Add tests for expect function.

test/test_treetensornetworks/test_expect.jl

@@ -0,0 +1,32 @@
+using ITensors
+using ITensorNetworks
+using Test
+
+@testset "MPS expect comparison with ITensors" begin
+  N = 25
+  s = siteinds("S=1/2", N)
+  a = random_mps(s; internal_inds_space=100)
+  b = MPS([a[v] for v in vertices(a)])
+  res_a = expect("Sz", a)
+  res_b = expect(b, "Sz")
+  res_a = [res_a[v] for v in vertices(a)]
+  @test res_a ≈ res_b rtol=1e-6 
+end
+
+@testset "TTN expect" begin
+    tooth_lengths = fill(5, 6)
+    c = named_comb_tree(tooth_lengths)
+    s = siteinds("S=1/2", c)
+    d = Dict()
+    magnetization = Dict()
+    for (i, v) in enumerate(vertices(s))
+      d[v] = isodd(i) ? "Up" : "Dn"
+      magnetization[v] = isodd(i) ? 0.5 : -0.5
+    end
+    states = v -> d[v]
+    state = TTN(s, states)
+    res=expect("Sz", state)   # just testing that this doesn't go via expect for arbitrary tensor networks.
+    @test all([isapprox(res[v], magnetization[v],atol=1e-8) for v in vertices(s)])
+end
+
+nothing