[julia-users] performance of two different array allocations

[Jeremy Kozdon]

In a class I'm teaching the students are using Julia and I couldn't for the 
life of me figure out why one of my students codes was allocating a lot of 
memory.

I finally paired it down the following example that I don't understand:

function version1(N)
  b = [1;zeros(N-1)]
  println(typeof(b))
  for k = 1:N
    for j = 1:N
      b[j] += k
    end
  end
end


function version2(N)
  b = zeros(N)
  b[1] = 1
  println(typeof(b))
  for k = 1:N
    for j = 1:N
      b[j] += k
    end
  end
end

N = 1000
println("compiling..")
@time version1(N)
version2(N)
println()
println()

println("Version 1")
@time version1(N)
println()

println("Version 2")
@time version2(N)

The output of this (without the compiling output) in v0.4.5 is:

Version 1
Array{Float64,1}
  0.092473 seconds (3.47 M allocations: 52.920 MB, 3.24% gc time)

Version 2
Array{Float64,1}
  0.001195 seconds (27 allocations: 8.828 KB)

Both version produce the same type for Array b, but in version1 every time 
through the loop allocation happens and in the version2 the only allocation 
is of the initial array.

I've not run into this one before (because I would never do version1), but 
as all of us that teach know students will always surprise you with their 
approaches.

Any help understanding what's going on would be appreciated.