Global variables in MUMPS (M) are a fundamental concept that allows data to be stored persistently and accessed across different routines and sessions. Unlike local variables, which are temporary and exist only within the scope of a single routine or session, global variables are stored in the database and can be accessed by any routine at any time.

1. Global Variables vs. Local Variables:

   • Local Variables: Temporary, exist only within the current routine or session.
   • Global Variables: Persistent, stored in the database, accessible across different routines and sessions.

2. Naming Conventions:

   • Global variable names start with a caret (^), followed by the name of the variable.
   • Example: ^GlobalVar

3. Structure:

   • Global variables can be simple or structured as arrays.
   • Example of a simple global variable: ^GlobalVar = "Hello"
   • Example of a structured global variable: ^GlobalArray(1) = "First Element"

4. Persistence:

   • Data stored in global variables remains available even after the program or system is restarted.

Explanation:

• SET ^Greeting = "Hello, World!": This line sets the global variable ^Greeting to the string "Hello, World!".
• WRITE ^Greeting: This line retrieves and prints the value of the global variable ^Greeting.

Explanation:

• SET ^Names(1) = "Alice": Sets the first element of the global array ^Names to "Alice".
• SET ^Names(2) = "Bob": Sets the second element of the global array ^Names to "Bob".
• SET ^Names(3) = "Charlie": Sets the third element of the global array ^Names to "Charlie".
• WRITE ^Names(1), !: Retrieves and prints the first element of the global array.
• WRITE ^Names(2), !: Retrieves and prints the second element of the global array.
• WRITE ^Names(3), !: Retrieves and prints the third element of the global array.

Task:

1. Create a global variable named ^Course and set its value to "MUMPS Programming".
2. Retrieve and print the value of the global variable ^Course.

Solution:

Task:

1. Create a global array named ^Students and set the following values:
   • ^Students(1) = "John"
   • ^Students(2) = "Jane"
   • ^Students(3) = "Doe"
2. Retrieve and print the values of the global array ^Students.

Solution:

• Forgetting the Caret (^): Always remember to prefix global variable names with a caret (^). For example, ^GlobalVar instead of GlobalVar.
• Overwriting Data: Be cautious when setting values to global variables, as they persist and can be overwritten by other routines.
• Proper Naming: Use meaningful names for global variables to avoid confusion and ensure code readability.

In this section, we introduced the concept of global variables in MUMPS, highlighting their persistence and accessibility across different routines and sessions. We covered the basic syntax for creating and retrieving global variables, both simple and structured. By practicing with the provided examples and exercises, you should now have a solid understanding of how to work with global variables in MUMPS. In the next section, we will delve into storing and retrieving data using global variables, further expanding on their practical applications.