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Programming-Idioms

  • C++
  • Python

Idiom #281 Use a Point as a map key

You have a Point with integer coordinates x and y. Create a map m with key type Point (or equivalent) and value type string. Insert "Hello" at position (42, 5).

from collections import namedtuple
Point = namedtuple('Point', 'x y')

p = Point(42, 5)

m = {p: "Hello"}

m = {Point(42, 5): "Hello"} would work just fine, as well! Naming the point in advance, has the advantage of using both p and Point(42, 5) as a key
m = dict()
p = Point(x=42, y=5)
m[p] = 'Hello'

Dictionary keys must be hashable objects. User-defined classes are hashable by default.
m = {Point(42, 5): 'Hello'}
class Point:
    def __init__(self, x, y):
        self.x, self.y = x, y
    def __hash__(self):
        return hash((self.x, self.y))
    def __eq__(self, p):
        return self.x == p.x and \
               self.y == p.y
m = {Point(42, 5): 'Hello'}

"... Hashability makes an object usable as a dictionary key and a set member, because these data structures use the hash value internally."
m[new Point(x: 42, y: 5)] = "Hello";
New implementation...
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programming-idioms.org 
Point = namedtuple('Point', 'x y')

p = Point(42, 5)

m = {p: "Hello"}
m = dict()
p = Point(x=42, y=5)
m[p] = 'Hello'
m = {Point(42, 5): 'Hello'}
class Point:
    def __init__(self, x, y):
        self.x, self.y = x, y
    def __hash__(self):
        return hash((self.x, self.y))
    def __eq__(self, p):
        return self.x == p.x and \
               self.y == p.y
m = {Point(42, 5): 'Hello'}
m[new Point(x: 42, y: 5)] = "Hello";
Map<Point, String> m = {};
m[Point(42, 5)] = "Hello";

m := map[Point]string{}
p := Point{x: 42, y: 5}
m[p] = "Hello"
type Point = (Integer, Integer)
m :: Map.Map Point String
m = Map.empty
m' = Map.insert (42, 5) "Hello" m
Map<Point, String> m = new HashMap<>();
m.put(new Point(42, 5), "Hello");
type
  TKey = record
    x, y: Integer;
    class operator =(L,R: TKey): Boolean;
    class operator <(L,R: TKey): Boolean;
    class operator >(L,R: TKey): Boolean;
  end;
  TMap = specialize TFPGMap<TKey, String>;

var
  m: TMap;
  Key: TKey;
begin
  Key.x := 42;
  Key.y := 5;
  m := TMap.Create;
  m.Add(Key, 'Hello World');
  m.Free;
end.

my %m;
my $p = Point->new(x => 42, y => 5);

$m{$p} = 'Hello';

Point = Struct.new(:x, :y)
m = {Point.new(42, 5) => "Hello"}
let mut map: HashMap<Point, String> = HashMap::new();
map.insert(Point { x: 42, y: 5 }, "Hello".into());