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‎modules/ROOT/pages/expressions/list-expressions.adoc‎

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@@ -32,105 +32,6 @@ CREATE (alice:Person {name:'Alice', age: 65, role: 'Project manager', skills: ['
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(daniel)-[:WORKS_FOR]->(eskil)
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----
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[[list-concatenation]]
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== List concatenation
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Cypher contains two list concatenation operators: `||` and `+`.
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`||` is xref:appendix/gql-conformance/index.adoc[GQL conformant], while `+` is not.
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.List concatenation using `||` and `+`
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[source, cypher]
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----
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RETURN [1,2] || [3,4] AS list1,
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[1,2] + [3,4] AS list2
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----
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.Result
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[role="queryresult",options="header,footer",cols="2*<m"]
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|===
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| list1 | list2
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| [1, 2, 3, 4] | [1, 2, 3, 4]
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2+d|Rows: 1
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|===
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.Concatenate two `LIST` properties
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[source, cypher]
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----
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MATCH (cecil:Person {name: 'Cecil'}), (cecilia:Person {name: 'Cecilia'})
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RETURN cecil.skills || cecilia.skills AS combinedSkills
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----
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.Result
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[role="queryresult",options="header,footer",cols="1*<m"]
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|===
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| combinedSkills
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| ["Java", "Python", "JavaScript", "TypeScript"]
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1+d|Rows: 1
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|===
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The `+` operator can add elements to the beginning or end of a `LIST` value.
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This is not possible using the `||` operator.
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.Add elements to the beginning and end of a `LIST`
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[source, cypher]
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----
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WITH [1, 2, 3, 4] AS list
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RETURN 0 + list AS newBeginning,
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list + 5 AS newEnd
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----
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.Result
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[role="queryresult",options="header,footer",cols="2*<m"]
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|===
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| newBeginning | newEnd
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| [0, 1, 2, 3, 4] | [1, 2, 3, 4, 5]
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2+d|Rows: 1
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|===
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.Add elements to the beginning of a `LIST` property
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[source, cypher]
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----
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MATCH (cecil:Person {name: 'Cecil'})
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SET cecil.skills = "Cypher" + cecil.skills
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RETURN cecil.skills AS skillsList
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----
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.Result
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[role="queryresult",options="header,footer",cols="1*<m"]
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|===
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| skillsList
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| ["Cypher", "Java", "Python"]
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1+d|Rows: 1
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|===
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If `NULL` is part of a concatenated `LIST`, `NULL` will be a part of the new `LIST`.
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.Concatenate `LIST` including `NULL`
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[source, cypher]
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----
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RETURN [1, 2] || [3, null] AS listWithNull
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----
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.Result
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[role="queryresult",options="header,footer",cols="1*<m"]
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|===
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| listWithNull
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| [1, 2, 3, NULL]
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1+d|Rows: 1
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|===
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For removing `NULL` values when concatenating `LIST` values, see xref:expressions/list-expressions.adoc#null-list-concatenation-list-comprehension[`NULL`, list concatenation, and list comprehension].
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[[list-element-access]]
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== List element access
@@ -202,6 +103,44 @@ RETURN nestedList[1] AS secondList
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1+d|Rows: 1
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|===
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.Access specific elements in a nested `LIST`
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[source, cypher]
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----
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WITH [[1, 2], [3, 4], [5, 6]] AS nestedList
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RETURN nestedList[1] AS secondList,
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nestedList[1][0] AS firstElementOfSecondList
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----
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.Result
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[role="queryresult",options="header,footer",cols="2*<m"]
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|===
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| secondList | firstElementOfSecondList
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| [3, 4] | 3
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2+d|Rows: 1
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|===
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The index in the list element access can be any expression, including a variable.
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.Access list elements using a variable as an index
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[source, cypher]
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----
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WITH [[1, 2], [3, 4], [5, 6]] AS nestedList, 2 AS listIndex
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RETURN nestedList[listIndex] AS thirdList,
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nestedList[listIndex][listIndex - 1] AS secondElementOfThirdList
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----
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.Result
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[role="queryresult",options="header,footer",cols="2*<m"]
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|===
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| thirdList | secondElementOfThirdList
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| [5, 6] | 6
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2+d|Rows: 1
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|===
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The xref:expressions/predicates/list-operators.adoc[`IN`] operator, which checks for `LIST` membership, can be used together with `[]` to test whether an element exists in a nested `LIST`.
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.Check for membership in a nested `LIST`
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1+d|Rows: 1
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|===
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Attempting to reference an element outside the bounds of the `LIST` will return `NULL`, as will attempting to access elements from an empty `LIST`.
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.Out-of-bounds and empty `LIST` access
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1+d|Rows: 1
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|===
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[[list-concatenation]]
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== List concatenation
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Cypher contains two list concatenation operators: `||` and `+`.
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`||` is xref:appendix/gql-conformance/index.adoc[GQL conformant], while `+` is not.
