reasons: first we negate single responsibility principle for point class, second we cannot extend modifiers separately, and last but most important here, the point class needs to accommodate alpha and beta. So we would have 10⁶ of alpha and beta. As we add more modifiers they need more fields (properties).

## Solution

We keep point class the same. It is extended only based on its initial concept, e.g., `Point3d` would be a derived class with field `veclocityZ`.

`Point` implements just a method called `Accept`:

``````class Point
{
public:
double Density;
double VelocityX;
double VelocityY;
void Accept(IVisitor& visitor){
visitor.Visit(this);
}
}
``````

A modifier needs to implement `IVisitor` abstract class (or interface):

``````class IVisitor {
public:
void Visit(Point& point) =0
}
``````

Now we can define modifiers:

``````class DensityUpdater: public IVisitor
{
public:
double Alpha;
void Visit(Point& point){
point.Density = Alpha * (point.VelocityX + point.VelocityY) / 2;
}
}
``````

And another one

``````class DensityInitializer: public IVisitor {
public:
double Beta;
void Visit(Point& point){
point.Density = Beta * (point.x + point.y);
}
}
``````

Now somewhere in a the code we can have

``````DensityInitializer densityInitializer;
DensityInitializer.Beta = 1000.0;

DensityUpdater densityUpdater;
densityUpdater.Alpha = 0.5;

for(auto& point:points){
point.Accept(densityInitializer);
}

// and somewhere else
for(auto& point:points){
point.Accept(densityUpdater);
}
``````

## Benefits

• Storage class (Point) is concerning itself not new modifiers,
• Modifiers not store data in storage class to be repeated numerously,
• Modifiers can have any shape and extend in any way independent of storage class (loosely coupled).

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