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--- user_guide:tutorials:properties [2019/02/11 16:24] – ↷ Page moved from user_guide:properties to user_guide:tutorials:properties oroehrig
+++ user_guide:tutorials:properties [2019/02/11 16:24] (current) – restored. oroehrig
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-====== Objects, Properties and Rules ======
+{{page>.:latest:@FILEID@}}
-==== Objects ====
-In polymake, there is two kinds of objects. A //Big Object// models a complex mathematical concept, like a Polytope or a SimplicialComplex, while a //small object// is an instance of one of the many data types commonly used in computer science, like Integers, Matrices, Sets or Maps. A big object consists of a collection of other objects (big or small) describing it, called //properties//, and functions to compute more properties from the ones already known, called //production rules//.
-To find out the type of an object ''$c'', enter
-<code>
-print $c->type->full_name;
-</code>
-To get a more detailed explanation of the ''polymake'' object model and properties, check out the [[.howto:scripting#most_important_interfaces|scripting guide]].
-You can save polymake objects to disc, as explained [[.howto:data|here]].
-==== Properties ====
-Each (big) object has a list of properties of various types.  When an object is 'born' it comes with an initial list of properties, and all other properties will be derived from those.  Let's look at example from the ''polytope'' application.  The following creates a 3-dimensional cube:
-<code>
-polytope > $c=cube(3);
-</code>
-To find out what the initial set of properties is, use the ''list_properties'' method.  It returns an array of strings.  The extra code is just there to print this list nicely.
-<code>
-polytope > print join(", ", $c->list_properties);
-CONE_AMBIENT_DIM, CONE_DIM, FACETS, AFFINE_HULL, VERTICES_IN_FACETS, BOUNDED
-</code>
-To see what a property contains, use the ''->'' syntax:
-<code>
-polytope > print $c->FACETS;
-1 0 0
--1 0 0
-0 1 0
-0 -1 0
-0 0 1
-0 0 -1
-</code>
-You can also get the content of all properties using the ''properties'' method:
-<code>
-polytope > $c->properties;
-name: c
-type: Polytope<Rational>
-description: cube of dimension 3
-CONE_AMBIENT_DIM
-CONE_DIM
-FACETS
-1 0 0
--1 0 0
-0 1 0
-0 -1 0
-0 0 1
-0 0 -1
-AFFINE_HULL
-VERTICES_IN_FACETS
-{0 2 4 6}
-{1 3 5 7}
-{0 1 4 5}
-{2 3 6 7}
-{0 1 2 3}
-{4 5 6 7}
-BOUNDED
-</code>
-==== Production Rules ====
-The object is changed if we ask for a property which has not been computed before.
-<code>
-polytope > print $c->VERTICES;
--1 -1 -1
-1 -1 -1
--1 1 -1
-1 1 -1
--1 -1 1
-1 -1 1
--1 1 1
-1 1 1
-polytope > print join(", ", $c->list_properties);
-CONE_AMBIENT_DIM, CONE_DIM, FACETS, AFFINE_HULL, VERTICES_IN_FACETS, BOUNDED, FEASIBLE, POINTED, N_FACETS, FULL_DIM, N_VERTICES, VERTICES, LINEALITY_SPACE
-</code>
-The property ''VERTICES'' was added, but a few others were computed on the way, too. ''polymake'' applied a sequence of //production rules// that add new properties to the object that can be computed from the properties the object already posesses.
-What properties //can// be computed for a given object depends on the set of rules defined for it. Here is a short sequence of commands which lets you find out.
-<code>
-polytope> $t=$c->type->full_name;
-polytope> print join(", ", sorted_uniq(sort { $a cmp $b } map { keys %{$_->properties} } $t, @{$t->super}));
-</code>
-Instead of showing the (lengthy) enumeration have a look at the [[release_docs:latest:polytope.html|documentation]] for a complete list of properties known for objects of the application ''polytope''.
-=== Schedules ===
-You may wonder what sequence of rules led to the computation of a property you request. There usually are several mathematical ways to compute a property. ''polymake'' uses a nice scheduling algorithm to find the most efficient procedure, and you can look at what it returns.
-Suppose we want to see which sequence of rules leads to the computation of the F_VECTOR.
-<code>
-polytope > $schedule=$c->get_schedule("F_VECTOR");
-polytope > print join("\n", $schedule->list);
-HASSE_DIAGRAM : RAYS_IN_FACETS
-F_VECTOR : HASSE_DIAGRAM
-</code>
-So if you ask for the f-vector, ''polymake'' will first compute the Hasse diagram from the vertex-facet-incidences, and then compute the f-vector form the Hasse diagram. Applying the schedule to the object yields the same as asking for the property right away:
-<code>
-polytope > $schedule->apply($c);
-polytope > print join(", ", $c->list_properties);
-POLYTOPE_AMBIENT_DIM, POLYTOPE_DIM, FACETS, VERTICES_IN_FACETS, BOUNDED, HASSE_DIAGRAM, F_VECTOR
-</code>
-As you can see, the things ''polymake'' needed to compute in order to get to the f-vector are stored in the object as well, so you don't have to recompute them later.
-If you're interested, read more about rule scheduling in the [[.howto:scripting#rule_planning|scripting guide]] and the article on [[.extend:rules|writing rules yourself]].