Capacity in maps

[Archos]

Which is the goal to have an optional capacity as argument in maps? If
can be set more elements than given by the capacity

http://play.golang.org/p/Pvd38xW2WT

[Archos]

And I know about: "The initial capacity does not bound its size: maps
grow to accommodate the number of items stored in them..."

But I don't find an use to have it

[mjarco]

The goal is to not resize map to often. If you know that you will store 10000 entries in map, it's better to allocate map with such (or greater) capacity.

[Archos]

On Apr 24, 2:25 pm, mjarco <mja...@gmail.com> wrote:
> The goal is to not resize map to often. If you know that you will store
> 10000 entries in map, it's better to allocate map with such (or greater)
> capacity.

Ok. And why cann't be got the initial capacity given in the map? (If
any)
The function cap() is not valid in maps.

[Canopée]

Speaking of that, is there somewhere a description of Go's
implementation of map ?

The link between capacity and consequences is very clear when you're
speaking of the canonical implementation of a hash table but a quick
look at the code made me think that Go's map is more complex than that.
What kind of map is it ?

[mjarco]

AFAIK maps are hash tables, but I haven't look into sources that's my impression after I http://research.swtch.com/randhash lecture

[Ian Lance Taylor]

Archos <raul...@sent.com> writes:

> Ok. And why cann't be got the initial capacity given in the map? (If
> any)
> The function cap() is not valid in maps.

It's hard to see why that would be an interesting number. The capacity
is an important number for a slice. But we don't want to constrain the
implementation of the map datatype by defining exactly what the capacity
of a map is. And if we don't define the capacity, then the cap function
is not useful.

It might be better to think of the optional second argument to make when
making a map not as the capacity, but as a hint as to the number of
elements expected to go into the map. And you can use len to get the
number of elements that are actually in the map.

Ian