More concise HashMap initialization

Question

I\'m using a HashMap to count the occurrences of different characters in a string:

let text = \"GATTACA\";
let mut counts: HashMap

        

Answer 1

You can use iterators to emulate the dictionary comprehension, e.g.

let counts = "ACGT".chars().map(|c| (c, 0_i32)).collect::>();

or even for c in "ACGT".chars() { counts.insert(c, 0) }.

Also, one can write a macro to allow for concise initialisation of arbitrary values.

macro_rules! hashmap {
    ($( $key: expr => $val: expr ),*) => {{
         let mut map = ::std::collections::HashMap::new();
         $( map.insert($key, $val); )*
         map
    }}
}

used like let counts = hashmap!['A' => 0, 'C' => 0, 'G' => 0, 'T' => 0];.