```dart
var ingredients = new Set();
ingredients.addAll(['gold', 'titanium', 'xenon']);
assert(ingredients.length == 3);
// Adding a duplicate item has no effect.
ingredients.add('gold');
assert(ingredients.length == 3);
// Remove an item from a set.
ingredients.remove('gold');
assert(ingredients.length == 2);

var ingredients = new Set();
ingredients.addAll(['gold', 'titanium', 'xenon']);
// Check whether an item is in the set.
assert(ingredients.contains('titanium'));
// Check whether all the items are in the set.
assert(ingredients.containsAll(['titanium', 'xenon']));

// 交际是两个 set 中都有的数据的子集：
var ingredients = new Set();
ingredients.addAll(['gold', 'titanium', 'xenon']);
// Create the intersection of two sets.
var nobleGases = new Set.from(['xenon', 'argon']);
var intersection = ingredients.intersection(nobleGases);
assert(intersection.length == 1);
assert(intersection.contains('xenon'));
```

## 判断 Iterable 集合（List, Set, 和 Map）是否为空的

```dart
// 可以使用 isEmpty 函数来判断集合是否为空的：
if (lunchBox.isEmpty) return 'so hungry...';
if (words.isNotEmpty) return words.join(' ');
var teas = ['green', 'black', 'chamomile', 'earl grey'];
assert(!teas.isEmpty);
```


dart.md

```js
const s = new Set();
[2, 3, 5, 4, 5, 2, 2].forEach(x => s.add(x));
for (let i of s) {
  console.log(i);
}
// 2 3 5 4

// 例一
const set = new Set([1, 2, 3, 4, 4]);
[...set]
// [1, 2, 3, 4]

// 例二
const items = new Set([1, 2, 3, 4, 5, 5, 5, 5]);
items.size // 5

// 例三
const set = new Set(document.querySelectorAll('div'));
set.size // 56

// 去除数组的重复成员
[...new Set(array)]

// 去除字符串里面的重复字符。
[...new Set('ababbc')].join('')
// "abc"

// 两个对象总是不相等的。
let set = new Set();
set.add({});
set.size // 1
set.add({});
set.size // 2


s.add(1).add(2).add(2);
// 注意2被加入了两次

s.size // 2

s.has(1) // true
s.has(2) // true
s.has(3) // false

s.delete(2);
s.has(2) // false


let set = new Set(['red', 'green', 'blue']);

for (let item of set.keys()) {
  console.log(item);
}
// red
// green
// blue

for (let item of set.values()) {
  console.log(item);
}
// red
// green
// blue

for (let item of set.entries()) {
  console.log(item);
}
// ["red", "red"]
// ["green", "green"]
// ["blue", "blue"]
```


js.md

花括号或 set() 函数可以用来创建集合。
注意：要创建一个空集合你只能用 set() 而不能用 {}， 因为后者是创建一个空字典

```sh
>>> basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
>>> print(basket)                      # show that duplicates have been removed
{'orange', 'banana', 'pear', 'apple'}
>>> 'orange' in basket                 # fast membership testing
True
>>> 'crabgrass' in basket
False

>>> # Demonstrate set operations on unique letters from two words
...
>>> a = set('abracadabra')
>>> b = set('alacazam')
>>> a                                  # unique letters in a
{'a', 'r', 'b', 'c', 'd'}
>>> a - b                              # letters in a but not in b
{'r', 'd', 'b'}
>>> a | b                              # letters in a or b or both
{'a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'}
>>> a & b                              # letters in both a and b
{'a', 'c'}
>>> a ^ b                              # letters in a or b but not both
{'r', 'd', 'b', 'm', 'z', 'l'}

# 类似于 列表推导式，集合也支持推导式形式
>>> a = {x for x in 'abracadabra' if x not in 'abc'}
>>> a
{'r', 'd'}
```

## 集合的比较

```python
(1, 2, 3)              < (1, 2, 4)
[1, 2, 3]              < [1, 2, 4]
'ABC' < 'C' < 'Pascal' < 'Python'
(1, 2, 3, 4)           < (1, 2, 4)
(1, 2)                 < (1, 2, -1)
(1, 2, 3)             == (1.0, 2.0, 3.0)
(1, 2, ('aa', 'ab'))   < (1, 2, ('abc', 'a'), 4)
```
