Reply-to: gamsworld@googlegroups.com

Hi Arne,

Thanks! It woks when I use integer variable change the model to MIP!

Greetings!

Jarek

On Tuesday, 21 January 2014 14:23:37 UTC+1, Arne Stolbjerg Drud wrote:

## Jarek

You should introduce an integer variable that defines the number of batches you are going to produce. You production is then number of batches times batch size,

With an integer variable your model becomes a MIP model instead of LP.

Good luck

Arne

\

Arne Stolbjerg Drud

ARKI Consulting & Development A/S

Bagsvaerdvej 246A, DK-2880 Bagsvaerd, Denmark

Phone: (+45) 44 49 03 23, Fax: (+45) 44 49 03 33, email: ad…@arki.dk

From: gams...@googlegroups.com [mailto:gams...@googlegroups.com] On Behalf Of Jarek

Sent: Tuesday, January 21, 2014 11:21 AM

To: gams...@googlegroups.com

Subject: Re: Constraint checking if sum of variables is a whole number. Help please.

Hi Renger,

Thanks for your reply.

I think the mod function is for dividing and calculating the divisive reminder: mod(arg1, arg2)

The error is:

**** $51,256

**** The following LP errors were detected in model Production:

**** 51 equation batch… the function MOD is called with non-constant arguments

In short there are 4 producers making 4 products. Costs and production times differ for them.

They can make particular products only in batches (products KO : 100, SP: 20, BL: 30, SU: 20).

I need to find optimal production plan for maximizing net profit.

I have problem with the batches restriction.

Maybe what I’m trying to do makes no sense at all…

Here is the program:

$Ontext

$Offtext

Sets

i products / KO, SP, BL, SU /

j producers / S1, S2, S3, S4 / ;

Parameters

d(i) Demand

/ KO 2400

SP 1200

BL 1400

SU 800 /

p(i) Price

/ KO 220

SP 800

BL 530

SU 1050 /

w(j) Available time of work for producers

/ S1 160

S2 160

S3 160

S4 160 /

b(i) Batches in which products must be produced

/ KO 100

SP 20

BL 30

SU 20 / ;

Table c(i,j) production costs: dollars per item per producer

S1 S2 S3 S4

KO 11.5 11.8 11.7 11.6

SP 36 35 37.5 36

BL 22.3 23 21.7 24

SU 35 40 40 32.5 ;

Table pf(i,j) performance: items per hour per producer

S1 S2 S3 S4

KO 20 16 18 19

SP 5 6 4 5

BL 7 6 8 5

SU 3 2 2 4 ;

Scalar tf tax factor /0.81/ ;

Variables

x(i,j) items produced

np net profit;

Positive Variable x(i,j) ;

Equations

profit the objective function

workTime(j) time of work per month

demand(i) demand for the items

batch(i) constraints because of the batches ;

profit … np =e= sum((i,j), x(i,j)*(p(i)-c(i,j))*tf) ;

workTime(j) … sum(i, (1/pf(i,j))*x(i,j)) =l= w(j) ;

demand(i) … sum(j, x(i,j)) =l= d(i) ;

batch(i) … mod(sum(j, x(i,j)), b(i)) =e= 0 ;

Model Production /all/

solve Production maximazing np using LP

display x.l

On Tuesday, 21 January 2014 08:52:07 UTC+1, Renger van Nieuwkoop wrote:

Hi Jarek

It would be useful, if you would report the error message. In your code example below, you do not divide by pk(i) (â€œ,â€ instead of â€œ/â€)

Cheers

Renger

From: gams...@googlegroups.com [mailto:gams...@googlegroups.com] On Behalf Of Jarek

Sent: Dienstag, 21. Januar 2014 00:03

To: gams...@googlegroups.com

Subject: Constraint checking if sum of variables is a whole number. Help please.

Hi Guys,

Please help. I’m desperate now. I’m writing short optimization program in GAMS as an assignment on my university and I’m running out of time.

I need to define constraint saying that sum of computed values of variables devided by some specific factor is a whole number.

i.e:

10/2 = 5 which is ok

3/2 = 1.5 which is not ok

I tried to use mod() function to check the reminder but it returnes error.

Something like that:

packet(i) … mod(sum(j, x(i,j)), pk(i)) =e= 0;

x(i,j) is the variable

pk(i) is the divisive factor

The constraint should be that the value of the computation is equal to 0 (which means the number is whole): … =e= 0

How can I do it properly?

Thanks!

## Jarek

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