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Version: 8

Vectors in Behavior Trees

This how-to guide explains how to create, modify, and manipulate vectors (dynamic arrays) in MoveIt Pro Behavior Trees using the built-in vector manipulation Behaviors. This is just an overview, for more details see the Doxygen docs.

Overview​

Vectors are dynamic arrays that can store multiple elements of the same type. MoveIt Pro provides several Behaviors for working with vectors, allowing you to create empty vectors, add/remove elements, access specific elements, and query vector properties. This is particularly useful for managing collections of poses, waypoints, or other data structures in your robotic applications.

All of these vector operations use the BT::Any type from BehaviorTree.cpp.

Key Behaviors​

ResetVector​

This Behavior creates an empty vector and stores it in the blackboard for use by other Behaviors.

Important Note

ResetVector is currently the required approach for creating new vectors.

GetSizeOfVector​

Use this Behavior to get the number of elements in a vector.

GetElementOfVector​

Retrieves a specific element from a vector using its index. Supports negative indexing (e.g., -1 for the last element), similarly to Python.

Negative Indexing:

  • -1 = last element
  • -2 = second-to-last element
  • -3 = third-to-last element
  • And so on...

InsertInVector​

Inserts a new element into a vector at the specified index. All elements after this index are shifted to make room for the new element.

Default Element Value

When configuring the element parameter, the default value is {json:{"__type":"string","value":"{element}"} which is intentionally designed to allow for BT::Any types. A better approach is in development.

Type Safety

The element you insert must be the same type as the existing elements in the vector. If the vector is empty, any type can be inserted to establish the vector's type.

PushBackVector​

Inserts a new element into a vector at the end of the vector, similar to C++'s push_back.

Type Safety

The element you push must be the same type as the existing elements in the vector. If the vector is empty, any type can be pushed to establish the vector's type.

ReplaceInVector​

Replaces an existing element in a vector at the specified index with a new element.

RemoveFromVector​

Removes an element from a vector at the specified index. All elements after the removed element are shifted down to fill the gap.

Practical Example​

For a complete example showing how to work with vectors in a Behavior Tree, check out the Vector and String Example inside of the lab_sim robot config. This example demonstrates creating a vector, adding poses, accessing elements, and modifying the vector.

Error Handling​

The vector manipulation Behaviors will return FAILURE status in these cases:

  • Index out of bounds - Accessing an index that doesn't exist
  • Type mismatch - Trying to insert/replace with incompatible element type
  • Empty vector access - Trying to access elements in an empty vector
  • Invalid input - Missing required input ports