I suspect you're opening the file and clearing it for each entry. Try opening it with mode "a" append instead of mode "w" write. You should have a close in there too when you're finished with it.

However. Writing the values to a topic as described in the tutorials would be recommended. Then you could use the provided tools such as the plot plugin of the rqt_gui to plot the change in height over time. Or RVIS to show the pose in real time.

Using 'a' doesn't work properly. It only saves a few samples (like 15000, but I need much more). For example, my drone starts at 2.15 metres and ends at 0 metres and it only saves from 2.15 to 2.14 :S. What can I do? Which are the tutorials to write a lot of samples in a topic?

What can I do?

Not opening and closing a file at a high rate. That is not what that infrastructure was design for.

If you must use a file, open it once, keep the file handle around and use it to write new data to it whenever you need to.

Even more reason to use a topic instead, which is what they're designed for.

How can I paste a lot of floats in a topic? I mean, how would the topic be defined? Imagine that a new value comes each 0.01 seconds, how can I append that value to the topic?

Please read the introduction to topics page, everything you're asking is answered there.

I already read that, but I'm asking because I couldn't find the solution to my problem

I guess the procedure would be:

• init your node
• create a NodeHandle object
• advertise(..) the float64 topic
• publish(..) msgs whenever appropriate

you don't "append" to a topic. It's not a file.

Can you please give me a link to a tutorial or something where that is explained?

I hate to repeat what was already said, but the basic ROS tutorials show you exactly this.

I searched the tutorials where that was explained and couldn’t find it. That’s why I’m asking here. Can you please tell me the exact link or tutorial?

I assume @PeteBlackerThe3rd is referring to: Writing a Simple Publisher and Subscriber (Python).

But it is not explained there! I read that tutorial more than 5 times

Perhaps you can be a bit more clear about what "is not explained there". What are you exactly looking for?

I need to save a variable quantity of floats in a topic. Floats are the distance between a drone and the hround, and it is updated 10 times per second, approximately

Topics do not 'save' data they communicate it to any listening nodes. A std_msgs/Float64 sends single 64 bit float values, it can send many of these individual values per second the rate is not fixed and will depend on the publisher.

But if I send multiple floats, are all going to be ‘saved’ (I don’t know how to say it better) or just the last one? I need to have them all, like when using rosbag

The topic itself will not 'save' anything, rosbag can be used to record topics and bandwidth allowing would record every float you publish if you start rosbag record before your node.