Add code for a multivariate discrete random variable

src/QuantEcon.jl

@@ -42,6 +42,7 @@ export
 
 # discrete_rv
     DiscreteRV,
+    MVDiscreteRV,
     draw,
 
 # mc_tools

src/discrete_rv.jl

@@ -78,3 +78,69 @@ function Base.rand!(out::AbstractArray{T}, d::DiscreteRV) where T<:Integer
 end
 
 @deprecate draw Base.rand
+
+
+struct MVDiscreteRV{TV1<:AbstractArray,TV2<:AbstractVector,K,TI<:Integer}
+    q::TV1
+    Q::TV2
+    dims::NTuple{K,TI}
+
+    function MVDiscreteRV{TV1,TV2,K,TI}(q::TV1, Q::TV2, dims::NTuple{K,TI}) where {TV1,TV2,K,TI}
+        abs(sum(q) - 1.0) > 1e-10 && error("q should sum to 1")
+        abs(Q[end] - 1.0) > 1e-10 && error("Q[end] should be 1")
+        length(Q) != prod(dims) && error("Number of elements is inconsistent")
+
+        new{TV1,TV2,K,TI}(q, Q, dims)
+    end
+end
+
+
+function MVDiscreteRV(q::TV1) where TV1<:AbstractArray
+    Q = cumsum(vec(q))
+    dims = size(q)
+
+    return MVDiscreteRV{typeof(q),typeof(Q),length(dims),eltype(dims)}(q, Q, dims)
+end
+
+
+"""
+Make a single draw from the multivariate discrete distribution.
+
+##### Arguments
+
+- `d::MVDiscreteRV`: The `MVDiscreteRV` type represetning the distribution
+
+##### Returns
+
+- `out::NTuple{Int}`: One draw from the discrete distribution
+"""
+function Base.rand(d::MVDiscreteRV)
+    x = rand()
+    i = searchsortedfirst(d.Q, x)
+
+    return ind2sub(d.dims, i)
+end
+
+"""
+Make multiple draws from the discrete distribution represented by a
+`MVDiscreteRV` instance
+
+##### Arguments
+
+- `d::MVDiscreteRV`: The `DiscreteRV` type representing the distribution
+- `k::Int`
+
+##### Returns
+
+- `out::Vector{NTuple{Int}}`: `k` draws from `d`
+"""
+Base.rand(d::MVDiscreteRV{T1,T2,K,TI}, k::V) where {T1,T2,K,TI,V} =
+    NTuple{K,TI}[rand(d) for i in 1:k]
+
+function Base.rand!(out::AbstractArray{NTuple{K,TI}}, d::MVDiscreteRV) where {K,TI}
+    @inbounds for I in eachindex(out)
+        out[I] = rand(d)
+    end
+
+    return out
+end

test/test_discrete_rv.jl

@@ -1,34 +1,69 @@
 @testset "Testing discrete_rv.jl" begin
 
-    # set up
-    n = 10
-    x = rand(n)
-    x ./= sum(x)
-    drv = DiscreteRV(x)
-
-    # test Q sums to 1
-    @test drv.Q[end] ≈ 1.0
-
-    # test lln
-    draws = rand(drv, 100_000)
-    c = counter(draws)
-    counts = Array{Float64}(n)
-    for i=1:n
-        counts[i] = c[i]
-    end
-    counts ./= sum(counts)
+    @testset "Testing univariate discrete rv" begin
+        # set up
+        n = 10
+        x = rand(n)
+        x ./= sum(x)
+        drv = DiscreteRV(x)
+
+        # test Q sums to 1
+        @test drv.Q[end] ≈ 1.0
+
+        # test lln
+        draws = rand(drv, 100_000)
+        c = counter(draws)
+        counts = Array{Float64}(n)
+        for i=1:n
+            counts[i] = c[i]
+        end
+        counts ./= sum(counts)
 
-    @test isapprox(Base.maximum(abs, counts - drv.q), 0.0; atol=1e-2)
+        @test isapprox(Base.maximum(abs, counts - drv.q), 0.0; atol=1e-2)
 
-    draws = Array{Int}(100_000)
-    rand!(draws, drv)
-    c = counter(draws)
-    counts = Array{Float64}(n)
-    for i=1:n
-        counts[i] = c[i]
+        draws = Array{Int}(100_000)
+        rand!(draws, drv)
+        c = counter(draws)
+        counts = Array{Float64}(n)
+        for i=1:n
+            counts[i] = c[i]
+        end
+        counts ./= sum(counts)
+
+        @test isapprox(Base.maximum(abs, counts - drv.q), 0.0; atol=1e-2)
     end
-    counts ./= sum(counts)
 
-    @test isapprox(Base.maximum(abs, counts - drv.q), 0.0; atol=1e-2)
+    @testset "Testing multivariate discrete rv" begin
+        # Do tests for various sizes
+        for dims in [(5, 3), (5, 10, 3), (5, 7, 5, 10)]
+            # How many dimensions
+            n = length(dims)
+
+            # Make some distributional matrix
+            q = rand(dims...)
+            q ./= sum(q)  # Normalize to sum to 1
+
+            # Create mv rv
+            rv = MVDiscreteRV(q)
+
+            # Make sure it doesn't draw numbers that don't make sense... Must
+            # be between 1 and n
+            for i in 1:n
+                @test rand(rv)[i] >= 1
+                @test rand(rv)[i] <= dims[i]
+            end
+
+            ndraws = 1_000_000
+            draws = rand(rv, ndraws)
+            counter = zeros(dims...)
+            for i in 1:ndraws
+                draw = draws[i]
+                counter[draw...] += 1.0
+            end
+            counter ./= ndraws
+            @test isapprox(Base.maximum(abs, counter - rv.q), 0.0; atol=1e-2)
+        end
+
+    end
 
 end  # testset