Fix and test rational exponent to_std output

Author: asterycs

src/stdstr.jl

@@ -42,7 +42,7 @@ function to_std_str(arg::Real)
 end
 
 function to_std_str(arg::Rational)
-    out = string(arg)
+    out = string(arg.num) * "/" * string(arg.den)
 
     return "(" * out * ")"
 end

test/StdTest.jl

@@ -276,6 +276,13 @@ end
     @test to_std(dc.UnaryOperation{dc.Abs}(mul(A, x))) == "abs(Ax)"
 end
 
+@testset "to_std output is correct with rational exponent" begin
+    x = Variable("x", Upper(1))
+
+    @test to_std(dc.Power(2, 1//3)) == "2^(1/3)"
+    @test to_std(dc.Power(x, 1//3)) == "x^(1/3)"
+end
+
 @testset "to_std output is correct with sgn" begin
     x = Variable("x", Upper(2))
     A = Variable("A", Upper(1), Lower(2))