# Re: [go-nuts] Pop out first element of a slice.

```Very good mathematical explanation. Thanks!

On Tuesday, September 20, 2016 at 7:24:39 PM UTC+3, Axel Wagner wrote:
>
> if we assume, that the number of elements N is roughly stable (so pops and
> pushes are roughly balanced):
>
> • If we append and reslice, we need to reallocate every N pops, because
> when space runs out, append will allocate 2N elements, so it has space for
> N new ones. After N pop/push sequences, it will run out of space and the
> capacity is again N, so it will allocate a new array with 2N elements, copy
> over N and continue. So every N pops, we need to allocate N elements and
> copy N elements, but don't incur any other costs.
> • If we shift down, then we ~never need to reallocate, but need to copy N
> elements on each pop, so over N pops we need to shift N² elements
>
> So, really, the question is how (N•copy(N)) compares  (alloc(N) +
> copy(N)). Which, as it seems to me, heavily depends on N, the memory you
> have, the GC pressure you have and your requirements.
>
> The answer, as always with these kinds of question is: Benchmark both on
> winner, pick that one. Otherwise, choose either.
> In general, I'd indeed assume that for somewhat filled fifo-queues the
> append-approach is faster. But I'd be willing to be surprised.
>
> On Tue, Sep 20, 2016 at 6:03 PM, Ian Davis <m...@iandavis.com
> <javascript:>> wrote:
>
>> On Tue, Sep 20, 2016, at 04:15 PM, Gabriel Adumitrachioaiei wrote:
>>
>> You might be right, but I just don't realize how. Since capacity will be
>> 2x or 1.5x as before, reallocating the slice will not happen often. Or do
>> you think that this would still be worse than copying almost all slice
>> everytime there is a pop ?
>>
>>
>> I think it depends on your use case. For the sql package I suspect the
>> use case is a long running process with only a few members in the slice and
>> a roughly level number over time. In that case the cost of copying will be
>> small and offset by the savings in not needing to allocate.
>>
>> However, if your use case is a a large slice that is iterated over via a
>> pop operation, with few or no pushes then it makes more sense to simply
>> reslice as you were doing in your original version.
>>
>> Ian
>>
>>
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>
>```
```
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