reduce函数

reduce()将表达式逐个应用于列表中的元素，然后和累加器中的当前结果累加，最后返回完整结果。该函数将遍历给定列表中的每个元素e，在e上运行表达式并和累加器的当前结果累加，将新的结果存储在累加器中。这个函数类似于函数式语言（如Lisp和Scala）中的fold或reduce方法。

openCypher兼容性

在openCypher中，reduce()函数没有定义。nGQL使用了Cypher方式实现reduce()函数。

语法

  1. reduce(<accumulator> = <initial>, <variable> IN <list> | <expression>)
参数说明
accumulator在遍历列表时保存累加结果。
initial为accumulator提供初始值的表达式或值。
variable为列表引入一个变量，决定使用列表中的哪个元素。
list列表或列表表达式。
expression该表达式将对列表中的每个元素运行一次，并将结果累加至accumulator。

示例

  1. nebula> RETURN reduce(totalNum = 10, n IN range(1, 3) | totalNum + n) AS r;
  2. +----+
  3. | r  |
  4. +----+
  5. | 16 |
  6. +----+
  8. nebula> RETURN reduce(totalNum = -4 * 5, n IN [1, 2] | totalNum + n * 2) AS r;
  9. +-----+
  10. | r   |
  11. +-----+
  12. | -14 |
  13. +-----+
  15. nebula> MATCH p = (n:player{name:"LeBron James"})<-[:follow]-(m) \
  16.         RETURN  nodes(p)[0].age AS src1,  nodes(p)[1].age AS dst2,  \
  17.         reduce(totalAge = 100, n IN nodes(p) | totalAge + n.age) AS sum;
  18. +------+------+-----+
  19. | src1 | dst2 | sum |
  20. +------+------+-----+
  21. | 34   | 31   | 165 |
  22. +------+------+-----+
  23. | 34   | 29   | 163 |
  24. +------+------+-----+
  25. | 34   | 33   | 167 |
  26. +------+------+-----+
  27. | 34   | 26   | 160 |
  28. +------+------+-----+
  29. | 34   | 34   | 168 |
  30. +------+------+-----+
  31. | 34   | 37   | 171 |
  32. +------+------+-----+
  34. nebula> LOOKUP ON player WHERE player.name == "Tony Parker" \
  35.         |  GO FROM $-.VertexID over follow \
  36.         WHERE follow.degree != reduce(totalNum = 5, n IN range(1, 3) | $$.player.age + totalNum + n) \
  37.         YIELD $$.player.name AS id, $$.player.age AS age, follow.degree AS degree;
  38. +---------------------+-----+--------+
  39. | id                  | age | degree |
  40. +---------------------+-----+--------+
  41. | "Tim Duncan"        | 42  | 95     |
  42. +---------------------+-----+--------+
  43. | "LaMarcus Aldridge" | 33  | 90     |
  44. +---------------------+-----+--------+
  45. | "Manu Ginobili"     | 41  | 95     |
  46. +---------------------+-----+--------+