skip show when denominator is one

NEWS.md

@@ -40,6 +40,7 @@ Standard library changes
 ------------------------
 
 * `startswith` now supports seekable `IO` streams ([#43055])
+* printing integral `Rational`s will skip the denominator in `Rational`-typed IO context (e.g. in `Arrays`) ([#45396])
 
 #### Package Manager
 

base/rational.jl

@@ -83,6 +83,11 @@ end
 
 function show(io::IO, x::Rational)
     show(io, numerator(x))
+
+    if isone(denominator(x)) && get(io, :typeinfo, Any) <: Rational
+        return
+    end
+
     print(io, "//")
     show(io, denominator(x))
 end

test/rational.jl

@@ -253,6 +253,10 @@ end
     rational2 = Rational(-4500, 9000)
     @test sprint(show, rational1) == "1465//8593"
     @test sprint(show, rational2) == "-1//2"
+    @test sprint(show, -2//2) == "-1//1"
+    @test sprint(show, [-2//2,]) == "Rational{Int64}[-1]"
+    @test sprint(show, MIME"text/plain"(), Union{Int, Rational{Int}}[7 3//6; 6//3 2]) ==
+        "2×2 Matrix{Union{Rational{Int64}, Int64}}:\n  7    1//2\n 2//1   2"
     let
         io1 = IOBuffer()
         write(io1, rational1)