Very detailed explanation, thank you!
On Tuesday, May 10, 2016 at 11:35:43 PM UTC+3, Yichao Yu wrote: > > On Tue, May 10, 2016 at 4:24 PM, Andrei Zh <faithle...@gmail.com > <javascript:>> wrote: > > From performance perspective, how bad it is to use `push!(array, > element)` > > compared to pre-allocating array and setting elements in it? > > If you know the size ahead of time, always pre allocate it. > Resizing the array reuse memory if possible and double the buffer > every time (until a relatively large threshold iirc) so the time > memory is reallocated is O(log(size of array)). > > Cost of not pre-allocating comes from realloc cost (which is a small > constant factor more expensive than pre-allocating) and disabling some > optimizations (e.g. a `push!` loop can't (or very hard to) be > vectorized (SIMD)). Chances are if it's hard to know the size ahead of > time, the compiler doesn't have much room to optimize anyway and since > the realloc cost is not particularly high, you might as well use a > loop to append elements. > > > > > More generally, how extending arrays works in Julia? (code in array.c is > > quite readable, but some high-level description is appreciated) >
