Draw two distinct random integers from `1:n`

General Usage Performance
, ,
16

Yes, that. It is not completely evident that those index transformations are more expensive than a call to rand() though. (unfortunately to get the indexes right I would spend all day…). But a simple guess suggests that it might be something of the same order:

julia> function iter(n,draw)
           for i in 1:1000
               draw(n)
           end
           nothing
       end
iter (generic function with 1 method)

julia> function draw2(n)
           a = rand(1:n^2 - n)
           b = mod(a,n) + div(n^3,n) # <--- some random stuff
           if b >= a
               b += 1
           end
           a, b
       end
draw2 (generic function with 1 method)

julia> @btime iter(3,$draw2)
  13.623 μs (0 allocations: 0 bytes)

julia> function draw3(n)
           a = rand(1:n)
           b = rand(1:n-1)
           if b >= a
               b += 1
           end
           a, b
       end
draw2 (generic function with 1 method)

julia> @btime iter(3,$draw3)
  14.473 μs (0 allocations: 0 bytes)