能否给一段ROS机器人turtlesim画圆形轨迹的示例代码？

可以，以下是一段ROS机器人turtlesim画圆形轨迹的示例代码：

#include <ros/ros.h>
#include <geometry_msgs/Twist.h>
#include <math.h>

int main(int argc, char** argv)
{
    ros::init(argc, argv, "turtle_circle");
    ros::NodeHandle nh;
    ros::Publisher pub = nh.advertise<geometry_msgs::Twist>("/turtle1/cmd_vel", 100);
    ros::Rate rate(10);

    double radius = 2.;
    double speed = 1.;
    double angular_speed = speed / radius;
    double angle = .;
    double time = .;
    double x = .;
    double y = .;

    while (ros::ok())
    {
        geometry_msgs::Twist msg;
        msg.linear.x = speed;
        msg.angular.z = angular_speed;

        pub.publish(msg);

        time += .1;
        angle = speed * time / radius;
        x = radius * cos(angle);
        y = radius * sin(angle);

        if (angle >= 2 * M_PI)
        {
            break;
        }

        rate.sleep();
    }

    return ;
}