The line now reads x[:,:] += 1.0f-5*dxdt
And the result is now correct, but memory usage increased. Shouldn't it go down if we're re-assigning to the same variable? On Wednesday, December 10, 2014 11:57:28 PM UTC-7, Isaiah wrote: > > `x += ...` is equivalent to writing `x = x + ...` which rebinds the > variable within that function. Instead, do an explicit array assignment > `x[:,:] = ...` > > This is discussed in the manual with a warning about type changes, but the > implication for arrays should probably be made clear as well: > http://julia.readthedocs.org/en/latest/manual/mathematical-operations/#updating-operators > > (there are some ongoing discussions about in-place array operators to > improve the situation) > > On Wed, Dec 10, 2014 at 7:44 PM, Mark Stock <mark.j...@gmail.com > <javascript:>> wrote: > >> Hello, n00b Julia user here. I have two functions that change the values >> of a passed-in array. One works (dxFromX), but for the other one >> (eulerStep) the caller does not see any changes to the array. Why is this? >> >> function dxFromX!(x,dxdt) >> fill!(dxdt,0.0) >> >> for itarg = 1:size(x,1) >> for isrc = 1:size(x,1) >> dx = x[isrc,1] - x[itarg,1] >> dy = x[isrc,2] - x[itarg,2] >> coeff = 1.0f0 / (dx^2 + dy^2 + 0.1f0) >> dxdt[itarg,1] -= coeff * dy >> dxdt[itarg,2] += coeff * dx >> end >> end >> end >> >> function eulerStep!(x) >> dxdt = zeros(x) >> print ("\ndxdt before\n",dxdt[1:5,:],"\n") >> dxFromX!(x,dxdt) >> print ("\ndxdt after has changed\n",dxdt[1:5,:],"\n") >> x += 1.0f-5*dxdt >> print ("\nx inside\n",x[1:5,:],"\n") >> end >> >> x = float32(rand(1024,2)) >> print ("\nx before\n",x[1:5,:],"\n") >> @time eulerStep!(x) >> print ("\nx after is unchanged!\n",x[1:5,:],"\n") >> >> I see the same behavior on 0.3.3 and 0.4.0, both release and debug >> binaries, on OSX and Linux. >> > >
