Fix a - x and add tests (#131)

* Fix a - x for constant a

* Add a few tests of op(a, x) for constants a

* Typo

Author: dpsanders

src/intervals/arithmetic.jl

@@ -77,7 +77,7 @@ function -(a::Interval{T}, b::T) where {T<:Real}
 end
 function -(b::T, a::Interval{T}) where {T<:Real}
     isempty(a) && return emptyinterval(T)
-    @round(b - a.lo, b - a.hi)
+    @round(b - a.hi, b - a.lo)
 end
 
 function +(a::Interval{T}, b::Interval{T}) where T<:Real

test/interval_tests/numeric.jl

@@ -39,6 +39,17 @@ setprecision(Interval, Float64)
     @test diam(a) == eps(1.e-20)
 end
 
+@testset "Arithmetic with constants" begin
+    x = 1..2
+
+    @test 0.1 + x == Interval(1.0999999999999999, 2.1)
+    @test 3.0 - x == 1..2
+    @test 3.1 - x == Interval(1.1, 2.1)
+    @test 0.1 * (1..1) == Interval(0.1, 0.1)
+    @test (1..1) / 10.0 == Interval(0.09999999999999999, 0.1)
+
+end
+
 @testset "Power tests" begin
     @test @interval(0,3) ^ 2 == Interval(0, 9)
     @test @interval(2,3) ^ 2 == Interval(4, 9)