## 4.4. List functions

List functions return lists of things — nodes in a path, and so on.

Further details and examples of lists may be found in Section 2.11, “Lists” and Section 2.7.10, “List operators”.

Functions:

### 4.4.1. keys()

keys returns a list containing the string representations for all the property names of a node, relationship, or map.

Syntax: keys(expression)

Returns:

 A list containing String elements.

Arguments:

Name Description

expression

An expression that returns a node, a relationship, or a map.

Considerations:

 keys(null) returns null.

Query.

MATCH (a)
WHERE a.name = 'Alice'
RETURN keys(a)

A list containing the names of all the properties on the node bound to a is returned.

Table 4.44. Result
keys(a)

1 row

["name","eyes","age"]

Try this query live.  CREATE (alice:Person:Developer {name:'Alice', age: 38, eyes: 'brown'}), (bob {name: 'Bob', age: 25, eyes: 'blue'}), (charlie {name: 'Charlie', age: 53, eyes: 'green'}), (daniel {name: 'Daniel', age: 54, eyes: 'brown'}), (eskil {name: 'Eskil', age: 41, eyes: 'blue', array: ['one', 'two', 'three']}), (alice)-[:KNOWS]->(bob), (alice)-[:KNOWS]->(charlie), (bob)-[:KNOWS]->(daniel), (charlie)-[:KNOWS]->(daniel), (bob)-[:MARRIED]->(eskil) MATCH (a) WHERE a.name = 'Alice' RETURN keys(a)

### 4.4.2. labels()

labels returns a list containing the string representations for all the labels of a node.

Syntax: labels(node)

Returns:

 A list containing String elements.

Arguments:

Name Description

node

An expression that returns a single node.

Considerations:

 labels(null) returns null.

Query.

MATCH (a)
WHERE a.name = 'Alice'
RETURN labels(a)

A list containing all the labels of the node bound to a is returned.

Table 4.45. Result
labels(a)

1 row

["Person","Developer"]

Try this query live.  CREATE (alice:Person:Developer {name:'Alice', age: 38, eyes: 'brown'}), (bob {name: 'Bob', age: 25, eyes: 'blue'}), (charlie {name: 'Charlie', age: 53, eyes: 'green'}), (daniel {name: 'Daniel', age: 54, eyes: 'brown'}), (eskil {name: 'Eskil', age: 41, eyes: 'blue', array: ['one', 'two', 'three']}), (alice)-[:KNOWS]->(bob), (alice)-[:KNOWS]->(charlie), (bob)-[:KNOWS]->(daniel), (charlie)-[:KNOWS]->(daniel), (bob)-[:MARRIED]->(eskil) MATCH (a) WHERE a.name = 'Alice' RETURN labels(a)

### 4.4.3. nodes()

nodes() returns a list containing all the nodes in a path.

Syntax: nodes(path)

Returns:

 A list containing Node elements.

Arguments:

Name Description

path

An expression that returns a path.

Considerations:

 nodes(null) returns null.

Query.

MATCH p =(a)-->(b)-->(c)
WHERE a.name = 'Alice' AND c.name = 'Eskil'
RETURN nodes(p)

A list containing all the nodes in the path p is returned.

Table 4.46. Result
nodes(p)

1 row

[Node[0]{name:"Alice",eyes:"brown",age:38},Node[1]{name:"Bob",eyes:"blue",age:25},Node[4]{array:["one","two","three"],name:"Eskil",eyes:"blue",age:41}]

Try this query live.  CREATE (alice:Person:Developer {name:'Alice', age: 38, eyes: 'brown'}), (bob {name: 'Bob', age: 25, eyes: 'blue'}), (charlie {name: 'Charlie', age: 53, eyes: 'green'}), (daniel {name: 'Daniel', age: 54, eyes: 'brown'}), (eskil {name: 'Eskil', age: 41, eyes: 'blue', array: ['one', 'two', 'three']}), (alice)-[:KNOWS]->(bob), (alice)-[:KNOWS]->(charlie), (bob)-[:KNOWS]->(daniel), (charlie)-[:KNOWS]->(daniel), (bob)-[:MARRIED]->(eskil) MATCH p = (a)-->(b)-->(c) WHERE a.name = 'Alice' AND c.name = 'Eskil' RETURN nodes(p)

### 4.4.4. range()

range() returns a list comprising all integer values within a range bounded by a start value start and end value end, where the difference step between any two consecutive values is constant; i.e. an arithmetic progression. The range is inclusive, and the arithmetic progression will therefore always contain start and — depending on the values of start, step and end — end.

