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The series_fill_forward function fills missing values (nulls) in a numeric dynamic array (series) by propagating the first known value forward through the array. This function is useful for handling gaps in time series data where you want to use the earliest available value to fill subsequent missing data points. You can use series_fill_forward when you have time series data with missing values and want to fill gaps using the first observed value. This is particularly useful for backward-looking analysis, historical data reconstruction, or when the initial data point is the best estimate for missing later values. Typical applications include financial data analysis, sensor data processing, and performance monitoring where the baseline value is most relevant.

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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.
In Splunk SPL, filling missing values forward is typically done with fillnull or eval expressions using filldown logic. To fill arrays forward, you usually need to expand arrays and apply the transformation row by row. In APL, series_fill_forward provides a direct, efficient way to perform forward filling on dynamic arrays.
... | fillnull value=0 | streamstats window=5 current=f first(field) as filled_field
In SQL, filling missing values forward requires window functions with FIRST_VALUE() or custom logic using LAG() with specific window specifications. Most SQL implementations focus on forward filling as the default behavior. In APL, series_fill_forward simplifies this operation by directly handling forward propagation of values in arrays.
SELECT FIRST_VALUE(value) OVER (ORDER BY timestamp ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS filled_value
FROM measurements;

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Usage

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Syntax

series_fill_forward(array)

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Parameters

ParameterTypeDescription
arraydynamicA dynamic array of numeric values that may contain null values.

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Returns

A dynamic array where null values are replaced by the first non-null value encountered when traversing the array forward.

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Use case examples

  • Log analysis
  • OpenTelemetry traces
  • Security logs
In log analysis, you can use series_fill_forward to fill missing request duration data using the first available value, which is useful for maintaining baseline performance metrics.Query
['sample-http-logs']
| summarize durations = make_list(req_duration_ms) by id
| extend filled_durations = series_fill_forward(durations)
Run in PlaygroundOutput
iddurationsfilled_durations
u123[150, null, null, 200][150, 150, 150, 200]
u456[null, 100, null, 300][100, 100, 100, 300]
This query fills missing request durations with the first available value, useful for maintaining baseline performance metrics in analysis.
  • series_fill_backward: Fills missing values by propagating the last known value backward. Use when you want to use the most recent available value to fill gaps.
  • series_fill_const: Fills missing values with a constant value. Use when you want to replace nulls with a specific default value.
  • series_fill_linear: Fills missing values using linear interpolation. Use when you want smooth transitions between known values.
  • series_equals: Compares each element to a specified value. Use for identifying specific values after filling operations.
  • series_greater: Returns elements greater than a specified value. Use for threshold analysis after filling missing data.