MaplePrimes - answers and comments on Question, "subsop" in "procedure" cannot work, weird!

assignment

jakubi — Fri, 21 Nov 2008 09:03:51 Z

It would be the same error with:

F:=proc(L)
L:=1:
end proc;

Without the assignment it works:

F:=proc(L)
subsop(1=NULL,L);
end proc;

L:=[x,y,z,d];
F(L);
                              [y, z, d]

with side effects

acer — Fri, 21 Nov 2008 09:11:32 Z

If you really, really want to,

> F:=proc(L::evaln)
>   L:=subsop(1=NULL,eval(L));
> end proc:

> L:=[x,y,z,d];
                               L := [x, y, z, d]
 
> F(L);
                                   [y, z, d]
 
> F(L);
                                    [z, d]
 
> F(L);
                                      [d]
 
> F(L);
                                      []

Alternatively,

> restart:

> F:=proc(L)
>   L:=subsop(1=NULL,eval(L));
> end proc:

> L:=[x,y,z,d];
                               L := [x, y, z, d]
 
> F('L');
                                   [y, z, d]
 
> F('L');
                                    [z, d]
 
> F('L');
                                      [d]
 
> F('L');
                                      []

Some people think that this sort of thing is evil.

acer

Thanks, acer, I find another

gepo — Fri, 21 Nov 2008 21:03:12 Z

Thanks, acer,

I find another way to solve this problem. here is the way:

> F:=proc(L)

>local LL;

> LL:=subsop(1=NULL,LL);

>L:=LL;
> end proc:

> L:=[x,y,z,d];
                               L := [x, y, z, d]

>L:= F(L);
                                   [y, z, d]
>L:= F(L);
                                    [z, d]
>L:= F(L);
                                      [d]
>F(L);
                                      []

By the way, shy it is evil? if I want to such thing, whether there are other methods that are not evil:)

to acer: Yes, it is

gepo — Fri, 21 Nov 2008 23:01:18 Z

to acer:

Yes, it is great.

Thanks so much.

to Robert Israel: yes,

gepo — Fri, 21 Nov 2008 23:15:15 Z

to Robert Israel:

yes, when you want to newline, shift+enter instead of enter can get:

> F:=proc(L)

>local LL;

> LL:=subsop(1=NULL,LL);
> end proc:

> L:=[x,y,z,d];
L := [x, y, z, d]

>L:= F(L);
                                   [y, z, d]
>L:= F(L);
                                    [z, d]
>L:= F(L);
                                      [d]
>F(L);
                                      []

actually, it is the same as:

F := proc(L) local LL; LL:=subsop(1=NULL,LL);(wihtout "enter")end proc;

Not about enter

Robert Israel — Sat, 22 Nov 2008 02:46:33 Z

That wasn't my point at all. Whether on one line or several, your code

> F := proc(L) local LL; LL:=subsop(1=NULL,LL); end proc;

will not work. The result of F(anything) is just NULL. Note that you haven't used L at all in the procedure.

The version you had before

> F:=proc(L)

>local LL;

>   LL:=subsop(1=NULL,LL);

>L:=LL;
> end proc:

won't work either, because the additional statement L := LL will generate an error " illegal use of a formal parameter".