Groups   210 of 99+      julia-users › copy assignment opeartion on existing and valid destination arrays 9 posts by 3 authors      Roy Wang    10 2 14   I often need a  copy assignment  type of operation to an existing destination array of the exact same element type and size. Let's only talk about arrays of concrete types, like a multi-dimensional array of floats. This is useful when I write optimization solvers, and I need to store vectors or matrices from the previous step. I usually pre-allocate a pair of arrays of the same type and size, x and x_next, then do:  x_next   x;   at the end of each iteration of my solver.  At first, I thought using copy    shallow copy  on them is fine to make sure they are separate entities, since floating point numbers and integers are concrete types in Julia. While I verified this is true  at least on arrays of Float64s , I looked at  around line 202 at the time of this post , and copy   seems to call copy!  similar a , a . To my understanding, this allocates a new destination array, fills it with the corresponding values from the source array, then assigns the pointer of this new destination array to x_next, and the garbage collector removes the old array that x_next was pointing to. This is a lot of work when I just want to traverse through x_next, and assign it the corresponding values from x. Please correct me if my understanding is wrong!  This is a really common operation. I'd appreciate it if someone can advise me whether there is already an existing method for doing this  or a better solution  before I write my own.  Cheers,  Roy     Roy Wang    10 2 14    This kind of routine is what I'm talking about...    copy assignment for vectors function copyassignment! a::Vector,b::Vector       assert length a     length b      for n 1:length a          a n  b n ;     end end  My questions: 1  Is there a standard function that does this? 2  Is there a better way to do this so it'll handle any type of multi-dimensional array of integers and floats without performance penalty?      John Myles White    10 2 14  Re:  julia-users  Re: copy assignment opeartion on existing and valid destination arrays  Why not use copy!    -- John      Roy Wang    10 2 14    oops, add   assert eltype a     eltype b  in the checks too, otherwise there might be an  inexact error  when mixing integers and floats.       Roy Wang    10 2 14  Re:  julia-users  Re: copy assignment opeartion on existing and valid destination arrays  Hey John,  Ah geez, copy!   was only 2 lines lower than copy   in abstractarray-jl. Thanks!      Roy Wang    10 2 14  Re:  julia-users  Re: copy assignment opeartion on existing and valid destination arrays  Is there a version of copy!   that uses  parallel? My x and x_next are usually huge.      David P. Sanders    10 4 14  Re:  julia-users  Re: copy assignment opeartion on existing and valid destination arrays  Hi, Are you sure that you need a copy operation? If I understand correctly what you are doing, you just need access to the x from the previous iteration.  Could you not just swap x and x_next and avoid the copy :  X, x_next   x_next, x   so you create them once only .      Roy Wang    10 21 14  Re:  julia-users  Re: copy assignment opeartion on existing and valid destination arrays  This is great, thanks!  I verified pointer X  is the same as what pointer x_next  used to be before running your line of code, and pointer x_next  after running your line of code is the same as pointer x .  I did not know of this about Julia!      David P. Sanders    10 21 14  Re:  julia-users  Re: copy assignment opeartion on existing and valid destination arrays    El martes, 21 de octubre de 2014 14:25:33 UTC-5, Roy Wang escribió: This is great, thanks!  I verified pointer X  is the same as what pointer x_next  used to be before running your line of code, and pointer x_next  after running your line of code is the same as pointer x .  I did not know of this about Julia!  I'm glad this was useful!  This is a common idiom  programming style   trick  in any language. Unfortunately, often people don't know  that this is possible or practical, and they waste a lot of time copying arrays unnecessarily when they could just be swapping the pointers, as you put it.  In most languages you would need to call an explicit function, like `swap` in Cpp, in order to do this. This idiom,    x, x_next   x_next, x    works in both Python and Julia. Yes, it is quite neat ;    The capital X that was in my original post was due to an auto-correct when typing from my phone. It should be small x to work with the code in your loop.   David.     On Saturday, 4 October 2014 11:05:33 UTC-4, David P. Sanders wrote: Hi, Are you sure that you need a copy operation? If I understand correctly what you are doing, you just need access to the x from the previous iteration.  Could you not just swap x and x_next and avoid the copy :  X, x_next   x_next, x   so you create them once only .