What are global variables? How are these variables declared?

Global variables are variables declared outside of any function or block of code, making them accessible and modifiable from anywhere within the program.

Understanding Global Variables

• Scope: The primary characteristic of a global variable is its global scope. This means that after a global variable is declared, you can access and change its value from any part of your program, including within functions, loops, and other code blocks.
• Lifetime: Global variables exist for the entire duration of the program's execution. They are created when the program starts and destroyed when the program terminates.

Declaring Global Variables

Global variables are declared at the outermost level of your code, outside of any functions or other blocks. The syntax for declaration depends on the programming language. Here are some examples:

C/C++:

#include <iostream>

int global_variable = 10; // Declaration of a global integer variable

void myFunction() {
  global_variable = 20; // Modifying the global variable inside a function
  std::cout << "Inside myFunction: " << global_variable << std::endl;
}

int main() {
  std::cout << "Before myFunction: " << global_variable << std::endl;
  myFunction();
  std::cout << "After myFunction: " << global_variable << std::endl;
  return 0;
}

Python:

global_variable = 10  # Declaration of a global variable

def my_function():
  global global_variable # Explicitly declare intent to use the global variable
  global_variable = 20
  print("Inside my_function:", global_variable)

print("Before my_function:", global_variable)
my_function()
print("After my_function:", global_variable)

JavaScript:

var globalVariable = 10; // Declaration of a global variable

function myFunction() {
  globalVariable = 20; // Modifying the global variable
  console.log("Inside myFunction: " + globalVariable);
}

console.log("Before myFunction: " + globalVariable);
myFunction();
console.log("After myFunction: " + globalVariable);

Key Considerations:

• Naming Conventions: Use meaningful names for global variables to improve code readability.
• Avoid Overuse: Excessive use of global variables can lead to tightly coupled code, making it difficult to maintain and debug. Consider using alternative approaches like passing variables as arguments to functions or using object-oriented principles.
• Scope Resolution: If a local variable within a function has the same name as a global variable, the local variable takes precedence within that function. In languages like Python, you must explicitly declare that you want to use the global variable within the function using the global keyword.

Summary

Global variables, declared outside of functions or blocks, possess global scope and program-wide accessibility, enabling widespread data sharing but potentially complicating maintenance.