Re: [News] LOGO-L> Indirect Assignment

[Frank Caggiano <caggiano@atlantic.net>]

Yehuda,

Lets say we have a bunch of boxes that we're going to store things in.
In order to reference the boxes we give them names; "box1", "box2",
"box3", etc.

So in box1 we put in a ball; in box2 a fire engine.  If I ask you to get
what's in box1 you get the ball; I ask you to put this block into box2.
These are DIRECT assignments. The object is placed directly into the
container.

Now lets say I have you put a doll into box4 then I have you put box4
(actually a slip of paper with "b" "o" "x" "4" writen on it) into box3.
Now If you go to retrive the article in box3 what do you find? Not the
article but a piece of paper telling you where it is. So then you go to
box4 and get the doll. This is INDIRECT assignment and can go on for as
many levels as you care to.

In logo terms this is:

make "box4 "doll
make "box3 "box4
show thing :box3	; Note use of keyword 'thing'
doll
show :box3
box4

To assign to box4 through box3 then:

make :box3 "dog  	; Note use of ':' before 'box3' rather than '"'
show :box3
box4
show thing :box3
dog

The keyword 'thing' says you do not want the contents of box3 but the
contents of the container named by box3.

This can get very confusing. Just think of containers in a row with
pointers from one to the next.  Follow the pointers until there are no
more and that is were the actual value goes (or is).

There is a quote from "Alice Through the Looking Glass" that was just
made for indirect assignments.
-----
"The name of the song is called 'Haddocks Eyes', said the knight.

"Oh, thats the name of the song, is it?" Alice said, trying to feel
interested.

"No, you don't understand", the Knight said looking a little vexed.
"Thats what the name is called. The name really is 'The Aged Aged Man.'"
---

It goes on for a few more levels of indirection. Its on p282 in my copy
if you are interested.

regards
-- 
Frank Caggiano
caggiano@atlantic.net
http://www.atlantic.net/~caggiano
---------------------------------------------------------------
Please post messages to the Logo forum to logo-l@gsn.org.  Mail
questions about the list administration to logofdn@gsn.org.  To
unsubscribe send    unsubscribe logo-l    to majordomo@gsn.org.