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On the arduino side, array messages have an extra variable added to their class definitions used to define their length. In our case, this means that the Float32MultiArray msg has a data and data_length field; in order to set the data field we first need to set the data_length field:

#include <ros.h>
#include <std_msgs/Float32MultiArray.h>

int array_length = 3;
float values[array_length] = {1.0, 2.0, 3.0};

ros::NodeHandle nh;
std_msgs::Float32MultiArray array_msg;
ros::Publisher pub("array_publisher", &array_msg);

void setup() {
  nh.initNode();
  nh.advertise(pub);
}

void loop(){
  array_msg.data_length = array_length;
  array_msg.data = values;
  pub.publish(&array_msg);
  nh.spinOnce();
  delay(1000);
}

For more information, see the array section of the rosserial limitations page.