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Map

Introduction

import 'map';

The Map<K, V> type is built into the language. The map module adds functions for copying a map, checking and removing keys, and retrieving its keys or values.

See Maps for map literals, indexing, assignment, and .length.

Notes

  • Maps keep their entries in insertion order.
  • keys and values return new arrays. Changing those arrays does not change the map.
  • copy creates a new map, but it does not copy records stored inside it.
  • These are module functions: use map.has(ages, "ada"), not ages.has("ada").

map.copy()

Map<K, V> map.copy(Map<K, V> m)

Returns a new map with the same entries and order.

import 'map';
 
Map<String, int> ages = { "ada": 36, "grace": 40 };
Map<String, int> copy = map.copy(ages);
 
copy["linus"] = 22;
 
print(ages.length);      // 2
print(copy.length);      // 3

map.has()

bool map.has(Map<K, V> m, K key)

Reports whether key is present in the map.

import 'map';
 
Map<String, int> ages = { "ada": 36 };
 
print(map.has(ages, "ada"));     // true
print(map.has(ages, "grace"));   // false

map.keys()

K[] map.keys(Map<K, V> m)

Returns the keys in insertion order as a new growable array. Use it with forEach to iterate over a map.

import 'map';
 
Map<String, int> ages = { "ada": 36, "grace": 40 };
 
forEach(map.keys(ages), name) {
    print(name, ages[name]);
}

map.remove()

bool map.remove(Map<K, V> m, K key)

Removes the entry for key and returns true. If the key was not present, the map is unchanged and the function returns false.

import 'map';
 
Map<String, int> ages = { "ada": 36, "grace": 40 };
 
print(map.remove(ages, "grace"));   // true
print(map.remove(ages, "grace"));   // false
print(ages.length);                 // 1

map.values()

V[] map.values(Map<K, V> m)

Returns the values in insertion order as a new growable array. The result uses the same order as map.keys(m).

import 'map';
import 'json';
 
Map<String, int> ages = { "ada": 36, "grace": 40 };
print(json.toText(map.values(ages)));   // [36,40]
JSONMath