how to read a text file and publish the data

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Follow the tutorial in Publishers and Subscribers to learn how to publish a message.

After that, follow these simple steps.

Step 1 : Read integers from file and store them in an array / vector

Step 2 : Advertise a publisher to a topic of your preference with message type std_msgs/Int64.msg

ros::Publisher pub = nh.advertise<std_msgs::Int64>("topic_name", 5);

Step 3 : Iterate through your integer array/vector and publish them.