WEBVTT

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Let's now do the second step for this activity.

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We already have a number publisher node.

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We are going to create a second node.

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So a second file where we have another node in which we are going to subscribe to this number topic.

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And then we are going to add every number that we receive to a counter.

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And we are going to publish this counter to a new topic.

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Okay.

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So now that we have the publisher working, we know that we should receive the correct data on the subscriber

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side.

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So let's create a new file.

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Let's go back to our.

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So package my pi pkg.

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And in this folder where we have all the files.

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And let's do a number counter dot pi which we also make executable number counter.

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And it's right there.

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I'm also going to remove the pi cache.

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No need for that.

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And let's start this node.

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That's going to be number counter.

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We are going to need the template.

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So let's put the template here and let's name it number counter node.

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Let's also modify it here.

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I'll remove that comment and the name of the node let's say also number counter.

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Okay.

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Let's keep things simple.

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And then what we are going to create a subscriber.

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So to create a subscriber we will need a name and a message type.

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What's the message or the interface type.

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Well it's very easy.

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It is going to be the exact same that we have on the publisher okay.

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Otherwise they won't be able to communicate with each other.

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So from example interfaces dot msg import int 64.

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And we've already added that inside the package dot XML.

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So no need to do anything else.

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Great.

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So I'm going to start with the subscriber.

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Okay.

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That's the first thing we need to do here.

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I'm going to create a subscriber here.

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Self dot.

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Let's name it simply number subscriber.

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Subscriber like that is equal to self dot create subscription.

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And what do we need to give first the message type which is going to be int 64.

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Then we have the topic name.

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What is the topic name.

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Well topic name you see here is number.

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So you write number exactly okay.

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If you have any typo here that's not going to work.

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And then we need to provide a callback okay.

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So I'm going to write a callback first here.

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Let's name it simply callback.

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So I start with callback.

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And then I'm going to put the name of the topic or something similar.

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So callback number we receive.

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So we have self and then we receive a message.

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And I'm just going to add this to specify that this message is an int 64.

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That's going to help with the auto completion.

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And well I'm first going to do a log just to make sure that this subscriber is working okay.

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If you have a doubt you can always do it step by step.

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So let's do self get logger info and we should be able to just publish the message like that message

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dot data.

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That should work.

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So then we can go back to our create subscription.

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Here we put self dot callback number.

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And then we have the queue size because it's a bit long I'm going to press enter here.

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All right.

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And we have our subscriber here.

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We have registered a callback which is that method.

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So when the node is spinning whenever we receive a message on the number topic, we are going to call

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this method.

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So let's try that.

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Actually, let's just try this very simple code.

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To try this, I'm going to go back to my setup.py and add a new executable named number counter is equal

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to my pi pkg dot file, name is number counter and the function name is main.

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Okay.

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And make sure you have a comma okay.

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Every time.

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Now that I have this let's go back.

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Let's go to our roster workspace and let's run the command.

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So it should be here call code build.

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We are going to do this package only and symlink install okay.

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It has been built.

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So right here I already have my publisher running okay.

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So you can study it again.

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Or you can just keep it running.

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And in that terminal I'm going to source.

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I'm going to source because I have just built the package.

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Okay.

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I have a new node, so I need to source it.

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And let's also source that one just in case we use it later.

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And let's do Ros to run my pi PG with number.

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And this time it's going to be number counter.

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Okay.

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So we make sure that we find the executable with the autocompletion.

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And then we make sure that it's working.

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And you see we have an error.

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And it seems the error is with the log.

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Yeah probably if I go back well probably I need to cast this as a string.

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Okay.

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Otherwise it doesn't really like it.

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So let's save and let's go back.

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I have used Simulink install and I have source this terminal, so I should be able to run it again.

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And this time it's working.

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Okay.

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We receive the number two every one second.

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You can verify with the log.

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That's every one second.

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So let's stop that.

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And now we know that this subscriber is working.

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So we have the publisher working.

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The subscriber is working.

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We don't really need this log anymore okay.

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I was just to test that.

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It's working.

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Now what we want to do is to add all the numbers that we get inside a counter, and then we want to

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publish that counter on a new topic.

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So let's first create a counter.

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That's quite easy.

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We do self dot counter is equal to zero here.

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And then we do self dot counter plus equal message dot data okay.

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We just add the data.

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So the number we receive to the counter which starts at zero as simple as that.

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And then we want to publish this counter.

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So let's create a new publisher for example here Let's do self dot.

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So we're going to publish what.

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A number count or number counter.

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Let's do number count.

