How to do it...

1. In the folder src/bin, create a file called hashmap.rs.
2. Add the following code, and run it with cargo run --bin hashmap:

1   use std::collections::HashMap; 
2 
3   fn main() {
4     // The HashMap can map any hashable type to any other
5     // The first type is called the "key"
6     // and the second one the "value"
7     let mut tv_ratings = HashMap::new();
8     // Here, we are mapping &str to i32
9     tv_ratings.insert("The IT Crowd", 8);
10    tv_ratings.insert("13 Reasons Why", 7);
11    tv_ratings.insert("House of Cards", 9);
12    tv_ratings.insert("Stranger Things", 8);
13    tv_ratings.insert("Breaking Bad", 10);
14 
15    // Does a key exist?
16    let contains_tv_show = tv_ratings.contains_key("House of
        Cards");
17    println!("Did we rate House of Cards? {}", contains_tv_show);
18    let contains_tv_show = tv_ratings.contains_key("House");
19    println!("Did we rate House? {}", contains_tv_show);
20 
21    // Access a value
22    if let Some(rating) = tv_ratings.get("Breaking Bad") {
23      println!("I rate Breaking Bad {} out of 10", rating);
24    }
25 
26    // If we insert a value twice, we overwrite it
27    let old_rating = tv_ratings.insert("13 Reasons Why", 9);
28    if let Some(old_rating) = old_rating {
29      println!("13 Reasons Why's old rating was {} out of 10",
          old_rating);
30    }
31    if let Some(rating) = tv_ratings.get("13 Reasons Why") {
32      println!("But I changed my mind, it's now {} out of 10",
          rating);
33    }
34 
35    // Remove a key and its value
36    let removed_value = tv_ratings.remove("The IT Crowd");
37    if let Some(removed_value) = removed_value {
38       println!("The removed series had a rating of {}",
           removed_value);
39    }
40 
41    // Iterating accesses all keys and values
42    println!("All ratings:");
43    for (key, value) in &tv_ratings {
44      println!("{}\t: {}", key, value);
45    }
46 
47    // We can iterate mutably
48    println!("All ratings with 100 as a maximum:");
49    for (key, value) in &mut tv_ratings {
50      *value *= 10;
51      println!("{}\t: {}", key, value);
52    }
53 
54    // Iterating without referencing the HashMap moves its
        contents
55    for _ in tv_ratings {}
56    // tv_ratings is not usable anymore

If you don't need to access both keys and values at the same time, you can iterate over either individually:

58    // Like with the other collections, you can preallocate a size
59    // to gain some performance
60    let mut age = HashMap::with_capacity(10);
61    age.insert("Dory", 8);
62    age.insert("Nemo", 5);
63    age.insert("Merlin", 10);
64    age.insert("Bruce", 9);
65 
66    // Iterate over all keys
67    println!("All names:");
68    for name in age.keys() {
69      println!("{}", name);
70    }
71 
72    // Iterate over all values
73    println!("All ages:");
74    for age in age.values() {
75      println!("{}", age);
76    }
77 
78    // Iterate over all values and mutate them
79    println!("All ages in 10 years");
80    for age in age.values_mut() {
81      *age += 10;
82      println!("{}", age);
83    }
84

You can use the entry API to assign default values to keys if they're not yet in the HashMap:

87    {
88      let age_of_coral = age.entry("coral").or_insert(11);
89      println!("age_of_coral: {}", age_of_coral);
90    }
91    let age_of_coral = age.entry("coral").or_insert(15);
92    println!("age_of_coral: {}", age_of_coral);
93 }