Transformed parameter expressions are used to transform the parameter
values sampled with `eval_rng()`

as a function of input data
(treatment strategies and patients) and time
intervals. These functions are used when evaluating an object of class
`model_def`

defined using `define_model()`

. The transformed parameters
are ultimately converted into tparams objects and used to simulate outcomes with an
economic model.

```
define_tparams(expr, times = NULL, ...)
eval_tparams(x, input_data, rng_params)
```

- expr
Expressions used to transform parameters. As with

`define_rng()`

, braces should be used so that the result of the last expression within the braces is evaluated. The expression must return a named list with the following possible elements:*tpmatrix*: The transition probability matrix used to simulate transition probabilities in the economic model. This should either be the output of`tpmatrix()`

or a 3-dimensional array as in`tparams_transprobs()`

.*utility*: The utility values to attach to states and used to simulate quality-adjusted life-years in the economic model. Either a vector (in which case utility is the same in each health state) or a`data.table`

/`data.frame`

/`matrix`

with a column for each (non-death) health state.*costs*: A named list of costs for each category used to simulate costs in the economic model. Each element of the list must be in the same format as`utility`

.

- times
Distinct times denoting the stopping time of time intervals.

- ...
Additional arguments to pass to the environment used to evaluate

`expr`

.- x
An object of class

`tparams_def`

.- input_data
An object of class expanded_hesim_data (as in

`eval_model()`

) expanded by the distinct times in`times`

.- rng_params
Random samples of the parameters returned by

`eval_rng()`

.

`define_tparams()`

returns an object of class `tparams_def`

,
which is a list containing the unevaluated "transformation" expressions
passed to `expr`

, `times`

, and any additional arguments passed to
`...`

. `eval_tparams()`

evaluates the `tparams_def`

object
and should return a list of transformed parameter objects.

`define_tparams()`

is evaluated when creating economic models as a
function of `model_def`

objects defined with `define_model()`

. Operations
are "vectorized" in the sense that they are performed for each unique combination
of `input_data`

and `params`

. `expr`

is evaluated in an environment including
each variable from `input_data`

, all elements of `rng_params`

, and a variable
`time`

containing the values from `times`

. The `time`

variable can be used
to create models where parameters vary as a function of time.
`eval_tparams()`

is not exported and is only meant for use within `eval_model()`

.