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Publisher is equal to self.

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Create publisher.

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And we need to decide because that's a new topic.

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We need to decide on a message type and a name.

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What's going to be the message type.

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So I didn't give you the message type.

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But.

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Well if we were using Int64 to publish a number and then we add all those numbers inside the counter.

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So that's still that's still a number.

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That's the same data type.

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Okay.

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So it's still an integer.

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So we can use the same.

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We can use int 64.

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All right.

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Let's not make it complicated.

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If we have the same data type we use the same message.

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And then we need to decide on a name.

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So I'm going to name it number count.

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And we need a size ten.

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Let's go back to a new line here.

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All right.

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So in our constructor we have what we have a publisher and we have a subscriber.

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Now how are we going to publish to this topic.

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So that's going to be the most interesting part of this activity is that we are not going to publish

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from a timer, because what we've seen so far, for example, in the number publisher, is that you

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create a publisher and then how to publish to this topic.

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Well, we create a timer and we say every x amount of time we just publish something.

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But you can publish from anywhere, so you can publish from a timer callback, but you can also publish

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from a subscriber callback.

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And here it makes sense because we want to publish the counter when it's updated.

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And when do we update the counter?

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Well, we update it when we receive a new number.

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So in this subscriber callback we are going to create a new message and to publish it.

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So here I can do.

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So I'm not going to do MSG I'm not going to name it.

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Message like that because we already have.

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Message here.

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So I'm going to do.

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New message that's going to be an INT 64.

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Because that's what we have here as well.

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And then new message dot data is equal to self counter.

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And then we can just do self dot number count.

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Publisher dot publish with the new message.

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All right.

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So this way we only publish when we receive a message here with this subscriber callback.

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And this way it's kind of a data pipeline.

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Okay.

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You receive a message and as soon as you receive a message, you can process it.

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You can do what you can do different kind of stuff.

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And then you send another message on another topic.

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And here, well, I just write this code here in the callback number, I could have created another

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function named for example publish number counter so you can do as you want.

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Here I'm just going to keep it like this.

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And let's add a log self get logger info.

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Number counter has been studied.

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All right let's save the file.

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And that's about it for the code.

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So to recap when we create the node here we create a publisher and a subscriber.

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We register a callback with the subscriber.

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Okay.

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So when the node is spinning, whenever we receive a message on the number topic, we're going to go

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to this callback.

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And when we go to this callback, we add the number to the counter.

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And we publish this counter to the number count topic.

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So that's also a very common structure in robotics.

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Now we can try it.

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So no need to compile anything.

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We've used Simulink install I'm just going to stop the publisher And let's start the subscribe.

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So the number counter again and you see number counter has been started.

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We have nothing actually we don't print anything here.

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So we will need to check the topics.

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So let's go here.

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Let's do Ros two topic list.

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And you see that we have a number topic and a number count topic.

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If I do Ros two topic equal number.

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Well we have nothing because now what is publishing on this number topic.

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Okay.

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And if I do number count we also have nothing.

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Okay.

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Because nobody is publishing on that.

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All right we need to start the number publisher.

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Okay.

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So number publisher has been started.

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And then the number counter is receiving the messages and adding them to a counter and publishing to

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the counter.

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So let's just print again.

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What do we have on the number topic.

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So you see that's what's published here.

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Data two.

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And then let's do a Ross two topic equal with the number count.

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And well at this time you see the counter is 54 and 56.

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And then plus two every time.

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All right.

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You can see if I stop for example the number publisher, we don't receive any new number here.

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So we don't publish any new counter data okay.

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If I start the number publisher again we continue okay, 7678, etc..

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Now if I stop the number counter, you see, if I stop the number counter, well, we don't publish

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anything else on the number count topic.

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And if I start it again.

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Then it's going to receive messages from there.

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And you see we have two four.

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So this counter starts again at zero.

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All right.

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If you start this node again, you see, when we start the node again, we reset the counter to zero.

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Great.

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Let's.

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So the two are running.

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Now I'm going to run our graph here.

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And you can see that's exactly what we had on the image for the activity.

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So maybe well you can choose node topics all.

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Maybe you will have that.

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So you can uncheck that things okay.

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And maybe also leaf topics.

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And you see we have our number publisher node publishing to the number topic.

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The number counter node is subscribing to the number topic and publishing to the number count topic.

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So we have a data pipeline.

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And well that's the end of this activity.

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As a reminder, you can find the complete Cplusplus solution at the end of this section.

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So you can download the whole workspace, including all the Python and C plus plus code for this section.