有条件地添加到HashMap的最佳方式，尽可能少的查找？ [英] Best way to conditionally add to a HashMap, with as few lookups as possible?

问题描述

使用哈希映射时，如果密钥不存在，通常需要添加键：值对。

$
$ b

如果！map.contains（ key）{
let val = create_val（）;
map.insert（key，val）;
some_creation_logic（val）;
} else {
let val = map [key];
some_update_logic（val）;
}

虽然这可以起作用，但它总是会进行2次查找。

最接近我可以使用 Entry.or_insert （或 or_insert_with ） ，计算长度以便 else 分支。

  let map_len_prev = map.len（）; 
 let val = map.or_insert_with（key，create_val）; 
如果map_len_prev！= map.len（）{
 some_creation_logic（val）; 
} else {
 some_update_logic（val）; 
 
 $ / code> 

是否有更清晰的方式在需要时插入值，同时保留对于它已经/不存在的情况运行两个逻辑分支？

c> Entry ：

 使用std :: collections :: hash_map :: Entry :: {占用， 空的}; 
 
 match map.entry（key）{
已占用（val）=> {
 some_update_logic（val.get（））; 
}，
空置（入口）=> {
 let val = entry.insert（create_val（））; 
 some_creation_logic（val）; 
} 
} 
  

When using hash maps, it is common to want to add a key:value pair if the key is not already present.

This reads well but isn't as optimal as it could be.

if !map.contains(key) {
    let val = create_val();
    map.insert(key, val);
    some_creation_logic(val);
} else {
    let val = map[key];
    some_update_logic(val);
}

While this works it will always do 2 lookups.

The closest I could get was to use Entry.or_insert (or or_insert_with), counting the length so the else branch can be taken.

let map_len_prev = map.len();
let val = map.or_insert_with(key, create_val);
if map_len_prev != map.len() {
    some_creation_logic(val);
} else {
    some_update_logic(val);
}

Is there a clearer way to insert a value when needed while keeping a way of running both branches of logic for cases it does/doesn't already exist?

解决方案

You can just match the Entry:

use std::collections::hash_map::Entry::{Occupied, Vacant}; 

match map.entry(key) {
    Occupied(val) => {
        some_update_logic(val.get());
    },
    Vacant(entry) => {
        let val = entry.insert(create_val());
        some_creation_logic(val);
    }
}

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