Re: [Maxima-discuss] Return a funcion with an if, from block

[Stavros M. <mac...@al...>]

Let's look at a simpler example. Consider:

f(x):= block([n],if n=0 then 1 else x);


In Maxima, if a variable *n* is not assigned a value, it evaluates to
itself (the symbol *n*).
The "=" operation checks for structural equality, not mathematical
equality. The symbol *n* is not equal to the number *0*, so the predicate
is false, and thus your function returns *x*. So you should use *equal*:

f(x):= block([n],if equal(n,0) then 1 else x);


Until *n* is defined, the test *equal(n,0) *is undefined, so an unevaluated
conditional is returned.

             -s

On Fri, Jul 1, 2022 at 3:39 PM Me Self <vla...@gm...> wrote:

> Hello everyone
>
> Is it possible to return a function with an if, from a block()? In a
> very simple example:
>
> (%i1) f(x) := block([y, r, n], r:map(rhs,allroots(x)), y:if n=0 then
> 1.618 else x*n)$
> (%i2) f(x+1);
> (%o2)   -1.0*n
> (%i3) g(n) := ''%$
> (%i4) g([0, 1, 2]);
> (%o4)   [0, -1.0, -2.0]
>
> f(x) takes a function (polynomial here, for simplicity) as an argument
> (see %i2) which is used, internally, to generate some numbers (a bogus
> allroots() here), and I need it to return a function of n, where n
> will take discrete values. As it is now, it simply evaluates n to be
> false (n is just n, not 0) so it spits out the rest of the
> conditional. What I want it to return is this (based off on the rest
> of the example code above):
>
> (%i2) f(x+1);
> (%o2)   if n=0 then 1.618 else -1*n
>
> I don't want to use makelist() as that requires a numerical 4th
> argument, which would mean f(x) will need an extra argument for f(x)
> -- but only one is needed.
>
> I have tried declaring y like this inside the block(): y:'(if n=0 then
> 1.618 else n*r), which returns:
>
> (%o2)   if n=0 then 1.618 else n*r
>
> but when evaluated:
>
> (%o4)   [0, r, 2*r]
>
> So, how can I make the block() return this:
>
> (%o2) f(x+1);
> (%o2)   if n=0 then 1.618 else n*r
>
> but which, when evaluated it returns:
>
> (%i3) g(n) := ''%$
> (%o4) g([0, 1, 2]);
> (%o4)   [1.618, -1.0, -2.0]
>
>
> Regards,
> Vlad
>
>
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