Which Of The Following Is Not A Legal Identifier

Which of the following is not a legal identifier? A Deep Dive into Programming Language Rules

Understanding legal identifiers is fundamental to programming. A legal identifier is a name used to identify a variable, function, class, or other program element. Different programming languages have specific rules governing what constitutes a valid identifier. This article will explore these rules, focusing on common pitfalls and providing examples to help you confidently identify illegal identifiers.

What Makes an Identifier "Legal"?

Before diving into examples of illegal identifiers, let's establish the general principles that define a legal identifier in most programming languages. These principles often overlap but can have subtle variations:

• Starting Characters: Identifiers typically must begin with a letter (uppercase or lowercase) or an underscore (_). Numbers are generally not allowed as the first character. This prevents ambiguity and improves readability.

• Subsequent Characters: After the initial character, identifiers can include letters (uppercase or lowercase), numbers (0-9), and underscores (_). Most languages prohibit spaces or other special characters (e.g., $, %, &, *).

• Case Sensitivity: Many languages (like C++, Java, Python, and JavaScript) are case-sensitive. This means myVariable and myvariable are considered distinct identifiers. Other languages (like Pascal) are not case-sensitive.

• Keywords: Reserved words (keywords) used by the programming language itself (e.g., if, else, for, while, int, float, class, etc.) cannot be used as identifiers. These keywords have special meanings within the language's syntax and using them as identifiers would lead to compilation or runtime errors.

• Length Restrictions: While many languages don't explicitly limit identifier length, there might be practical limits imposed by the compiler or interpreter. Extremely long identifiers can be cumbersome and reduce readability.

• Special Characters: As previously mentioned, the use of special characters beyond the underscore is generally restricted. Characters such as !, @, #, $, %, ^, &, *, (, ), -, +, =, [, ], {, }, \, |, ;, :, <, >, ?, /, and . are typically not allowed within identifiers.

Common Mistakes Leading to Illegal Identifiers

Let's examine some common scenarios that often result in illegal identifiers, highlighting the specific rules being violated:

1. Starting with a Number

1stVariable; // Illegal in most languages

This is illegal because identifiers usually cannot begin with a number. The first character must be a letter or an underscore.

2. Including Spaces

my variable; // Illegal

Spaces are not permitted within identifiers. If you need to represent multiple words, use camel case (e.g., myVariable) or underscores (e.g., my_variable).

3. Using Special Characters

my$variable;  // Illegal in most languages
my-variable; // Often illegal
my.variable; // Often illegal - might be valid in certain contexts (like member access)

Special characters, with the notable exception of the underscore, are usually forbidden.

4. Using Keywords

int int = 10; // Illegal; 'int' is a keyword

Keywords are reserved words with special meaning in the language. Using them as identifiers leads to syntax errors.

5. Case Sensitivity Issues (in case-sensitive languages)

let myVar = 10;
let myvar = 20; // This is a different variable in JavaScript

In case-sensitive languages, myVar and myvar are distinct identifiers.

6. Overly Long Identifiers

While there's no strict limit in many languages, extremely long identifiers can make code less readable and more prone to errors. Avoid excessively long names.

Language-Specific Nuances

While the general principles outlined above are common, there can be subtle differences in how various programming languages handle identifiers.

Python: Python is relatively permissive, allowing underscores and letters/numbers, with the requirement that the identifier doesn't start with a number.

Java: Java follows similar rules to Python, emphasizing case sensitivity and prohibiting spaces and most special characters. Keywords are strictly reserved.

C++: Similar to Java, C++ has strict rules about starting characters and keyword usage. It's case sensitive.

JavaScript: JavaScript rules are largely consistent with Java and C++. Case sensitivity is crucial.

C#: C# has similar rules to Java and C++. It has a wide range of keywords that cannot be used as identifiers.

Practical Examples: Identifying Illegal Identifiers

Let's analyze several examples to determine their legality as identifiers in a typical C++/Java/JavaScript-like language:

1. _privateVariable: Legal. Begins with an underscore, followed by alphanumeric characters.

2. 123variable: Illegal. Starts with a number.

3. my variable: Illegal. Contains a space.

4. my-variable: Illegal. Contains a hyphen.

5. myVariable: Legal. Uses camel case.

6. my_variable: Legal. Uses underscores.

7. int: Illegal. int is a keyword.

8. for: Illegal. for is a keyword.

9. class: Illegal. class is a keyword.

10. _123: Legal. Starts with an underscore, followed by numbers.

11. myVariable_1: Legal. Alphanumeric characters with an underscore.

12. MyVariable: Legal. Different from myVariable in case-sensitive languages.

13. My$Variable: Illegal. Contains an illegal character ($).

Best Practices for Choosing Identifiers

Beyond just legality, choosing good identifiers is crucial for code readability and maintainability. Follow these best practices:

• Meaningful Names: Choose names that clearly reflect the identifier's purpose. Avoid cryptic abbreviations.

• Consistent Naming Conventions: Adopt a consistent style (camel case, snake case, Pascal case) throughout your codebase.

• Avoid Single-Letter Identifiers: Unless the context is very clear (e.g., loop counters i, j, k), avoid single-letter identifiers for variables and functions.

• Use Descriptive Names: A longer, descriptive name is often better than a short, ambiguous one. It significantly improves the code's understandability for both you and others who might work on it later.

• Avoid Names That Could Be Confused: Choose names that are unlikely to be confused with each other or with keywords. Consider visual distinctions to help prevent accidental misuse.

Conclusion

Understanding legal identifiers is a crucial skill for any programmer. By adhering to the rules of your chosen programming language, choosing meaningful names, and following best practices, you can write cleaner, more maintainable, and less error-prone code. Remember that seemingly small errors in identifier selection can lead to significant problems during compilation, runtime, and debugging. Paying careful attention to these details is essential for effective software development.