What is the difference between global and local variables in programming?

Global variables are accessible throughout the entire program, while local variables are only accessible within the function or block they are declared in.

In more detail, a global variable is a variable that is declared outside all functions or blocks. Because of its scope, it can be accessed and modified by any part of the code, making it globally available. This can be useful in certain situations, such as when multiple functions need to access the same data. However, it also means that any part of the program can change the variable, which can lead to unexpected behaviour and bugs that are difficult to track down.

On the other hand, a local variable is a variable that is declared within a function or a block. Its scope is limited to that function or block, meaning it can't be accessed or modified by code outside of that function or block. This can help to prevent bugs, as it ensures that the variable can only be changed in a controlled manner. However, it also means that the variable can't be accessed by other parts of the program, which can be limiting in some situations.

In general, it's considered good practice to use local variables as much as possible, and only use global variables when necessary. This is because local variables help to keep code modular and easy to understand, as each function or block of code has its own set of variables that it works with. Understanding how these variables interact with standard algorithms is crucial. For a deeper understanding of how different programming constructs operate with variables, see Introduction to Fundamental Programming Constructs.

Additionally, defining procedures effectively often involves making decisions about using local or global variables, which are crucial for maintaining the scope and functionality of the code. More on this can be found in the section Defining and Using Procedures.

Global variables, on the other hand, can make code more difficult to understand and debug, as they can be changed by any part of the program. It's also important to understand the broader implications of variable usage in programming through practical examples in Understanding and Applying Standard Algorithms.

A-Level Computer Science Tutor Summary: In programming, global variables can be used anywhere in the program, making them convenient but risky as they can cause unexpected issues. Local variables are safer, restricted to the function or block where they're declared, which reduces errors. It's usually better to stick with local variables for clearer and more secure code, saving global variables for when you really need them.