How to Define a Constraint Using If-Then-Else for Vegetable Growth?

[Alexx1]

How can we define a constraint saying:

if b is larger than zero then y has to equal 1
if b is equal to zero then y has to equal 0

?

It concerns the following problem:
b[k] = amount of vegetable i growing on patch k
y[k] = 1 if vegetable i is grown on patch k
= 0 otherwise

So we want to say that if the amount of vegetable i on patch k is larger than zero (i.e. b[k] > 0), vegetable i must be assigned to patch k (i.e. y[k] = 1)
I was thinking that it could be something like that:

for all i
sum(k) b[k]*y[k] = b[k]

But I don't know if this is correct or not, since you could cancel out both terms b[k]

What do you guys think?

Kind regards,
Alex

 

[I like Serena]

Hi Alexx1!

Perhaps like this?
y[k] = (b[k] > 0);

 

[Michael Redei]

If you know that b[k] will never be negative, you have y[k] = sgn( b[k] ).

 

[Alexx1]

The thing is I need a constraint to use in the program
IBM ILOG CPLEX Optimization Studio

None of both constraints is accepted in CPLEX

 

[Michael Redei]

According to Wikipedia's entry on OPL (the programming lanuage used in the program you mention), it does support an IF-THEN-ELSE construct. Why don't you use that?

 

[Alexx1]

According to Wikipedia's entry on OPL (the programming lanuage used in the program you mention), it does support an IF-THEN-ELSE construct. Why don't you use that?

I would prefer stating it in a constraint only using =/>=/<=/</>
Do you know an alternative?

 

[Michael Redei]

Here's some examples from IBM's manual on OPL:

int value = ...; 
int signValue = (value>0) ? 1 : (value<0) ? -1 : 0;
int absValue = (value>=0) ? value : -value;

It seems that OPL supports a "... ? ... : ..." construct. Have you tried that? E.g.

y[k] = (b[k] > 0) ? 1 : 0;

 

[jim mcnamara]

That is a ternary operator like the one in the C language:

                   if()               then             else
int variable=(boolean statement) ? [when true] : [when false];

The example on line two above "concatenates" or nests operators

int signValue = (value>0) ? 1 :[/color] (value<0) ? -1 : 0;[/color]

Maybe that will help...