series_log

This page explains how to use the series_log function in APL.

The series_log function computes the natural logarithm (base e) of each element in a numeric dynamic array (series). This performs element-wise logarithmic transformation across the entire series.

You can use series_log when you need to apply logarithmic transformations to time-series data. This is particularly useful for normalizing exponentially distributed data, linearizing exponential growth patterns, compressing wide value ranges, or preparing data for analysis that assumes log-normal distributions.

For users of other query languages

If you come from other query languages, this section explains how to adjust your existing queries to achieve the same results in APL.

Splunk SPL users

In Splunk SPL, you typically use the log() function within an eval command to calculate logarithms. In APL, series_log applies the logarithm operation to every element in an array simultaneously.

Splunk example

... | eval log_value=log(value)

ANSI SQL users

In SQL, you use the LOG() or LN() function to calculate natural logarithms on individual rows. In APL, series_log operates on entire arrays, applying the logarithm operation element-wise.

SQL example

SELECT LN(value) AS log_value
FROM measurements;

Usage

Syntax

series_log(array)

Parameters

Parameter	Type	Description
array	dynamic	A dynamic array of numeric values. Values must be positive.

Returns

A dynamic array where each element is the natural logarithm of the corresponding input element. Returns null for non-positive values.

Use case examples

Log analysis

In log analysis, you can use series_log to normalize request durations that follow an exponential distribution, making patterns easier to visualize and analyze.

Query

['sample-http-logs']
| summarize durations = make_list(req_duration_ms) by id
| extend log_durations = series_log(durations)
| take 5

Run in Playground

Output

id	durations	log_durations
u123	[50, 100, 500, 1000]	[3.91, 4.61, 6.21, 6.91]
u456	[25, 75, 200, 800]	[3.22, 4.32, 5.30, 6.68]

This query applies logarithmic transformation to request durations, compressing the range and making it easier to compare values across different scales.

List of related functions

• series_pow: Raises series elements to a power. Use as the inverse operation to logarithms when working with exponentials.
• series_abs: Returns the absolute value of each element. Use before series_log to ensure positive values.
• series_magnitude: Computes the magnitude of a series. Use when you need Euclidean norm instead of logarithmic transformation.
• log: Scalar function for single values. Use for individual calculations instead of array operations.