Weird Stuff (Basic)

hussey

I use PowerBasic in work, varient of basic

anyhow

I had this weird thing happening to me

This works
IF INSTR(g$, "DUBLIN") AND CheckEx(g$) OR INSTR(h$, "DUBLIN") AND CheckEx(h$) OR INSTR(i$, "DUBLIN") AND CheckEx(i$) THEN

This doesn't
IF ( INSTR(g$, "DUBLIN") AND CheckEx(g$) ) OR ( INSTR(h$, "DUBLIN") AND CheckEx(h$) ) OR ( INSTR(i$, "DUBLIN") AND CheckEx(i$) ) THEN

note the brackets - The second one is techinally correct
but for some reason
in the INSTR(h$,"DUBLIN") returns 8
and CheckEx(h$) return 1

and the statement (INSTR(h$, "DUBLIN") AND CheckEx(h$) ) returns 0

any ideas why??

by any chance is it trying to 'AND' (as in bitwise) 100+001 = 000
and in the first one it is a Logical AND??

Spunj

Instr doesn't return a true or false (0 or 1), it returns the position at which it found the searchstring or 0 if it didn't find. You need a boolean value here so see if it exists by checking that it was found anywhere:

Your code should (using the 2nd sample) look more like :
OR ( INSTR(h$, "DUBLIN") > 0 AND CheckEx(h$) )
assuming CheckEx returns true or false.

That way you are doing boolean logic and not trying to do an AND between 0 and 8

hussey

Originally posted by Spunj

Your code should (using the 2nd sample) look more like :
OR ( INSTR(h$, "DUBLIN") > 0 AND CheckEx(h$) )
assuming CheckEx returns true or false.

bangs head on table, oh bloody course!!
stupid me

Thanks
its odvious now

Talliesin

Maybe.

Funny. Most languages (including most versions of basic) give AND a higher precedence than OR, so the two would be exactly the same.

If PowerBasic doesn't have this then it's possible that the flaw is as you describe.

The key to using bitwise And and Or as logical operators is to cast results to boolean. In languages that use the same operators for bitwise and boolean mathematics generally true is the same as -1 (rather than 1 as is common in other languages). VB would be an example of this.

The reason being that on a two's complement machine then if you treat 0 (zero) as false then:

The machine sees: NOT(0) = -1
Boole says: NOT false = True.

The machine sees: X And -1 = X
Boole says: X And True = X

The machine sees: X OR -1 = -1
Boole says: X OR True = True.

The machine sees X XOR -1 = NOT X
Boole says: X XOR True = NOT X

Hence -1 is what we want.

Cast the result of each CheckEx to Boolean and you should be ok.

If CheckEx is your own you could make it return -1 rather than 1.

If powerbasic doesn't has a boolean cast (like CBool in VB) then you can create one with a function that goes something like this (I'm using VB syntax, you may have to adopt it):

hussey

in power basic
AND has higher precidence over OR

I knew the code was teh same, but I wanted the brackets so it would be more clearer

I had CheckEx(x$) return 1 if true and 0 if false

Spunj

Aye Tallesin, but he had already said CheckEx returns 1, so the abs value of the vbTrue doesn't matter

Edit:
It might, silly me, nevermind...been away from vb for too long :P

Talliesin

Argh, simultaneous post.

That matches my point about casting one of them to boolean (>0 casts to bool for any positive number, <>0 casts for any number).

Spunj

hehe..at least the problem is solved, thats the main thing!