Скачать или смотреть Understanding the undefined Terminal Size in Gnuplot: Why It Happens and Its Solution

Discover why Gnuplot returns `undefined variables` for terminal sizes after startup and learn how to resolve this issue effectively.
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Why is Terminal Size Undefined after Gnuplot Startup?

Gnuplot is a powerful tool for drawing plots and visualizations from the command line. Often, users encounter puzzling behaviors that can confuse even experienced users. One such issue arises when, after starting Gnuplot, you try to access terminal size variables, specifically GPVAL_TERM_XSIZE and GPVAL_TERM_YSIZE, only to be met with an error stating "undefined variable." Let’s dive into the details of why this occurs and how to rectify it.

The Issue at Hand

When Gnuplot starts, a default terminal is indeed set (for example, wxt), and default sizes (like 640,384) are configured. However, if you attempt to retrieve the terminal size immediately using the commands:

[[See Video to Reveal this Text or Code Snippet]]

you will receive an error message:

[[See Video to Reveal this Text or Code Snippet]]

What Causes This Problem?

Terminal Initialization: At startup, Gnuplot prints the name of the current terminal to the console. However, the actual terminal does not open until the first plot command is executed. This delayed initialization means that the relevant GPVAL_TERM_* variables, which are dependent on plot commands, remain undefined before any plotting occurs.

Dependency on Plot Commands: The GPVAL_TERM_* variables are designed to reflect the state of the terminal after the first plot command. They are updated only after executing a plot, which means until then, they cannot be accessed without returning an error.

Attempting to Specify a Size

Trying to set the terminal size before plotting, such as:

[[See Video to Reveal this Text or Code Snippet]]

will not solve the issue. You will still encounter the same error because the terminal had not yet been initialized.

The Solution: Plotting to Initialize Variables

To resolve this issue, the simplest and most straightforward solution is to perform a dummy plot. This means you can issue any basic plot command (even if it involves no actual data) to initialize the terminal state:

[[See Video to Reveal this Text or Code Snippet]]

This command uses NaN (Not a Number) to create an empty plot, effectively kicking off the terminal setup and allowing Gnuplot to populate the GPVAL_TERM_* variables. After executing this, GPVAL_TERM_XSIZE and GPVAL_TERM_YSIZE will be defined and can be accessed without error.

Conclusion

Understanding how Gnuplot initializes terminal variables is essential for troubleshooting issues like undefined variables. Remember, the takeaway is that these variables only come to life after the first plotting command. If you ever find yourself facing similar issues, a simple dummy plot can unlock the door to Gnuplot’s powerful visualization capabilities.

By following these steps, you can ensure smoother operations when working with Gnuplot, eliminating confusion over undefined variables and enhancing your plotting efficiency.