I need to do a thing where I iterate through a mutable query and compare it against data from itself, but rust won't let me have a mutable query AND use data from it too, what can I do about that? Do I have to clone the query to compare it? How to clone a query?
#Calculating a query against itself
static marten
torn schooner
iterate the query once, collecting needed data into a Vec. Iterate again and mutate based on values in the Vec
static marten
but I have to do that every frame, surely that'd hurt performance a lot
junior garden
collecting into a vec once?
maybe if you have a billion things it'd be a problem
probably not even then
unkempt gate
There's also https://docs.rs/bevy_ecs/latest/bevy_ecs/system/struct.Query.html#method.iter_combinations_mut if you have to compare everything to everything
It's O(n^k) though so only use that if you need to and don't have a massive amount of entities
Otherwise collecting to a vec seems fine, if you're worried about performance you can also keep the Vec allocation across frames by sticking it in a Local
static marten
actually yea it might be faster to collect into a vec since I'm only getting the data once, I assume ``iter_cominbations_mut()` has to get the data every time its run which'd probably be quite a bit slower
static marten
Otherwise collecting to a vec seems fine, if you're worried about performance yo...
keep the Vec allocation?
unkempt gate
keep the `Vec` allocation?
If you do query.iter().collect() it will allocate a new vec every frame. You can instead add a Local<Vec<T>> system parameter and use extend and clear instead of collect so that once the vec is allocated you can just reuse that allocation
unkempt gate
Local is a bevy concept that means that data is local to a system instead of being stored in a resource
iter.collect is roughly equal to let collection = Vec::new(); collection.extend(iter);. So if you reuse the Vec, clear will remove the items but keep the capacity from before
static marten
ohh ok
so if I put it into a local it'll just be stored locally in my system
and bevy will just reuse the already allocated memory
(resource is just a global variable, right?)
static marten
I cannot figure out how to use iter.collect with my queries
steel star
I cannot figure out how to use `iter.collect` with my queries
You can't store the references the query returns. Try .iter().cloned().collect()
static marten
You can't store the references the query returns. Try `.iter().cloned().collect(...
it just keeps telling me the value of type cannot be build from std iter??