Asking Excel Solver to use binaries, but selects other values

I am trying to get a linear program solved using solver.  The values that 
Solver needs to enter are contrained as binary, but excel still try to find 
values that are neither 0 or 1.
0
Stephane (12)
6/23/2005 4:29:13 PM
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Stephane,

I assume that when you say it is selecting values that are neither 0 or 1, 
they are values that are numerically close to 0 or 1 (such as 0.99998). If 
this is the case, you may want to consider the following approach:

1. Have the Solver constrain your decision variables as binary.
2. Have another set of cells which uses the decision variable to produce a 
result. When the underlying decision variable is close enough to a 0 or 1, 
have it return exactly a 0 or 1 (and not the numerical approximation 
generated by the Solver).
3. Use this underlying result instead of the underlying decision variable in 
your objective function.



"Stephane" wrote:

> I am trying to get a linear program solved using solver.  The values that 
> Solver needs to enter are contrained as binary, but excel still try to find 
> values that are neither 0 or 1.
0
6/23/2005 8:23:11 PM
Actually, Solver is trying values such as 0.52, 0.17, and so on.  I will try 
to use other cells and convert back the values to 0 or 1.

Thanks for the quick answer,

Stephane 

"Excel Best Practices Resource Center" wrote:

> Stephane,
> 
> I assume that when you say it is selecting values that are neither 0 or 1, 
> they are values that are numerically close to 0 or 1 (such as 0.99998). If 
> this is the case, you may want to consider the following approach:
> 
> 1. Have the Solver constrain your decision variables as binary.
> 2. Have another set of cells which uses the decision variable to produce a 
> result. When the underlying decision variable is close enough to a 0 or 1, 
> have it return exactly a 0 or 1 (and not the numerical approximation 
> generated by the Solver).
> 3. Use this underlying result instead of the underlying decision variable in 
> your objective function.
> 

0
Stephane (12)
6/23/2005 8:33:03 PM
No, please don't second guess Solver.  It won't help Solver find a 
solution and you will likely land up at a wrong result.

Solver uses a well-established technique called branch and bound.  This 
technique 'relaxes' the integer/binary constraints and then 
reestablishes them towards the end of the analysis.

If Solver shows you a non-binary result it probably also pops up an 
alert of one kind or another -- maybe, about how it cannot find a 
solution?

Have you specified your problem as 'linear?'  In the Solver dialog box 
click the Options button.  Those kinds of problems are the most easily 
solved by the techniques programmed into Solver.  Of course, it is up 
to you to ensure that the requirements of linearity (as defined in 
Operations Research) are met.


-- 
Regards,

Tushar Mehta
www.tushar-mehta.com
Excel, PowerPoint, and VBA add-ins, tutorials
Custom MS Office productivity solutions

In article <1A9EE47E-8354-4336-96FC-CE9249B358EF@microsoft.com>, 
Stephane@discussions.microsoft.com says...
> Actually, Solver is trying values such as 0.52, 0.17, and so on.  I will try 
> to use other cells and convert back the values to 0 or 1.
> 
> Thanks for the quick answer,
> 
> Stephane 
> 
> "Excel Best Practices Resource Center" wrote:
> 
> > Stephane,
> > 
> > I assume that when you say it is selecting values that are neither 0 or 1, 
> > they are values that are numerically close to 0 or 1 (such as 0.99998). If 
> > this is the case, you may want to consider the following approach:
> > 
> > 1. Have the Solver constrain your decision variables as binary.
> > 2. Have another set of cells which uses the decision variable to produce a 
> > result. When the underlying decision variable is close enough to a 0 or 1, 
> > have it return exactly a 0 or 1 (and not the numerical approximation 
> > generated by the Solver).
> > 3. Use this underlying result instead of the underlying decision variable in 
> > your objective function.
> > 
> 
> 
0
6/23/2005 10:25:09 PM
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