Syntax: range(start, end [, step])

Returns:

 A list of Integer elements.

Arguments:

Name Description

start

An expression that returns an integer value.

end

An expression that returns an integer value.

step

A numeric expression defining the difference between any two consecutive values, with a default of 1.

Query.

RETURN range(0, 10), range(2, 18, 3)

Two lists of numbers in the given ranges are returned.

Table 4.47. Result
range(0, 10) range(2, 18, 3)

1 row

[0,1,2,3,4,5,6,7,8,9,10]

[2,5,8,11,14,17]

Try this query live.  CREATE (alice:Person:Developer {name:'Alice', age: 38, eyes: 'brown'}), (bob {name: 'Bob', age: 25, eyes: 'blue'}), (charlie {name: 'Charlie', age: 53, eyes: 'green'}), (daniel {name: 'Daniel', age: 54, eyes: 'brown'}), (eskil {name: 'Eskil', age: 41, eyes: 'blue', array: ['one', 'two', 'three']}), (alice)-[:KNOWS]->(bob), (alice)-[:KNOWS]->(charlie), (bob)-[:KNOWS]->(daniel), (charlie)-[:KNOWS]->(daniel), (bob)-[:MARRIED]->(eskil) RETURN range(0, 10), range(2, 18, 3)

### 4.4.5. reduce()

reduce() returns the value resulting from the application of an expression on each successive element in a list in conjunction with the result of the computation thus far. This function will iterate through each element e in the given list, run the expression on e — taking into account the current partial result — and store the new partial result in the accumulator. This function is analogous to the fold or reduce method in functional languages such as Lisp and Scala.

Syntax: reduce(accumulator = initial, variable IN list | expression)

Returns:

 The type of the value returned depends on the arguments provided, along with the semantics of expression.

Arguments:

Name Description

accumulator

A variable that will hold the result and the partial results as the list is iterated.

initial

An expression that runs once to give a starting value to the accumulator.

list

An expression that returns a list.

variable

The closure will have a variable introduced in its context. We decide here which variable to use.

expression

This expression will run once per value in the list, and produce the result value.

Query.

MATCH p =(a)-->(b)-->(c)
WHERE a.name = 'Alice' AND b.name = 'Bob' AND c.name = 'Daniel'
RETURN reduce(totalAge = 0, n IN nodes(p)| totalAge + n.age) AS reduction

The age property of all nodes in the path are summed and returned as a single value.

Table 4.48. Result
reduction

1 row

117

Try this query live.  CREATE (alice:Person:Developer {name:'Alice', age: 38, eyes: 'brown'}), (bob {name: 'Bob', age: 25, eyes: 'blue'}), (charlie {name: 'Charlie', age: 53, eyes: 'green'}), (daniel {name: 'Daniel', age: 54, eyes: 'brown'}), (eskil {name: 'Eskil', age: 41, eyes: 'blue', array: ['one', 'two', 'three']}), (alice)-[:KNOWS]->(bob), (alice)-[:KNOWS]->(charlie), (bob)-[:KNOWS]->(daniel), (charlie)-[:KNOWS]->(daniel), (bob)-[:MARRIED]->(eskil) MATCH p = (a)-->(b)-->(c) WHERE a.name = 'Alice' AND b.name = 'Bob' AND c.name = 'Daniel' RETURN reduce(totalAge = 0, n IN nodes(p) | totalAge + n.age) AS reduction

### 4.4.6. relationships()

relationships() returns a list containing all the relationships in a path.

