# array_reverse



Use the `array_reverse` function in APL to reverse the order of elements in an array. This function is useful when you need to transform data where the sequence matters, such as reversing a list of events for chronological analysis or processing lists in descending order.

## Usage [#usage]

### Syntax [#syntax]

```kusto
array_reverse(array_expression)
```

### Parameters [#parameters]

* `array_expression`: The array you want to reverse. This array must be of a dynamic type.

### Returns [#returns]

Returns the input array with its elements in reverse order.

## Use case examples [#use-case-examples]

<Tabs>
  <Tab title="Log analysis">
    Use `array_reverse` to inspect the sequence of actions in log entries, reversing the order to understand the initial steps of a user's session.

    **Query**

    ```kusto
    ['sample-http-logs']
    | summarize paths = make_list(uri) by id
    | project id, reversed_paths = array_reverse(paths)
    ```

    [Run in Playground](https://play.axiom.co/axiom-play-qf1k/query?initForm=%7B%22apl%22%3A%22%5B'sample-http-logs'%5D%20%7C%20summarize%20paths%20%3D%20make_list\(uri\)%20by%20id%20%7C%20project%20id%2C%20reversed_paths%20%3D%20array_reverse\(paths\)%22%7D)

    **Output**

    | id    | reversed\_paths                      |
    | ----- | ------------------------------------ |
    | U1234 | \['/home', '/cart', '/product', '/'] |
    | U5678 | \['/login', '/search', '/']          |

    This example identifies a user’s navigation sequence in reverse, showing their entry point into the system.
  </Tab>

  <Tab title="OpenTelemetry traces">
    Use `array_reverse` to analyze trace data by reversing the sequence of span events for each trace, allowing you to trace back the sequence of service calls.

    **Query**

    ```kusto
    ['otel-demo-traces']
    | summarize spans = make_list(span_id) by trace_id
    | project trace_id, reversed_spans = array_reverse(spans)
    ```

    [Run in Playground](https://play.axiom.co/axiom-play-qf1k/query?initForm=%7B%22apl%22%3A%22%5B'otel-demo-traces'%5D%20%7C%20summarize%20spans%20%3D%20make_list\(span_id\)%20by%20trace_id%20%7C%20project%20trace_id%2C%20reversed_spans%20%3D%20array_reverse\(spans\)%22%7D)

    **Output**

    | trace\_id | reversed\_spans           |
    | --------- | ------------------------- |
    | T12345    | \['S4', 'S3', 'S2', 'S1'] |
    | T67890    | \['S7', 'S6', 'S5']       |

    This example reveals the order in which service calls were made in a trace, but in reverse, aiding in backtracking issues.
  </Tab>

  <Tab title="Security logs">
    Apply `array_reverse` to examine security events, like login attempts or permission checks, in reverse order to identify unusual access patterns or last actions.

    **Query**

    ```kusto
    ['sample-http-logs']
    | where status == '403'
    | summarize blocked_uris = make_list(uri) by id
    | project id, reversed_blocked_uris = array_reverse(blocked_uris)
    ```

    [Run in Playground](https://play.axiom.co/axiom-play-qf1k/query?initForm=%7B%22apl%22%3A%22%5B'sample-http-logs'%5D%20%7C%20where%20status%20%3D%3D%20'403'%20%7C%20summarize%20blocked_uris%20%3D%20make_list\(uri\)%20by%20id%20%7C%20project%20id%2C%20reversed_blocked_uris%20%3D%20array_reverse\(blocked_uris\)%22%7D)

    **Output**

    | id    | reversed\_blocked\_uris               |
    | ----- | ------------------------------------- |
    | U1234 | \['/admin', '/settings', '/login']    |
    | U5678 | \['/account', '/dashboard', '/login'] |

    This example helps identify the sequence of unauthorized access attempts by each user.
  </Tab>
</Tabs>

## List of related functions [#list-of-related-functions]

* [array\_length](/apl/scalar-functions/array-functions/array-length): Returns the number of elements in an array.
* [array\_shift\_right](/apl/scalar-functions/array-functions/array-shift-right): Shifts array elements to the right.
* [array\_shift\_left](/apl/scalar-functions/array-functions/array-shift-left): Shifts array elements one position to the left, moving the first element to the last position.

## Other query languages [#other-query-languages]

<LanguageComparisons>
  <Accordion title="Splunk SPL users">
    In Splunk, reversing an array isn’t a built-in function, so you typically manipulate the data manually or use workarounds. In APL, `array_reverse` simplifies this process by reversing the array directly.

    <CodeGroup>
      ```sql Splunk example
      # SPL doesn’t have a direct array_reverse equivalent.
      ```

      ```kusto APL equivalent
      let arr = dynamic([1, 2, 3, 4, 5]);
      print reversed_arr = array_reverse(arr)
      ```
    </CodeGroup>
  </Accordion>

  <Accordion title="ANSI SQL users">
    Standard ANSI SQL lacks an explicit function to reverse an array; you generally need to create a custom solution. APL’s `array_reverse` makes reversing an array straightforward.

    <CodeGroup>
      ```sql SQL example
      -- ANSI SQL lacks a built-in array reverse function.
      ```

      ```kusto APL equivalent
      let arr = dynamic([1, 2, 3, 4, 5]);
      print reversed_arr = array_reverse(arr)
      ```
    </CodeGroup>
  </Accordion>
</LanguageComparisons>