Topic: LISP equality test gotcha

[MP]

Had a bug in one of my programs (introduced by a quick edit I had done this morning). Fortunately I found it and fixed it quick but in some circumstances it could be a bugger to trace the following (seemingly) illogical behavior.

<clears throat> ahem (symbolically):

Code: [Select]

(setq
    var1 nil
    var2 0
)

(< var1 var2)
In a strongly typed language this would throw an uninitialized variable error.

Not in LISP. It happily returns T (or nil if you reverse the terms).

Buyer beware.

[CAB]

Wow, thanks.
Now if I can only remember that for future use. 

[Lee Mac]

Thanks Michael   Good to know.

While we're on the topic, I discovered this recently...

Code: [Select]

(setq l1 '(1 2) l2 '(1 2))

(eq l1 l2)  ==>  nil

(= l1 l2)  ==>  nil

(equal l1 l2)  ==>  T

Similarly:

Code: [Select]

e1
<Entity name: 7ef03798>

e2
<Entity name: 7ef03798>

(eq e1 e2)  ==>  T

(= e1 e2)  ==>  nil

(equal e1 e2)  ==>  T

I'm sure there are many other examples....

[T.Willey]

Lee,

The only time I use ' = ' is on strings, and I never use ' eq ', always ' equal '.

[JohnK]

Had a bug in one of my programs (introduced by a quick edit I had done this morning). Fortunately I found it and fixed it quick but in some circumstances it could be a bugger to trace the following (seemingly) illogical behavior.

<clears throat> ahem (symbolically):
...

huh? That could be a real pain to find.

Code: [Select]

(cond
  ((and var1 var2)
    (< var1 var2))
  )I mean, Ive never really thought to run a conditional for every equality test.

whoa? *thought* didnt we hit on this subject that one time we discussed the "anaphoiric if" (Im gonna go look up my notes)?

[ElpanovEvgeniy]

Code: [Select]

(< -1e+99 var1 var2)
(< nil var1 var2)


[MP]

huh? That could be a real pain to find.

Code: [Select]

(cond
  ((and var1 var2)
    (< var1 var2))
  )I mean, Ive never really thought to run a conditional for every equality test.

And I'd never do that ^ kind of checking. The problem I had a programming error, not an execution error. If, in the course of normal execution, it was expected behavior that var1 could be assigned nil then I'd branch long before the conditional test noted. In my actual program I had commented out some code and replaced it with new code. Unfortunately I also commented out the variable assignment code, introducing the programming bug.

[MP]

On eq vs equal, generally speaking I use eq when comparing numbers or strings, otherwise equal.

[Lee Mac]

On eq vs equal, generally speaking I use eq when comparing numbers or strings, otherwise equal.

I would usually use 'eq' for strings, and '=' for numbers, and otherwise equal - but, to be honest, I wouldn't normally put myself in a situation where I would compare entity names or VLA-objects.

[MP]

[CAB]

Lee,

Code: [Select]

(equal l1 l2)  ==>  T

(equal l1 l2)  ==>  T

With this font it looked like eleven & twelve.  

[alanjt]

Lee,

Code: [Select]

(equal l1 l2)  ==>  T

(equal l1 l2)  ==>  T

With this font it looked like eleven & twelve.  

At first, I thought the same thing. Needless to say, I was rather confused.

[JCTER]

[]http://www.karolinagames.com/horatio/images/90d31b379587cb1bf205881c9a1209ee.jpg[/img]

No offense, but... worst... use of that meme... ever.
 

[uncoolperson]

anyone else confused about the two sets of sunglasses?

[JCTER]

anyone else confused about the two sets of sunglasses?

Because he's always wearing sunglasses... even when he does his dramatic putting-on of his sunglasses.  He's -that- Miami.