int globalVariable; // default initial value is 0

void setup() {
  Serial.begin(9600);

  // Output the initial values of the global, static, and local variables
  Serial.print("Global Variable: ");
  Serial.println(globalVariable);  // Output: 0

  static int staticVariable;  // default initial value is 0
  Serial.print("Static Variable: ");
  Serial.println(staticVariable);  // Output: 0

  int localVariable;  // undefined initial value
  Serial.print("Local Variable: ");
  Serial.println(localVariable);  // Output: undefined (garbage value)

  register int registerVariable;  // undefined initial value
  Serial.print("Register Variable: ");
  Serial.println(registerVariable);  // Output: undefined (garbage value)
}

void loop() {
  // Your code here
}

/*In this program:

globalVariable and staticVariable are initialized to zero by default.
localVariable and registerVariable have undefined initial values because 
they are automatic storage class variables, so their initial values are 
whatever happens to be in the memory locations assigned to them.
----------------------------------------------------------------------------------------*/

/*The observed behavior could be due to a couple of factors. The initialization of variables to 0 
might not be guaranteed by the language specification, but could occur due to specific compiler behavior,
or the runtime environment of the Arduino platform. It's important to note that relying on such behavior 
can lead to non-portable code.

In a strict sense, according to the C/C++ standards, local variables of automatic storage duration
are not initialized to any value by default and will contain indeterminate values until they are 
explicitly initialized. However, some compilers or runtime environments may choose to initialize 
these variables to zero for safety or debugging purposes.

Moreover, the Arduino environment might have certain initializations in its startup code that 
clear memory before transferring control to your setup and loop functions, which could explain 
the observed behavior.

To demonstrate the proper usage and to ensure portable, reliable behavior, it's a good practice to always
 initialize your variables before use. Here's how you could modify your program to follow this practice:

int globalVariable;  // default initial value is 0

void setup() {
  Serial.begin(9600);

  // Output the initial values of the global, static, and local variables
  Serial.print("Global Variable: ");
  Serial.println(globalVariable);  // Output: 0

  static int staticVariable;  // default initial value is 0
  Serial.print("Static Variable: ");
  Serial.println(staticVariable);  // Output: 0

  int localVariable = 0;  // explicitly initialized
  Serial.print("Local Variable: ");
  Serial.println(localVariable);  // Output: 0

  register int registerVariable = 0;  // explicitly initialized
  Serial.print("Register Variable: ");
  Serial.println(registerVariable);  // Output: 0
}

void loop() {
  // Your code here
}

*/