[R] combining mathematical notation and value substitution

[Thomas Lumley]

On Sun, 22 Jun 2003, Faheem Mitha wrote:
> If I'm doing this correctly, R does not seem to think it is a call.
>
> > is.call("Monotonic Multigamma run (" * n == len * ", " * theta == t1
> * ").")
> Error in "Monotonic Multigamma run (" * n :
>         non-numeric argument to binary operator

R is trying to *evaluate*
  "Monotonic Multigamma run ("* n==len etc
which doesn't work.  Remember, is.call(), like any normal function, will
be passed the *value* of its arguments.

You could try
  is.call(quote("Monotonic Multigamma run("*n==len))
which is TRUE.

> It considers it a valid R expression though.
>
> > (mode(expression("Monotonic Multigamma run (" * n == len * ", " * theta
> == t1 * ").")))
> [1] "expression"
>

That's because expression() returns an expression.

>
> The clearest description I have seen of a call is in S Poetry, where it
> says
>
> "Mode call represents objects that are calls to a function. The first
> component of a call is the name (mode name) of the function being called.
> The rest of the call is the arguments given."
>
> This certainly is how calls are constructed using call(...), but I'm not
> sure how it fits in with an expression like the one above. What is the
> function being called in that case, for example?

Well, we can find out. It must be either * or ==, but it isn't immediately
obvious which one ends up at the top level

> thing <- quote("Monotonic Multigamma Run ("*n==len* ", " * theta
==t1*").")
> mode(thing)
[1] "call"
> length(thing)
[1] 3
> thing[[1]]
==
> thing[[2]]
"Monotonic Multigamma Run (" * n == len * ", " * theta
> thing[[3]]
t1 * ")."
> mode(thing[[2]])
[1] "call"
> mode(thing[[3]])
[1] "call"
> thing[[2]][[1]]
==
> thing[[3]][[1]]
*

So it is a call to ==, with two arguments, each itself a call.  The first
arguemetn is also a call to == and the second is a call to *. And so on in
a tree structure.


> Maybe I'd understand this stuff better if I knew Lisp. I'm very ignorant
> about programming language paradigms. I've looked at S Poetry, and S
> Programming. While they were helpful, I don't feel I've got a clear
> picture. Further recommended reading?
>

I don't know about further reading.  I think this is probably easier to
learn when you have a specific problem to solve.  I learned mostly by
porting the survival package.


	-thomas