LoopingInAMPL < OpsRes

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---+<a name="top"></a> Looping in AMPL

<ol>
<li>[[#for][<tt>for</tt> Loops]]
<li>[[#let][<tt>let</tt> Loops]]
<li>[[#condition][Conditional Loops]]
</ol>

---++<a name="for"></a><tt>for</tt> Loops

In AMPL you can create basic _for_ loops by [[SetsInAMPL][creating a set automatically]] and looping over elements in the set. To loop over a set you use the =for= keyword and the =in= keyword.
<pre>
for {&lt;element&gt; in &lt;SET&gt;} &lt;a statement&gt;;
</pre>

Now =&lt;a statement&gt;= is repeated for each element in =&lt;SET&gt;=. If you need to do more than one statement you can use ={= and =}=:
<pre>
for {&lt;element&gt; in &lt;SET&gt;}
{
  &lt;some statements&gt;
}
</pre>

%RED%Examples coming soon!%ENDCOLOR%

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---++<a name="let"></a><tt>let</tt> Loops

In AMPL you can [[PrintingInAMPL][display and/or print over specified subsets]] and you can also "loop" over a =let= statement. To assign values for an entire set you use the =let= keyword, the set and the <tt>:=</tt> operator:
<pre>
let {e in SET} &lt;parameter, usually involving e&gt; := &lt;expression, often involving e&gt;;
</pre>

Consider the following possibility for the American Steel problems ([[AmericanSteelTransshipmentProblem][the American Steel transshipment problem]] and [[AmericanSteelPlanningProblem][the American Steel planning problem]] respectively). American Steel can get a 5\% discount with their transportation provider out of Chicago as long as they commit to at least 1000 tonnes along each route. This change can be easily incorporated using two looping =let= statements:
<pre>
let {n in NODES : ('Chicago', n) in ARCS} Cost['Chicago', n] := 0.95 * Cost['Chicago', n];
let {n in NODES : ('Chicago', n) in ARCS} Min['Chicago', n] := max(1000, Min['Chicago', n]);
</pre>

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---++<a name="condition"></a> Conditional Loops

AMPL also provides syntax for repeating a loop depending on a [[LogicInAMPL][condition]] (often known as _while_ loops). These loops use a condition, the =repeat= keyword, the =while= or =until= keyword and the ={= and =}= operators:
<pre>
repeat while &lt;expression&gt; {
  &lt;loop statements;&gt;
};

repeat until &lt;expression&gt; {
  &lt;loop statements;&gt;
};
</pre>

These statements check the expression *before* executing the loop statements. The first one continues while the expression is true, the second terminates when the expression is true. AMPL provides loops that check the expression after the loop completes:
<pre>
repeat {
  &lt;loop statements;&gt;
} while &lt;expression&gt;;

repeat {
  &lt;loop statements;&gt;
} until &lt;expression&gt;;
</pre>
As before, the first loop continues while the expression is true, the second terminates when the expression is true.

%RED%Examples coming soon!%ENDCOLOR%

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-- Main.TWikiAdminGroup - 18 Mar 2008
Topic revision: r5 - 2008-03-18 - TWikiAdminUser