Lab02 fixes (#12)

Author: nmheim

docs/src/lecture_02/Lab02Ecosystem.jl

@@ -44,15 +44,15 @@ end
 
 function agent_step!(a::Animal, w::World)
     incr_energy!(a,-1)
-    if rand() <= food_prob(a)
+    if rand() <= foodprob(a)
         dinner = find_food(a,w)
         eat!(a, dinner, w)
     end
     if energy(a) <= 0
         kill_agent!(a,w)
         return
     end
-    if rand() <= reproduction_prob(a)
+    if rand() <= reprprob(a)
         reproduce!(a,w)
     end
     return a
@@ -63,21 +63,22 @@ mutable struct Grass <: Plant
     size::Int
     max_size::Int
 end
+Grass(id,m) = Grass(id, rand(1:m), m)
 
 mutable struct Sheep <: Animal
     id::Int
     energy::Float64
     Δenergy::Float64
-    reproduction_prob::Float64
-    food_prob::Float64
+    reprprob::Float64
+    foodprob::Float64
 end
 
 mutable struct Wolf <: Animal
     id::Int
     energy::Float64
     Δenergy::Float64
-    reproduction_prob::Float64
-    food_prob::Float64
+    reprprob::Float64
+    foodprob::Float64
 end
 
 id(a::Agent) = a.id  # every agent has an ID so we can just define id for Agent here
@@ -89,8 +90,8 @@ grow!(a::Plant) = a.size += 1
 # get field values
 energy(a::Animal) = a.energy
 Δenergy(a::Animal) = a.Δenergy
-reproduction_prob(a::Animal) = a.reproduction_prob
-food_prob(a::Animal) = a.food_prob
+reprprob(a::Animal) = a.reprprob
+foodprob(a::Animal) = a.foodprob
 
 # set field values
 energy!(a::Animal, e) = a.energy = e
@@ -118,23 +119,22 @@ end
 
 function eat!(a::Sheep, b::Grass, w::World)
     incr_energy!(a, size(b)*Δenergy(a))
-    kill_agent!(b,w)
+    b.size = 0
 end
 function eat!(wolf::Wolf, sheep::Sheep, w::World)
     incr_energy!(wolf, energy(sheep)*Δenergy(wolf))
     kill_agent!(sheep,w)
 end
 eat!(a::Animal,b::Nothing,w::World) = nothing
 
-kill_agent!(a::Plant, w::World) = a.size = 0
 kill_agent!(a::Animal, w::World) = delete!(w.agents, id(a))
 
 function find_food(a::Animal, w::World)
     as = filter(x->eats(a,x), w.agents |> values |> collect)
     isempty(as) ? nothing : sample(as)
 end
 
-eats(::Sheep,::Grass) = true
+eats(::Sheep,g::Grass) = size(g) > 0
 eats(::Wolf,::Sheep) = true
 eats(::Agent,::Agent) = false
 

docs/src/lecture_02/lab.md

@@ -236,8 +236,10 @@ input types and is our first practical example of [*multiple dispatch*](https://
 ```
 Implement a function `eat!(::Sheep, ::Grass, ::World)` which increases the sheep's
 energy by $\Delta E$ multiplied by the size of the grass.
+
 After the sheep's energy is updated the grass is eaten and its size counter has
 to be set to zero.
+
 Note that you do not yet need the world in this function. It is needed later
 for the case of wolves eating sheep.
 ```@raw html
@@ -248,9 +250,8 @@ for the case of wolves eating sheep.
 ```@example non_parametric_agents
 function eat!(a::Sheep, b::Grass, w::World)
     incr_energy!(a, size(b)*Δenergy(a))
-    kill_agent!(b,w)
+    b.size = 0
 end
-kill_agent!(a::Plant, w::World) = a.size = 0
 nothing # hide
 ```
 ```@raw html
@@ -281,9 +282,12 @@ eat!(grass,sheep,world);
 <div class="admonition-body">
 ```
 Next, implement a `Wolf` with the same properties as the sheep ($E$, $\Delta
-E$, $p_r$, and $p_f$) as well as the correspoding `eat!` method which increases
+E$, $p_r$, and $p_f$) as well as its `eat!` method. The `eat!` method for wolves increases
 the wolf's energy by `energy(sheep)*Δenergy(wolf)` and kills the sheep (i.e.
 removes the sheep from the world).
+Both `eat!` and `agent_step!` need to be able to remove agents from the world
+so it makes sense to create another function `kill_agent!(::Animal,::World)`.
+Please implement it as well to make your `agent_step!` work.
 
 Hint: You can use `delete!` to remove agents from the dictionary in your world.
 
@@ -343,7 +347,7 @@ function find_food(a::Animal, w::World)
     isempty(as) ? nothing : sample(as)
 end
 
-eats(::Sheep,::Grass) = true
+eats(::Sheep,g::Grass) = size(g) > 0
 eats(::Wolf,::Sheep) = true
 eats(::Agent,::Agent) = false
 ```
@@ -362,7 +366,8 @@ random `Grass` from all available `Grass` agents.
 ```
 
 Implement the method `find_food(::Sheep, ::World)` which first returns either a
-`Grass` (sampled randomly from all `Grass`es) or returns `nothing`.
+`Grass` (sampled randomly from all `Grass`es with a size larger than zero) or
+returns `nothing`.
 
 1. Hint: For the functional programming way of coding this can use `filter` and
    `isa` to filter for a certain type and `StatsBase.sample` to choose a random
@@ -382,7 +387,7 @@ using StatsBase  # needed for `sample`
 # you can install it by typing `]add StatsBase` in the REPL
 
 function find_food(a::Sheep, w::World)
-    as = filter(x->isa(x,Grass), w.agents |> values |> collect)
+    as = filter(x->isa(x,Grass) && size(x)>0, w.agents |> values |> collect)
     isempty(as) ? nothing : sample(as)
 end
 ```
@@ -460,7 +465,7 @@ function find_food(a::Animal, w::World)
     isempty(as) ? nothing : sample(as)
 end
 
-eats(::Sheep,::Grass) = true
+eats(::Sheep,g::Grass) = size(g) > 0
 eats(::Wolf,::Sheep) = true
 eats(::Agent,::Agent) = false
 ```