Re: [Maxima-discuss] weird behavior of apply

[Kris K. <kat...@gm...>]

Hi Dimiter,

On 09/29/2014 12:30 PM, Dimiter Prodanov wrote:
> Hello,
> 
> I define a function
> 
> dy(y):=if y#0 and y#0.0 then  1/(2*y)   else 'nan;
> 
> Then I wanted to substitute it's first argument by a value. The idea is
> to use pass a function having a variable number of arguments as a method
> parameter and then to evaluate it only on certain arguments.
> 
> 
> I stumbled on the following weird behavior:
> 
> apply(dy, [y=0]);
> expt: undefined: 0 to a negative exponent.
>  -- an error. 
> 

The list given to apply shouldn't include the parameter name; what's
happening is that the equation "y=0" is getting passed to dy and that's
causing the problem.

(%i3) trace(dy)$

(%i4) apply(dy,[y=0]);
1 Enter dy [y = 0]
expt: undefined: 0 to a negative exponent.
#0: A traced function(y=y = 0)
 -- an error. To debug this try: debugmode(true);

(%i5) apply(dy,[0]);
1 Enter dy [0]
1 Exit  dy nan
(%o5) nan

> also here
> 
> at(dy(y),[y=0]);
> 
> expt: undefined: 0 to a negative exponent.
>  -- an error. 
> 

Here the problem is that dy(y) is evaluated before the y=0 is done.

(%i6) at(dy(y),[y=0]);
1 Enter dy [y]
1 Exit  dy 1/(2*y)
expt: undefined: 0 to a negative exponent.
 -- an error. To debug this try: debugmode(true);

I hope this helps.

Cheers,
Kris Katterjohn