# Parameters

## Static parameters

Static parameters are values which can be defined before the evaluation of an expression. These
parameters can be accesed using the **Parameters** dictionary of the **Expression** instance.

```typescript
const e = new Expression('2 * [x] ^ 2 + 5 * [y]');
e.Parameters['x'] = 5;
e.Parameters['y'] = 1;

console.log(e.Evaluate());
```

Parameters can be useful when a value is unknown at compile time, or when performance is important
and the parsing can be saved for further calculations.

## Expression parameters

Expressions can be split into several ones by defining expression parameters. Those parameters are
not simple values but **Expression** instances themselves.

```typescript
const volume = new Expression('[surface] * h');
const surface = new Expression('[l] * [L]');
volume.Parameters['surface'] = surface;
surface.Parameters['l'] = 1;
surface.Parameters['L'] = 2;
```

## Dynamic parameters

Sometimes parameters can be even more complex to evaluate and need a dedicated method to be
evaluated. This can be done by intercepting there evaluation using the **EvaluateParameter** event
published on **Expression** instances. Thus, each time a parameter is not defined in the dictionary,
this event is called to try to resolve the value.

```typescript
const e = new Expression("Round(Pow([Pi], 2) + Pow([Pi], 2) + [X], 2)");

e.Parameters["Pi2"] = new Expression("Pi * [Pi]");
e.Parameters["X"] = 10;

e.EvaluateParameter["Pi"] = (args: ParameterArgs)
{
    args.Result = 3.14;
};
```

## Square brackets parameters

Parameters in between square brackets can contain special characters like spaces, dots, and also
start with digits.

```typescript
const e = new Expression('[My First Parameter] + [My Second Parameter]');
```

## Multi-valued parameters

When parameters are IEnumerable and the **EvaluationOptions.IterateParameters** is used, the result
is an **IList** made of the evaluation of each value in the parameter.

```typescript
const e = new Expression("(a * b) ^ c", EvaluateOption.IterateParameters);
 e.Parameters["a"] = new int[] { 1, 2, 3, 4, 5 };
 e.Parameters["b"] = new int[] { 6, 7, 8, 9, 0 };
 e.Parameters["c"] = 3;

 foreach (var result in (IList)e.Evaluate())
 {
     console.log(result);
 }

 //  216
 //  2744
 //  13824
 //  46656
 //  0
```