What is the use case for actually wanting to alter function parameters? In scientific programming, you often need routines to change values in more than one da

What is the use case for actually wanting to alter function parameters? In scientific programming, you often need routines to change values in more than one data structure (basically because the routine's invariants relate physical values that are represented in different data structures, typically arrays). So in a language like Fortran, where functions can only return one thing, you often call subroutines, passing in a number of mutable data structures whose contents all may need to be updated. In case you're interested in more details on Fortran (take these with a grain of salt, since it's been a few years since I used it daily)... In general, if you pass in "Type x" you're really passing in what C would call "Type x". The language makes this opaque, and some compilers will actually copy in and copy out if [Type] isn't too large. You can tag the ones that the function can't change with "parameter", which acts like the first "const" in "const Type const x". Subroutines allow you to change those arguments, but functions treat every argument as a parameter, and a conforming compiler should complain if you try to modify them. Finally, modern Fortran also provides explicit pointers, though they don't act quite like you'd expect if you come from a C background.