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.List concatenation using `||` and `+`
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[source, cypher]
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----
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RETURN [1,2] || [3,4] AS list1,
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[1,2] + [3,4] AS list2
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----
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.Result
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[role="queryresult",options="header,footer",cols="2*<m"]
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|===
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| list1 | list2
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| [1, 2, 3, 4] | [1, 2, 3, 4]
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2+d|Rows: 1
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|===
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.Concatenate two `LIST` properties
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[source, cypher]
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----
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MATCH (cecil:Person {name: 'Cecil'}), (cecilia:Person {name: 'Cecilia'})
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RETURN cecil.skills || cecilia.skills AS combinedSkills
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----
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.Result
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[role="queryresult",options="header,footer",cols="1*<m"]
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|===
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| combinedSkills
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| ["Java", "Python", "JavaScript", "TypeScript"]
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1+d|Rows: 1
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|===
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If `NULL` is part of a concatenated `LIST`, `NULL` will be a part of the new `LIST`.
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.Concatenate `LIST` including `NULL`
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[source, cypher]
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----
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RETURN [1, 2] || [3, null] AS listWithNull
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----
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.Result
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[role="queryresult",options="header,footer",cols="1*<m"]
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|===
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| listWithNull
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| [1, 2, 3, NULL]
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1+d|Rows: 1
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|===
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For removing `NULL` values when concatenating `LIST` values, see xref:expressions/list-expressions.adoc#null-list-concatenation-list-comprehension[`NULL`, list concatenation, and list comprehension].
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[[add-elements]]
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=== Add elements to a list
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The `+` operator can add elements to the beginning or end of a `LIST` value.
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This is not possible using the `||` operator.
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.Add elements to the beginning and end of a `LIST`
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[source, cypher]
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----
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WITH [1, 2, 3, 4] AS list
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RETURN 0 + list AS newBeginning,
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list + 5 AS newEnd
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----
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.Result
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[role="queryresult",options="header,footer",cols="2*<m"]
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|===
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| newBeginning | newEnd
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| [0, 1, 2, 3, 4] | [1, 2, 3, 4, 5]
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2+d|Rows: 1
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|===
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To insert a `LIST` value into a nested `LIST`, the added `LIST` must itself be nested.
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If the added `LIST` is not nested, its elements are treated as individual elements, whereas if it is nested, it maintains the `LIST` structure within the nested `LIST`.
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.Add `LIST` values to a nested `LIST`
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[source, cypher]
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----
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WITH [[1, 2], [3, 4]] AS nestedList
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RETURN nestedList + [5, 6] AS nonNestedAddition
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nestedList + [[5, 6]] AS nestedAddition
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----
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.Result
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[role="queryresult",options="header,footer",cols="2*<m"]
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|===
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| nonNestedAddition | nestedAddition
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| [[1, 2], [3, 4], 5, 6] | [[1, 2], [3, 4], [5, 6]]
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2+d|Rows: 1
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|===
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.Add elements to the beginning of a `LIST` property
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[source, cypher]
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----
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MATCH (cecil:Person {name: 'Cecil'})
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SET cecil.skills = "Cypher" + cecil.skills
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RETURN cecil.skills AS skillsList
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----
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.Result
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[role="queryresult",options="header,footer",cols="1*<m"]
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|===
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| skillsList
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| ["Cypher", "Java", "Python"]
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1+d|Rows: 1
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|===
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[[list-comprehension]]
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== List comprehension
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List comprehension is used to create new `LIST` values by iterating over existing `LIST` values and transforming the elements based on certain conditions or operations.
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This process effectively maps each element in the original `LIST` to a new value.
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The result is a new `LIST` that consists of the transformed elements.
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.Syntax
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[source, syntax]
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1+d|Rows: 1
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|===
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This below query iterates over the `skills` property of each Person node and creates a new `LIST` by xref:expressions/string-operators.adoc[concatenating the `STRING`] `" expert"` to each element in `skills`.
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The next example shows how to map a `LIST` using its indexes with a list comprehension.
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The xref:functions/list.adoc#functions-range[`range()`] function is used to generate the indexes from `0` to the last valid index of the `LIST`, and then each index is combined with its corresponding `LIST` value into a `STRING` value.
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The result is a `LIST` of `STRING` values formatted as `'index: value'`.
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.Map list elements using indexes
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[source, cypher]
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----
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WITH [1,2,3,4] AS list
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RETURN [listIndex IN range(0, size(list)-1) | toString(listIndex) || ': ' || toString(list[listIndex])] AS mappedListElements
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----
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.Result
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[role="queryresult",options="header,footer",cols="1*<m"]
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|===
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| mappedListElements
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| ["0: 1", "1: 2", "2: 3", "3: 4"]
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1+d|Rows: 1
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|===
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The below query iterates over the `skills` property of each `Person` node and creates a new `LIST` by xref:expressions/string-operators.adoc[concatenating the `STRING`] `" expert"` to each element in `skills`.
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.Modify `LIST` properties using list comprehension
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[source, cypher]
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RETURN [person IN collect(p) WHERE 'Python' IN person.skills | person.name] AS pythonExperts
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----
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.Result
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[role="queryresult",options="header,footer",cols="1*<m"]
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|===
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1+d|Rows: 1
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|===
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