Using EQV Operator

Summary:

Use FB's EQV operator to make logical decisions.

Using EQV Operator

rickclark58 04 Jun 2007 16:08

A little-known trick you can use in games is to use the EQV operator to test whether two expressions are equivalent. For example, suppose you have two monsters and you need to determine if they should fight based on their hit-point values. You can use the following technique to determine if they should fight.

If (monster1_HP >= HP_threshold) EQV (monster2_HP >= HP_threshold) then
    Fight
Else
    Don't Fight
End If

By using this technique you have condensed a multi-level If ladder into a single statement.

Re: Using EQV Operator

u9 05 Jun 2007 12:28

I've never understood the EQV operator. Isn't it the same as writing:

If (monster1_HP >= HP_threshold) = (monster2_HP >= HP_threshold) then
    ...

Or is it equal to using the logical and operator like so:

If (monster1_HP >= HP_threshold) and (monster2_HP >= HP_threshold) then
    ...

or what is the difference? Reading your code I would expect a Fight if both monsters either are above HP_treshold or if both are below it.

Re: Using EQV Operator

rickclark58 05 Jun 2007 15:15

Yeah it is a little confusing at first. My example isn't the best one either. :)

Here is the truth table for EQV:

OP1	OP2	Result
0	0	1
1	0	0
0	1	0
1	1	1

As you can see if both inputs are either 1 or 0 then the result is 1. That is, if the inputs are equivalent, (both are either TRUE or FALSE) then the OP returns 1.

Compare that to the AND operator.

OP1	OP2	Result
0	0	0
1	0	0
0	1	0
1	1	1

With the AND both input must be 1 before it returns 1.

The EQV operators is useful for comparing inputs and testing if they are similar to each other. You can't really do that with AND.

Re: Using EQV Operator

u9 05 Jun 2007 22:08

Ok thanks. But so it is the same as my first line

If (monster1_HP >= HP_threshold) = (monster2_HP >= HP_threshold) then
    ...

right? … Which gets me thinking, can you not write the code above in BASIC? I mean is it legal to compare two booleans using the = operator?

e.g. if true = true then would give the same as if false = false then, yes? I'm only asking because i usually use C++ and i was surprised when i found there was an EQV operator in VBscript :)

Re: Using EQV Operator

rickclark58 05 Jun 2007 23:44

Yeah, that would work.

dim as integer monster1_hp = 10, monster2_hp = 20, hp_threshold = 5

If (monster1_HP >= HP_threshold) = (monster2_HP >= HP_threshold) then
    print "Let's fight!"
else
    Print "Let's run!"
end if
sleep

Results in:

Let's fight!