(r2) 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]]
</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>

---++<a name="let"></a><tt>let</tt> Loops
%RED%Coming soon!%ENDCOLOR%

<h1>Under Construction></h1>

&#8220;Looping&#8221; let Statements
In the same way you can printf over specified subsets, you can use a let statement to &#8220;loop&#8221; over a set and change the data.
Syntax: To change data values for a set you use the let keyword, the set and the := operator
let {e in SET} <parameter, usually involving e> :=
               <expression, often involving e>;
Consider the following possibility for American Steel. 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 &#8220;looping&#8221; let statements.
let {n in NODES : (&#8216;Chicago&#8217;, n) in ARCS}
  Cost[&#8216;Chicago&#8217;, n] := 0.95 * Cost[&#8216;Chicago&#8217;, n];
let {n in NODES : (&#8216;Chicago&#8217;, n) in ARCS}
  Min[&#8216;Chicago&#8217;, n] := max(1000, Min[&#8216;Chicago&#8217;, n]);



Conditional Loops
AMPL allows us to repeat a loop depending on a condition.
Syntax: We can loop subject to a condition using the repeat keyword, the while or until keyword and the { and } operators.
repeat while <expression> { <loop statements;> };
repeat until <expression> { <loop statements;> };
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.
repeat { <loop statements;> } while <expression>;
repeat { <loop statements;> } until <expression>;
These statements check the expression after executing the loop statements. The first one continues while the expression is true, the second terminates when the expression is true.



???Using symbolic parameters to do conditional loops over sets (see symbolic parameters)???
-- Main.TWikiAdminGroup - 18 Mar 2008
Topic revision: r2 - 2008-03-18 - TWikiAdminUser
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