queries/variables

• C

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  #include <variables.h>
  #include <stdio.h>
   
  int main(int argc, char *argv[]) {
      ecs_world_t *ecs = ecs_init_w_args(argc, argv);
   
      ECS_TAG(ecs, Eats);
      ECS_TAG(ecs, Healthy);
   
      ecs_entity_t Apples = ecs_entity(ecs, { .name = "Apples" });
      ecs_entity_t Salad = ecs_entity(ecs, { .name = "Salad" });
      ecs_entity_t Burgers = ecs_entity(ecs, { .name = "Burgers" });
      ecs_entity_t Pizza = ecs_entity(ecs, { .name = "Pizza" });
      ecs_entity_t Chocolate = ecs_entity(ecs, { .name = "Chocolate" });
   
      ecs_add(ecs, Apples, Healthy);
      ecs_add(ecs, Salad, Healthy);
   
      ecs_entity_t bob = ecs_entity(ecs, { .name = "Bob" });
      ecs_add_pair(ecs, bob, Eats, Apples);
      ecs_add_pair(ecs, bob, Eats, Burgers);
      ecs_add_pair(ecs, bob, Eats, Pizza);
   
      ecs_entity_t alice = ecs_entity(ecs, { .name = "Alice" });
      ecs_add_pair(ecs, alice, Eats, Salad);
      ecs_add_pair(ecs, alice, Eats, Chocolate);
      ecs_add_pair(ecs, alice, Eats, Apples);
   
      // Here we're creating a query that in the query DSL would look like this:
      //   Eats($this, $Food), Healthy($Food)
      ecs_query_t *q = ecs_query(ecs, {
          .terms = {
              // Query variables are like wildcards, but enforce that the entity
              // substituted by the wildcard is the same across terms.
              //
              // For example, in this query it is not guaranteed that the entity
              // substituted by the * for Eats is the same as for Healthy:
              //   (Eats, *), Healthy(*)
              //
              // By replacing * with $food, both terms will be constrained to
              // match the same entity for the wildcard.
              { .first.id = Eats,    .second.name = "$food" },
              { .first.id = Healthy, .src.name    = "$food" }
          }
      });
   
      // Lookup the index of the variable. This will let us quickly lookup its
      // value while we're iterating.
      int food_var = ecs_query_find_var(q, "food");
   
      // Iterate the query
      ecs_iter_t it = ecs_query_iter(ecs, q);
      while (ecs_query_next(&it)) {
          ecs_entity_t food = ecs_iter_get_var(&it, food_var);
          for (int i = 0; i < it.count; i ++) {
              printf("%s eats %s\n", ecs_get_name(ecs, it.entities[i]),
                  ecs_get_name(ecs, food));
          }
      }
   
      ecs_query_fini(q);
   
      // Output
      //  Bob eats Apples
      //  Alice eats Apples
      //  Alice eats Salad
   
      return ecs_fini(ecs);
  }

• C++

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  #include <variables.h>
  #include <iostream>
   
  struct Eats { };
  struct Healthy { };
   
  int main(int, char *[]) {
      flecs::world ecs;
   
      flecs::entity Apples = ecs.entity("Apples").add<Healthy>();
      flecs::entity Salad = ecs.entity("Salad").add<Healthy>();
      flecs::entity Burgers = ecs.entity("Burgers");
      flecs::entity Pizza = ecs.entity("Pizza");
      flecs::entity Chocolate = ecs.entity("Chocolate");
   
      ecs.entity("Bob")
          .add<Eats>(Apples)
          .add<Eats>(Burgers)
          .add<Eats>(Pizza);
   
      ecs.entity("Alice")
          .add<Eats>(Salad)
          .add<Eats>(Chocolate)
          .add<Eats>(Apples);
   
      // Here we're creating a query that in the query DSL would look like this:
      //   Eats($this, $Food), Healthy($Food)
      flecs::query<> q = ecs.query_builder()
          // Identifiers that start with $ are query variables. Query variables
          // are like wildcards, but enforce that the entity substituted by the
          // wildcard is the same across terms.
          //
          // For example, in this query it is not guaranteed that the entity
          // substituted by the * for Eats is the same as for Healthy:
          //   (Eats, *), Healthy(*)
          //
          // By replacing * with $food, both terms will be constrained to
          // match the same entity for the wildcard.
          .with<Eats>("$food")
          .with<Healthy>().src("$food")
          .build();
   
      // Lookup the index of the variable. This will let us quickly lookup its
      // value while we're iterating.
      int food_var = q.find_var("food");
   
      // Iterate the query
      q.each([&](flecs::iter& it, size_t index) {
          std::cout 
              << it.entity(index).name()
              << " eats " << it.get_var(food_var).name()
              << "\n";
      });
   
      // Output:
      //  Bob eats Apples
      //  Alice eats Apples
      //  Alice eats Salad
  }