How can I update a value in a mutable HashMap? - hashmap

Question

Here is what I am trying to do:
use std::collections::HashMap;
fn main() {
let mut my_map = HashMap::new();
my_map.insert("a", 1);
my_map.insert("b", 3);
my_map["a"] += 10;
// I expect my_map becomes {"b": 3, "a": 11}
}
But this raises an error:
Rust 2015
error[E0594]: cannot assign to immutable indexed content
--> src/main.rs:8:5
|
8 | my_map["a"] += 10;
| ^^^^^^^^^^^^^^^^^ cannot borrow as mutable
|
= help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `std::collections::HashMap<&str, i32>`
Rust 2018
error[E0594]: cannot assign to data in a `&` reference
--> src/main.rs:8:5
|
8 | my_map["a"] += 10;
| ^^^^^^^^^^^^^^^^^ cannot assign
I don't really understand what that means, since I made the HashMap mutable. When I try to update an element in a vector, I get the expected result:
let mut my_vec = vec![1, 2, 3];
my_vec[0] += 10;
println! {"{:?}", my_vec};
// [11, 2, 3]
What is different about HashMap that I am getting the above error? Is there a way to update a value?

Answer 1

Indexing immutably and indexing mutably are provided by two different traits: Index and IndexMut, respectively.
Currently, HashMap does not implement IndexMut, while Vec does.
The commit that removed HashMap's IndexMut implementation states:
This commit removes the IndexMut impls on HashMap and BTreeMap, in
order to future-proof the API against the eventual inclusion of an
IndexSet trait.
It's my understanding that a hypothetical IndexSet trait would allow you to assign brand-new values to a HashMap, and not just read or mutate existing entries:
let mut my_map = HashMap::new();
my_map["key"] = "value";
For now, you can use get_mut:
*my_map.get_mut("a").unwrap() += 10;
Or the entry API:
*my_map.entry("a").or_insert(42) += 10;

Answer 2

Considering:
let mut m = std::collections::HashMap::new();
m.insert("a", 1);
m.insert("b", 3);
let k = "c";
If the key already exists:
m.insert(k, 10 + m[k] );
If the key not exists:
You may update a value of the key:
m.insert(k, 10 + if m.contains_key(k) { m[k] } else { 0 });
Or first insert a key only if it doesn't already exist:
m.entry(k).or_insert(0);
m.insert(k, 200 + m[k]);
Or update a key, guarding against the key possibly not being set:
*m.entry(k).or_insert(0) += 3000;
Finally print the value:
println!("{}", m[k]); // 3210
See:
https://doc.rust-lang.org/std/collections/struct.HashMap.html

Answer 3

I will share my own Answer because I had this issue but I was working with Structs so, that way in my case was a little bit tricky
use std::collections::HashMap;
#[derive(Debug)]
struct ExampleStruct {
pub field1: usize,
pub field2: f64,
}
fn main() {
let mut example_map = HashMap::new();
&example_map.insert(1usize, ExampleStruct { field1: 50, field2: 184.0});
&example_map.insert(6usize, ExampleStruct { field1: 60, field2: 486.0});
//First Try
(*example_map.get_mut(&1).unwrap()).field1 += 55; //50+55=105
(*example_map.get_mut(&6).unwrap()).field1 -= 25; //60-25=35
//Spliting lines
let op_elem = example_map.get_mut(&6);
let elem = op_elem.unwrap();
(*elem).field2 = 200.0;
let op_ok_elem = example_map.get_mut(&1);
let elem = op_ok_elem.unwrap_or_else(|| panic!("This msg should not appear"));
(*elem).field2 = 777.0;
println!("Map at this point: {:?}", example_map);
let op_err_elem = example_map.get_mut(&8);
let _elem = op_err_elem.unwrap_or_else(|| panic!("Be careful, check you key"));
println!("{:?}", example_map);
}
You can play with this on Rust Playground

Answer 4

You can do this with .and_modify
let mut my_map = HashMap::new();
my_map.insert("a", 1);
my_map.entry("a").and_modify(|k| *k += 10);
assert_eq!(my_map[&"a"], 11);