fix and unfix

[stockage...@gmail.com]

hi

could you please give us examples about fix unfix, what is the role of these two keywords ? 

i didn't understand the explanation in the book 

[stockage...@gmail.com]

if I fix a variable like this  in the model:  fix varname;   

without precising the value in the model, which value will take ?

[AMPL Google Group]

The "fix" command fixes variables at specified values. You can think of fixing a variable at a value as being equivalent to adding a constraint that the variable must equal the value. The unfix command reverses the effect of fix, so that the variable is not fixed anymore.

A command of the form "fix varname;" fixes the named variable at its current value:

• Before any solve, the current value is usually the initial value specified in a var statement, or it is 0 if no initial value has been specified.
• After a solve, the current value is normally the variable's value in the solution returned by the solver.

A variable's current value can also be modified using a data or let statement. If you not sure about a variable's current value, you can use a "display varname;" statement to show it.

A command of the form "fix varname := expression;" fixes the variable at the value given by the expression.

--
Robert Fourer
am...@googlegroups.com

{#HS:1705109371-107303#}

On Tue, Nov 23, 2021 at 8:27 AM UTC, AMPL Modeling Language <am...@googlegroups.com> wrote:

if I fix a variable like this in the model: fix varname;

without precising the value in the model, which value will take ?