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* Replace underscores with hyphens * remove temporary code * fix style check * fix collapse
263 lines
9.7 KiB
Markdown
263 lines
9.7 KiB
Markdown
---
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toc_priority: 57
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toc_title: Higher-Order
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---
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# Higher-order Functions {#higher-order-functions}
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## `->` operator, lambda(params, expr) function {#operator-lambdaparams-expr-function}
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Allows describing a lambda function for passing to a higher-order function. The left side of the arrow has a formal parameter, which is any ID, or multiple formal parameters – any IDs in a tuple. The right side of the arrow has an expression that can use these formal parameters, as well as any table columns.
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Examples: `x -> 2 * x, str -> str != Referer.`
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Higher-order functions can only accept lambda functions as their functional argument.
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A lambda function that accepts multiple arguments can be passed to a higher-order function. In this case, the higher-order function is passed several arrays of identical length that these arguments will correspond to.
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For some functions, such as [arrayCount](#higher_order_functions-array-count) or [arraySum](#higher_order_functions-array-count), the first argument (the lambda function) can be omitted. In this case, identical mapping is assumed.
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A lambda function can’t be omitted for the following functions:
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- [arrayMap](#higher_order_functions-array-map)
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- [arrayFilter](#higher_order_functions-array-filter)
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- [arrayFill](#higher_order_functions-array-fill)
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- [arrayReverseFill](#higher_order_functions-array-reverse-fill)
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- [arraySplit](#higher_order_functions-array-split)
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- [arrayReverseSplit](#higher_order_functions-array-reverse-split)
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- [arrayFirst](#higher_order_functions-array-first)
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- [arrayFirstIndex](#higher_order_functions-array-first-index)
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### arrayMap(func, arr1, …) {#higher_order_functions-array-map}
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Returns an array obtained from the original application of the `func` function to each element in the `arr` array.
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Examples:
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``` sql
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SELECT arrayMap(x -> (x + 2), [1, 2, 3]) as res;
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```
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``` text
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┌─res─────┐
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│ [3,4,5] │
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└─────────┘
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```
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The following example shows how to create a tuple of elements from different arrays:
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``` sql
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SELECT arrayMap((x, y) -> (x, y), [1, 2, 3], [4, 5, 6]) AS res
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```
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``` text
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┌─res─────────────────┐
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│ [(1,4),(2,5),(3,6)] │
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└─────────────────────┘
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```
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Note that the first argument (lambda function) can’t be omitted in the `arrayMap` function.
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### arrayFilter(func, arr1, …) {#higher_order_functions-array-filter}
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Returns an array containing only the elements in `arr1` for which `func` returns something other than 0.
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Examples:
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``` sql
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SELECT arrayFilter(x -> x LIKE '%World%', ['Hello', 'abc World']) AS res
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```
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``` text
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┌─res───────────┐
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│ ['abc World'] │
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└───────────────┘
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```
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``` sql
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SELECT
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arrayFilter(
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(i, x) -> x LIKE '%World%',
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arrayEnumerate(arr),
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['Hello', 'abc World'] AS arr)
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AS res
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```
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``` text
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┌─res─┐
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│ [2] │
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└─────┘
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```
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Note that the first argument (lambda function) can’t be omitted in the `arrayFilter` function.
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### arrayFill(func, arr1, …) {#higher_order_functions-array-fill}
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Scan through `arr1` from the first element to the last element and replace `arr1[i]` by `arr1[i - 1]` if `func` returns 0. The first element of `arr1` will not be replaced.
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Examples:
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``` sql
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SELECT arrayFill(x -> not isNull(x), [1, null, 3, 11, 12, null, null, 5, 6, 14, null, null]) AS res
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```
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``` text
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┌─res──────────────────────────────┐
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│ [1,1,3,11,12,12,12,5,6,14,14,14] │
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└──────────────────────────────────┘
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```
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Note that the first argument (lambda function) can’t be omitted in the `arrayFill` function.
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### arrayReverseFill(func, arr1, …) {#higher_order_functions-array-reverse-fill}
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Scan through `arr1` from the last element to the first element and replace `arr1[i]` by `arr1[i + 1]` if `func` returns 0. The last element of `arr1` will not be replaced.
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Examples:
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``` sql
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SELECT arrayReverseFill(x -> not isNull(x), [1, null, 3, 11, 12, null, null, 5, 6, 14, null, null]) AS res
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```
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``` text
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┌─res────────────────────────────────┐
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│ [1,3,3,11,12,5,5,5,6,14,NULL,NULL] │
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└────────────────────────────────────┘
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```
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Note that the first argument (lambda function) can’t be omitted in the `arrayReverseFill` function.
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### arraySplit(func, arr1, …) {#higher_order_functions-array-split}
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Split `arr1` into multiple arrays. When `func` returns something other than 0, the array will be split on the left hand side of the element. The array will not be split before the first element.
