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reduce函数Graph

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Graph

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

语法Graph

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。

说明：返回值的类型取决于提供的参数，以及表达式的语义。

示例Graph

nebula> RETURN reduce(totalNum = 10, n IN range(1, 3) | totalNum + n) AS r;
+----+
| r  |
+----+
| 16 |
+----+

nebula> RETURN reduce(totalNum = -4 * 5, n IN [1, 2] | totalNum + n * 2) AS r;
+-----+
| r   |
+-----+
| -14 |
+-----+

nebula> MATCH p = (n:player{name:"LeBron James"})<-[:follow]-(m) \
        RETURN  nodes(p)[0].age AS src1,  nodes(p)[1].age AS dst2,  \
        reduce(totalAge = 100, n IN nodes(p) | totalAge + n.age) AS sum;
+------+------+-----+
| src1 | dst2 | sum |
+------+------+-----+
| 34   | 31   | 165 |
+------+------+-----+
| 34   | 29   | 163 |
+------+------+-----+
| 34   | 33   | 167 |
+------+------+-----+
| 34   | 26   | 160 |
+------+------+-----+
| 34   | 34   | 168 |
+------+------+-----+
| 34   | 37   | 171 |
+------+------+-----+

nebula> LOOKUP ON player WHERE player.name == "Tony Parker" \
        |  GO FROM $-.VertexID over follow \
        WHERE follow.degree != reduce(totalNum = 5, n IN range(1, 3) | $$.player.age + totalNum + n) \
        YIELD $$.player.name AS id, $$.player.age AS age, follow.degree AS degree;
+---------------------+-----+--------+
| id                  | age | degree |
+---------------------+-----+--------+
| "Tim Duncan"        | 42  | 95     |
+---------------------+-----+--------+
| "LaMarcus Aldridge" | 33  | 90     |
+---------------------+-----+--------+
| "Manu Ginobili"     | 41  | 95     |
+---------------------+-----+--------+
最后更新: March 24, 2021
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