Overview of Segment Scan Functions

Table of Contents

• Function Categories
  • Basic Scan Functions
  • Conditional Scan Functions
  • Low-Level Functions
• Common Parameters
• Usage Guidelines
• Related Pages

This page provides an overview of all scan (prefix-sum) functions available for segment operations, helping to understand the differences between variants and choose the right function for your needs.

Function Categories

Basic Scan Functions

These functions compute scan within specified segments:

Function	Segments Scanned	Scan Type
Segments_inclusiveScanAllSegments	All segments	Inclusive
Segments_exclusiveScanAllSegments	All segments	Exclusive
Segments_inclusiveScanSegments_range (range)	Segments [begin, end)	Inclusive
Segments_exclusiveScanSegments_range (range)	Segments [begin, end)	Exclusive
Segments_inclusiveScanSegments_with_segment_indices (array)	Segments in array	Inclusive
Segments_exclusiveScanSegments_with_segment_indices (array)	Segments in array	Exclusive

When to use:

• Use *AllSegments when you need to scan all segments
• Use range overload when scanning a contiguous range of segments
• Use array overload when you have a specific, non-contiguous set of segment indices

Conditional Scan Functions

These functions add segment-level conditions, including only segments that satisfy the condition in the scan:

Function	Segments Scanned	Scan Type
Segments_inclusiveScanAllSegmentsIf	All segments	Inclusive with segment filter
Segments_exclusiveScanAllSegmentsIf	All segments	Exclusive with segment filter
Segments_inclusiveScanSegmentsIf_range	Segments [begin, end)	Inclusive with segment filter
Segments_exclusiveScanSegmentsIf_range	Segments [begin, end)	Exclusive with segment filter

When to use:

• Use these when you want to skip certain elements within segments
• Example: Compute prefix-sum only for positive values, or only for elements meeting specific criteria

Low-Level Functions

Function	Purpose
Segments_inclusiveScanSegment	Computes inclusive scan for a single segment view
Segments_exclusiveScanSegment	Computes exclusive scan for a single segment view

When to use:

• Use these when you have a SegmentView object and need fine-grained control
• Useful when implementing custom segment algorithms

Common Parameters

All scan functions share these common parameters:

• segments: The segments container to scan
• fetch: Lambda that retrieves element values (see Fetch Lambda)
• reduce: Function object that combines values (see Reduction Function Object)
• write: Lambda that stores the scan results (see Write Lambda)
• launchConfig: Configuration for parallel execution (optional)

Conditional variants additionally require:

• condition: Lambda that determines if an element should be included (see Condition Lambda)

Usage Guidelines

In-place vs. out-of-place:

• All scan implementations support in-place operations
• Your write lambda can modify the same array that fetch reads from
• For out-of-place scans, write to a different array

Performance considerations:

• Scan operations are sequential within each segment
• Different segments are processed in parallel
• Use element conditions (*If variants) to skip unnecessary elements
• Choose appropriate reduction function objects for best performance

Related Pages

• Lambda Functions for Segment Scan Operations - Detailed lambda function signatures
• Function objects for reduction operations - Available reduction operations