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Examples:
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``` sql
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SELECT arraySplit((x, y) -> y, [1, 2, 3, 4, 5], [1, 0, 0, 1, 0]) AS res
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```
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``` text
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┌─res─────────────┐
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│ [[1,2,3],[4,5]] │
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└─────────────────┘
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```
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Note that the first argument (lambda function) can’t be omitted in the `arraySplit` function.
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### arrayReverseSplit(func, arr1, …) {#higher_order_functions-array-reverse-split}
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Split `arr1` into multiple arrays. When `func` returns something other than 0, the array will be split on the right hand side of the element. The array will not be split after the last element.
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Examples:
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``` sql
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SELECT arrayReverseSplit((x, y) -> y, [1, 2, 3, 4, 5], [1, 0, 0, 1, 0]) AS res
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```
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``` text
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┌─res───────────────┐
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│ [[1],[2,3,4],[5]] │
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└───────────────────┘
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```
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Note that the first argument (lambda function) can’t be omitted in the `arraySplit` function.
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### arrayCount(\[func,\] arr1, …) {#higher_order_functions-array-count}
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Returns the number of elements in the arr array for which func returns something other than 0. If ‘func’ is not specified, it returns the number of non-zero elements in the array.
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### arrayExists(\[func,\] arr1, …) {#arrayexistsfunc-arr1}
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Returns 1 if there is at least one element in ‘arr’ for which ‘func’ returns something other than 0. Otherwise, it returns 0.
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### arrayAll(\[func,\] arr1, …) {#arrayallfunc-arr1}
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Returns 1 if ‘func’ returns something other than 0 for all the elements in ‘arr’. Otherwise, it returns 0.
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### arraySum(\[func,\] arr1, …) {#higher-order-functions-array-sum}
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Returns the sum of the ‘func’ values. If the function is omitted, it just returns the sum of the array elements.
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### arrayFirst(func, arr1, …) {#higher_order_functions-array-first}
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Returns the first element in the ‘arr1’ array for which ‘func’ returns something other than 0.
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Note that the first argument (lambda function) can’t be omitted in the `arrayFirst` function.
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### arrayFirstIndex(func, arr1, …) {#higher_order_functions-array-first-index}
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Returns the index of the first element in the ‘arr1’ array for which ‘func’ returns something other than 0.
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Note that the first argument (lambda function) can’t be omitted in the `arrayFirstIndex` function.
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### arrayCumSum(\[func,\] arr1, …) {#arraycumsumfunc-arr1}
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Returns an array of partial sums of elements in the source array (a running sum). If the `func` function is specified, then the values of the array elements are converted by this function before summing.
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Example:
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``` sql
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SELECT arrayCumSum([1, 1, 1, 1]) AS res
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```
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``` text
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┌─res──────────┐
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│ [1, 2, 3, 4] │
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└──────────────┘
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```
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### arrayCumSumNonNegative(arr) {#arraycumsumnonnegativearr}
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Same as `arrayCumSum`, returns an array of partial sums of elements in the source array (a running sum). Different `arrayCumSum`, when then returned value contains a value less than zero, the value is replace with zero and the subsequent calculation is performed with zero parameters. For example:
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``` sql
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SELECT arrayCumSumNonNegative([1, 1, -4, 1]) AS res
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```
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``` text
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┌─res───────┐
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│ [1,2,0,1] │
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└───────────┘
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```
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### arraySort(\[func,\] arr1, …) {#arraysortfunc-arr1}
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Returns an array as result of sorting the elements of `arr1` in ascending order. If the `func` function is specified, sorting order is determined by the result of the function `func` applied to the elements of array (arrays)
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The [Schwartzian transform](https://en.wikipedia.org/wiki/Schwartzian_transform) is used to improve sorting efficiency.
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Example:
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``` sql
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SELECT arraySort((x, y) -> y, ['hello', 'world'], [2, 1]);
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```
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``` text
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┌─res────────────────┐
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│ ['world', 'hello'] │
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└────────────────────┘
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```
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For more information about the `arraySort` method, see the [Functions for Working With Arrays](array-functions.md#array_functions-sort) section.
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### arrayReverseSort(\[func,\] arr1, …) {#arrayreversesortfunc-arr1}
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Returns an array as result of sorting the elements of `arr1` in descending order. If the `func` function is specified, sorting order is determined by the result of the function `func` applied to the elements of array (arrays).
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Example:
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``` sql
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SELECT arrayReverseSort((x, y) -> y, ['hello', 'world'], [2, 1]) as res;
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```
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``` text
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┌─res───────────────┐
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│ ['hello','world'] │
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└───────────────────┘
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```
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For more information about the `arrayReverseSort` method, see the [Functions for Working With Arrays](array-functions.md#array_functions-reverse-sort) section.
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[Original article](https://clickhouse.tech/docs/en/query_language/functions/higher_order_functions/) <!--hide-->
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