Re: [Maxima-discuss] substitution problem

[Eli D. <pu...@el...>]

Hi, Robert. Thanks. I feel stupid now. Maybe it's because I was writing 
R code recently, where you have "quoting functions".

Anyway, I was just using print to demonstrate. The function I was 
actually using was makelist. I was trying to do something like

sublis([ n = a.bunch.of.stuff ], makelist( 
a.bunch.of.stuff.with.an.n.in.it, i, 0, n-1 ));

and I was getting an error because n-1 didn't evaluate to a number. I 
suppose I could use a temporary variable or a utility function, but this 
seemed like the tidiest way to do it.

Eli

On 7/7/20 12:53 AM, Robert Dodier wrote:

> On Mon, Jul 6, 2020 at 8:43 PM Eli Damon <pu...@el...> wrote:
>
>> (%i1) sublis([n=3],print("f(n)","=",f(n)));
>> f(n) = f(n)
>> (%o1) f(3)
> Eli, it turns out this is exactly the behavior one should expect.
> sublist is an ordinary function, i.e. one which evaluates its
> arguments. So print("f(n)","=",f(n)) is evaluated, and it evaluates to
> f(n), and has the side effect of printing "f(n) = f(n)". Now sublis is
> called with arguments [n = 3] and f(n), so it returns f(3), which you
> see as %o1.
>
> I guess that you want to print an expression without evaluating it and
> also print it evaluated. The built-in functions display and ldisplay
> are a little bit like that, but not exactly. Here is an attempt to do
> just that.
>
> mydisplay (e, [vv]) ::= buildq ([e, e1: sublis (vv, e)], print ('e = 
> 'e1));
>
> It's a little delicate to get just the right amount of evaluation ...
> note that mydisplay is a macro, defined by ::= , which is a function
> which quotes its arguments, and its return value is evaluated by the
> caller. Note also the presence of single quotes in print('e = 'e1),
> that prevents e and e1 from being evaluated by the caller. Here's what
> I get for the above input.
>
> (%i35) mydisplay (f(n), n = 3);
> f(n) = f(3)
> (%o35) f(n) = f(3)
>
> mydisplay collects any arguments aside from e and puts them all in the 
> list vv.
>
> (%i36) mydisplay (g(m, n), n = a, m = 2*a);
> g(m, n) = g(2 a, a)
> (%o36) g(m, n) = g(2 a, a)
>
> I don't know whether mydisplay should print e = e1 or just return it.
> I guess which one is more useful depends on what you need to do. Maybe
> you can say more about what your goal is.
>
> There are probably other variations which are useful in some context.
> E.g. f(n) = f(3) = 1234 or whatever depending on the definition of f.
>
> Hope this helps,
>
> Robert Dodier