[julia-users] copy!() vs element-by-element copying

[Alan Crawford]

I have written the following test to see the difference between going 
copying element-by-element and using copy!(x,y):

function test1(x,y)
for i in eachindex(x)
    @inbounds x[i] = y[i]
end
end

function test2(x,y)
copy!(x,y)
end

function test()
NumObs = 1000
x = rand(NumObs)
y = rand(NumObs)
test1(x,y)
test2(x,y)
println("Test 1") 
@time(for z in 1:1e5 test1(x,y) end)
println("Test 2")
@time(for z in 1:1e5 test2(x,y) end)
end
test()


I get the following timings:

Test 1

  0.031750 seconds

Test 2

  0.009360 seconds

So it seems copy!() is quite a bit faster ... 

I ran this test because I would like to copy an element of a vector y into 
an element of vector x, but x and y are not the same everywhere - so i 
can’t do copy!(x,y). Moreover, since copy!() only works on arrays, I can’t 
do copy!(x[i],y[i]). 

So my question is whether there is a way to get the speed of copy!() , but 
for copying a Float to a Float? Seems like i am probably missing something 
fairly simple...

Thanks
Alan