collect_with

Trait CollectWith

Source 
pub trait CollectWith: Iterator {
    // Provided methods
    fn collect_with<T>(self, capacity: impl FnOnce(usize) -> usize) -> T
       where T: ExtendWithCapacity<Self::Item>,
             Self: Sized { ... }
    fn collect_with_exact<T>(self, capacity: impl FnOnce(usize) -> usize) -> T
       where T: ExtendWithCapacity<Self::Item>,
             Self: Sized { ... }
}
Expand description

Trait for collecting iterator elements with flexible capacity calculation

Provided Methods§

Source

fn collect_with<T>(self, capacity: impl FnOnce(usize) -> usize) -> T
where T: ExtendWithCapacity<Self::Item>, Self: Sized,

Collect elements using a capacity calculated from a closure

  • capacity
    • Closure that calculates capacity based on iterator size hints
§About the Final Capacity Size

For example, (0..10).collect_with::<Vec<_>>(|_size_bound| 2)

  1. (0..10).size_hint() returns (10, Some(10)).
  2. _size_bound is 10.
  3. The closure returns 2, the final capacity is max(10, 2) = 10.
  4. The vector is created with Vec::with_capacity(10).
§Example
use collect_with::CollectWith;

let s = [vec!["a"], vec!["b", "c", "d"]]
  .into_iter()
  .flatten()
  .collect_with::<String>(|size| match size {
    0 => 8,
    n => n,
  });
assert_eq!(s.len(), 4);
assert_eq!(s.capacity(), 8);

The collection may allocate more capacity than calculated if needed. If you need an exact capacity size, please use collect_with_exact()

Source

fn collect_with_exact<T>(self, capacity: impl FnOnce(usize) -> usize) -> T
where T: ExtendWithCapacity<Self::Item>, Self: Sized,

Collect elements using exact capacity calculated from a closure.

  • capacity
    • Closure that calculates exact capacity requirement

The collection will strictly use the calculated capacity without overallocation.

Implementors§

Source§

impl<I: Iterator> CollectWith for I

Implement CollectWith trait for Iterator