select

R2026b

Select subset of messages in ros2bagreader

Syntax

bagsel = select(bag)

bagsel = select(bag,Name=Value)

Description

bagsel = select(bag) returns a ros2bagreader object, bagsel, that contains all messages in the ros2bagreader object, bag.

This function creates a copy of the ros2bagreader object or returns a new ros2bagreader object that contains the specified message selection.

bagsel = select(bag,Name=Value) provides additional options specified by one or more name-value arguments. For example, Topic="/scan" selects a subset of the messages filtered by the topic /scan.

The select function does not wait for the full message list when filtering by Topic or TopicType. When filtering by Time, the function waits until message indexing completes for the queried time range before returning results.

Examples

Read Messages from ROS 2 Bag Log File

Extract the ZIP file that contains the ROS 2 bag log file and specify the full path to the log folder.

unzip("ros2_netwrk_bag.zip");

folderPath = fullfile(pwd,"ros2_netwrk_bag");

Create a ros2bagreader object that contains all the messages in the log file.

bagReader = ros2bagreader(folderPath);

Get information about the contents of the ros2bagreader object.

baginfo = ros2("bag","info",folderPath)

baginfo = struct with fields:
                 Path: 'C:\Users\user\OneDrive - MathWorks\Documents\MATLAB\ExampleManager\user.Bdoc26b\ros-ex96596996\ros2_netwrk_bag'
      FileInformation: "ros2_netwrk_bag.db3"
              Version: '1'
            StorageId: 'sqlite3'
             Duration: 207.9020
                Start: [1×1 struct]
                  End: [1×1 struct]
                 Size: 16839538
             Messages: 166867
                Types: [4×1 struct]
               Topics: [4×1 struct]
    CompressionFormat: 'none'
      CompressionMode: 'none'

Get all the messages in the ros2bagreader object.

msgs = readMessages(bagReader);

Select a subset of the messages, filtered by topic.

bagSel = select(bagReader,"Topic","/odom")

bagSel = 
  ros2bagreader with properties:

           FilePath: 'C:\Users\user\OneDrive - MathWorks\Documents\MATLAB\ExampleManager\user.Bdoc26b\ros-ex96596996\ros2_netwrk_bag'
      AvailableBags: "ros2_netwrk_bag.db3"
          StartTime: 1.6020e+09
            EndTime: <preparing>
    AvailableTopics: [1×3 table]
    AvailableFrames: {0×1 cell}
        MessageList: <preparing>
        NumMessages: 5275

Get the messages in the selection.

msgsFiltered = readMessages(bagSel);

Input Arguments

bag — Messages in ros2bagreader object

ros2bagreader object

Messages in the ros2bagreader object, specified as a ros2bagreader object.

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Before R2021a, use commas to separate each name and value, and enclose Name in quotes.

Example: select(bag,Topic="/scan") selects a subset of the messages, filtered by the topic /scan.

MessageType — ROS 2 message type

string scalar | character vector | cell array of string scalars | cell array of character vectors

ROS 2 message type, specified as a string scalar, character vector, cell array of string scalars, or cell array of character vectors. Multiple message types can be specified with a cell array.

Example: select(bag,MessageType={"sensor_msgs/CameraInfo","sensor_msgs/LaserScan"})

Data Types: char | string | cell

Time — Start and end times of ROS 2 bag selection

n-by-2 vector

Start and end times of the ROS 2 bag selection, specified as an n-by-2 vector.

Example: select(bag,Time=[bag.MessageList(1,1).Time,bag.MessageList(2,1).Time])

Data Types: double

Topic — ROS 2 topic name

string scalar | character vector | cell array of string scalars | cell array of character vectors

ROS 2 topic name, specified as a string scalar, character vector, cell array of string scalars, or cell array of character vectors. Multiple topic names can be specified with a cell array.

Example: select(bag,Topic={"/scan","/clock"})

Data Types: char | string | cell

Output Arguments

bagsel — Copy or subset of ROS 2 bag messages

ros2bagreader object

Copy or subset of ROS 2 bag messages, returned as a ros2bagreader object.

Version History

Introduced in R2021a

R2026b: Filter by topic without waiting for full message list

The select function does not wait for the full message list when filtering by Topic or TopicType. When filtering by Time, the function waits until message indexing completes for the queried time range before returning results.