18.2 Fixed sizes and indexing out of bounds
Stan’s matrices, vectors, and array variables are sized when they are declared and may not be dynamically resized. Function arguments do not have sizes, but these sizes are fixed when the function is called and the container is instantiated. Also, declarations may be inside loops and thus may change over the course of running a program, but each time a declaration is visited, it declares a fixed size object.
When an index is provided that is out of bounds, Stan throws a rejection error and computation on the current log density and gradient evaluation is halted and the algorithm is left to clean up the error. All of Stan’s containers check the sizes of all indexes.