| Prev | Getting Started – Table of Contents | Next |

Part 3: Customizing Simulation Output

In Part 2 you created a node called square_wave_closed_system that used the standard output stream std::cout to produce the following output.

y = 0
y = 1
y = 0
y = 1
y = 0
y = 1
y = 0
y = 1
y = 0
y = 1
y = 0
y = 1

However the use of std::cout within a node is not recommended practice. One problem with std::cout is that it does not indicate the point in simulated time at which each output occurred. Let’s find a better way to extract information from the simulation.

Find the file square_wave_closed_system.h and delete the following line of code, which imports the standard output stream.

#include <iostream>

Now, find the line below…

    std::cout << "y = " << float64(phase) << std::endl;

…and replace it with the following and save.

    print((string_builder() << "y = " << float64(phase)).str());

Here the string_builder object is used to create a std::string value of the form "y = #". The print function outputs the string along with contextual information about the simulation. If you rebuild and run the first_simulation executable, you should see the following output.

0|0|$time:time_point()
0|0|top$print:y = 0
1|0|$time:time_point() + 7_s
1|0|top$print:y = 1
2|0|$time:time_point() + 10_s
2|0|top$print:y = 0
3|0|$time:time_point() + 17_s
3|0|top$print:y = 1
4|0|$time:time_point() + 20_s
4|0|top$print:y = 0
5|0|$time:time_point() + 27_s
5|0|top$print:y = 1
6|0|$time:time_point() + 30_s
6|0|top$print:y = 0
7|0|$time:time_point() + 37_s
7|0|top$print:y = 1
8|0|$time:time_point() + 40_s
8|0|top$print:y = 0
9|0|$time:time_point() + 47_s
9|0|top$print:y = 1
10|0|$time:time_point() + 50_s
10|0|top$print:y = 0
11|0|$time:time_point() + 57_s
11|0|top$print:y = 1

Notice that each row begins with two numbers. The first number associates each row with a particular point in simulated time. The rows that begin with 0, for example, are associated with the time at the beginning of the simulation. The rows that begin with 3 are associated with the time point at 17 seconds from the beginning. The second number is a counter of steps within each time point, starting from 0. In this example, there is only one step per time point, so the counter is always at 0.

We still see the printed text "y = 0" or "y = 1", but it is associated with a node named top. This is the name that is always given to the top-level node in a simulation. In this example, the top node is the only node, but in more complex examples it is useful to see which node produced each output.

Although using the print function is recommended over using std::cout, it is still not the best practice. Ideally, the signal level would be made available through a message output port, and operations outside of the node would determine how the values are presented. This will be covered in the next part of the tutorial.

Let’s explore one more output customization feature.

In the square_wave.h file, find the instruction…

sim.process_remaining_events();

…and immediately above it insert the following line and save.

sim.top.print_on_event();

Now when the first_simulation executable is rebuilt and run, the type of each event is printed. You can see that the initialization event occurs first, followed by a succession of planned events, followed by the finalization event. This extra information helps one locate the function responsible for each printed value.

0|0|$time:time_point()
0|0|top$initialization
0|1|top$planned
0|1|top$print:y = 0
1|0|$time:time_point() + 7_s
1|0|top$planned
1|0|top$print:y = 1
2|0|$time:time_point() + 10_s
2|0|top$planned
2|0|top$print:y = 0
3|0|$time:time_point() + 17_s
3|0|top$planned
3|0|top$print:y = 1
4|0|$time:time_point() + 20_s
4|0|top$planned
4|0|top$print:y = 0
5|0|$time:time_point() + 27_s
5|0|top$planned
5|0|top$print:y = 1
6|0|$time:time_point() + 30_s
6|0|top$planned
6|0|top$print:y = 0
7|0|$time:time_point() + 37_s
7|0|top$planned
7|0|top$print:y = 1
8|0|$time:time_point() + 40_s
8|0|top$planned
8|0|top$print:y = 0
9|0|$time:time_point() + 47_s
9|0|top$planned
9|0|top$print:y = 1
10|0|$time:time_point() + 50_s
10|0|top$planned
10|0|top$print:y = 0
11|0|$time:time_point() + 57_s
11|0|top$planned
11|0|top$print:y = 1
12|0|$time:time_point() + 60_s
12|0|top$finalization

The print and print_on_event features are just two of several mechanisms that SyDEVS provides for customizing simulation output. In Part 4, you’ll learn how to take a node and give it a set of ports, which opens up additional possibilities for communicating data.