Syntax: relationships(path)

Returns:

 A list containing Relationship elements.

Arguments:

Name Description

path

An expression that returns a path.

Considerations:

 relationships(null) returns null.

Query.

MATCH p =(a)-->(b)-->(c)
WHERE a.name = 'Alice' AND c.name = 'Eskil'
RETURN relationships(p)

A list containing all the relationships in the path p is returned.

Table 4.49. Result
relationships(p)

1 row

[:KNOWS[0]{},:MARRIED[4]{}]

Try this query live.  CREATE (alice:Person:Developer {name:'Alice', age: 38, eyes: 'brown'}), (bob {name: 'Bob', age: 25, eyes: 'blue'}), (charlie {name: 'Charlie', age: 53, eyes: 'green'}), (daniel {name: 'Daniel', age: 54, eyes: 'brown'}), (eskil {name: 'Eskil', age: 41, eyes: 'blue', array: ['one', 'two', 'three']}), (alice)-[:KNOWS]->(bob), (alice)-[:KNOWS]->(charlie), (bob)-[:KNOWS]->(daniel), (charlie)-[:KNOWS]->(daniel), (bob)-[:MARRIED]->(eskil) MATCH p = (a)-->(b)-->(c) WHERE a.name = 'Alice' AND c.name = 'Eskil' RETURN relationships(p)

### 4.4.7. reverse()

reverse() returns a list in which the order of all elements in the original list have been reversed.

Syntax: reverse(original)

Returns:

 A list containing homogeneous or heterogeneous elements; the types of the elements are determined by the elements within original.

Arguments:

Name Description

original

An expression that returns a list.

Considerations:

 Any null element in original is preserved.

Query.

WITH [4923,'abc',521, NULL , 487] AS ids
RETURN reverse(ids)

Table 4.50. Result
reverse(ids)

1 row

[487,<null>,521,"abc",4923]

Try this query live.  CREATE (alice:Person:Developer {name:'Alice', age: 38, eyes: 'brown'}), (bob {name: 'Bob', age: 25, eyes: 'blue'}), (charlie {name: 'Charlie', age: 53, eyes: 'green'}), (daniel {name: 'Daniel', age: 54, eyes: 'brown'}), (eskil {name: 'Eskil', age: 41, eyes: 'blue', array: ['one', 'two', 'three']}), (alice)-[:KNOWS]->(bob), (alice)-[:KNOWS]->(charlie), (bob)-[:KNOWS]->(daniel), (charlie)-[:KNOWS]->(daniel), (bob)-[:MARRIED]->(eskil) WITH [4923,'abc',521, null, 487] AS ids RETURN reverse(ids)

### 4.4.8. tail()

tail() returns a list lresult containing all the elements, excluding the first one, from a list list.

Syntax: tail(list)

Returns:

 A list containing heterogeneous elements; the types of the elements are determined by the elements in list.

Arguments:

Name Description

list

An expression that returns a list.

Query.

MATCH (a)
WHERE a.name = 'Eskil'
RETURN a.array, tail(a.array)

The property named array and a list comprising all but the first element of the array property are returned.

Table 4.51. Result
a.array tail(a.array)

1 row

["one","two","three"]

["two","three"]

Try this query live.  CREATE (alice:Person:Developer {name:'Alice', age: 38, eyes: 'brown'}), (bob {name: 'Bob', age: 25, eyes: 'blue'}), (charlie {name: 'Charlie', age: 53, eyes: 'green'}), (daniel {name: 'Daniel', age: 54, eyes: 'brown'}), (eskil {name: 'Eskil', age: 41, eyes: 'blue', array: ['one', 'two', 'three']}), (alice)-[:KNOWS]->(bob), (alice)-[:KNOWS]->(charlie), (bob)-[:KNOWS]->(daniel), (charlie)-[:KNOWS]->(daniel), (bob)-[:MARRIED]->(eskil) MATCH (a) WHERE a.name = 'Eskil' RETURN a.array, tail(a.array)