This vignette attends to give a brief insight on the different
statements that may be used to build a structural model, compatible with
CAMPSIS.

The number of model statement types proposed in `campsismod`

is rather limited, with the underlying idea of keeping our model simple
and the translation to RxODE and mrgsolve as clean as possible. However,
in a near future, new types of model statements will likely be
supported.

Equations are described by 3 fields:

- lhs: the left-hand side string, which corresponds to the variable name
- rhs: the right-hand side string, which corresponds to the formula
- comment: a possible comment

Example:

The equivalent text form is:

`## KA=THETA_KA * exp(ETA_KA) # This is my KA parameter`

Similarly, Ordinary differential equations (ODE’s) are described by 3 fields as well:

- lhs: the left-hand side string, which corresponds to the variable name, must start with ‘A_’
- rhs: the right-hand side string, which corresponds to the derivative formula
- comment: a possible comment

Example:

The equivalent text form is:

`## d/dt(A_DEPOT)=-KA * A_DEPOT # This is my depot compartment`

Line breaks can be added to a model to add clear separations between blocks of equations. It does not have any field.

Example:

The equivalent text form is obviously a separation line.

Comments can be specified at any place in the model. They have a unique field:

- x: the comment you’d like to add

Example:

The equivalent text form is:

`## # This is my first comment`

If-statements allow a variable to take different values according to different specified conditions.

- condition: the given condition
- equation: an equation (see definition above)
- comment: a possible comment

Example:

`ifStatement <- IfStatement("COV==1", Equation("COV_EFFECT", "0.2"), comment="This is an if statement")`

The equivalent text form is:

`## if (COV==1) COV_EFFECT=0.2 # This is an if statement`

A common use of the if-statements is to add covariate effects into the model. Here is an example:

```
main <- MainRecord()
main <- main %>%
add(Equation("COV_EFFECT", "0")) %>% # Initialisation
add(IfStatement("COV==1", Equation("COV_EFFECT", "0.1"))) %>% # Covariate value is 1 in dataset
add(IfStatement("COV==2", Equation("COV_EFFECT", "0.2"))) %>% # Covariate value is 2 in dataset
add(IfStatement("COV==3", Equation("COV_EFFECT", "0.3"))) # Covariate value is 3 in dataset
```

The equivalent text would then be:

```
## [MAIN]
## COV_EFFECT=0
## if (COV==1) COV_EFFECT=0.1
## if (COV==2) COV_EFFECT=0.2
## if (COV==3) COV_EFFECT=0.3
```

Please note that mrgsolve require all extra variables (like COV_EFFECT in the previous example) to be initialised to a predefined value, which makes a lot of sense in general.

Package `campsismod`

does not check the equations from a
grammar point of view. This work is delegated to the simulation engine
(RxODE or mrgsolve) and C compiler.

In general, respecting the few rules listed below will give you a successful compatibility with both engines:

- All formula’s and if-statement conditions must be written in C code
- Functions included in the Math library of C are compatible (e.g. cos, sin, etc)
- Use the pow function for writing exponents (^ is accepted in RxODE, not in mrgsolve)
- Scientific notation for numbers is accepted (e.g. 2.51e-01)
- Initialise all extra variables (see example in the section above)