Identifiers in C: Definition, Rules, Examples & Best Practices

In the real world, we give names to everything, people, places, and things, to identify them. Programming is no different. When you create a variable, function, or array in your code, you need to give it a unique name. This brings us to a fundamental question for any new programmer: what are identifiers in C?

Simply put, they are the user-defined names you create to label the different parts of your program. Mastering the rules for creating valid Identifiers in C is a crucial first step in your coding journey. This tutorial will break down everything you need to know, from naming conventions to the difference between identifiers and keywords.

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What Are Identifiers in C?

In C programming, an identifier is a name given to a variable, function, array, constant, or any other user-defined item. You use identifiers to refer to the data or functions you've created, allowing the program to perform operations on them.

Think of an identifier as the "name tag" for an entity in your program. This name allows the program to access and manipulate that entity.

Must Explore: C Tutorial for Beginners: Learn C Programming Step-by-Step

Why Are Identifiers Important in C?

Identifiers are one of the core elements of programming in C. Without them, you wouldn’t be able to create variables, functions, or even access data. Let’s break down why they’re important:

1. Clarity: Properly named identifiers make your code much easier to understand. For instance, naming a variable `age` tells the reader that this variable stores an age, whereas `x` doesn't provide any information.

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2. Organization: By using descriptive identifiers, you can structure your code so it’s easier to follow and maintain. This organization is crucial, especially when working with large codebases.

3. Avoiding Conflicts: Identifiers help avoid naming conflicts. By following naming conventions and ensuring identifiers are unique, you reduce the risk of accidental clashes with other variables, functions, or even keywords.

4. Maintainability: When you or someone else revisits the code later, good identifiers make it clear what each part of the program does, making maintenance and debugging much easier.

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However, if you’re going to use Linux operating system or macOS to learn identifiers in C programming, then you should understand the correct process for that. And, we’ve got the guides for both.

• How to Compile and Run a C Program in Linux Terminal
• Comprehensive Guide to C Compiler for Mac: Installation, Alternatives, and Debugging Tips

Types of Identifiers in C

There are various types of identifiers in C, each serving a specific purpose. Here are the most common types of identifiers you’ll encounter:

1. Variable Identifiers

A variable identifier represents a storage location that holds data. In C, a variable is a named memory location used to store a value that can change during the execution of the program.

Example:

int age = 25; // 'age' is a variable identifier representing an integer.

printf("Age: %d\n", age); // Output: Age: 25

2. Function Identifiers

A function identifier is used to refer to a function, which is a block of code that performs a specific task. Functions are a fundamental building block in C programming, and their identifiers help the program know where to go to execute a specific set of instructions.

Example

void printMessage() {  // 'printMessage' is the identifier for the function
        printf("Hello, World!\n");
    }

    int main() {
        printMessage();  // Calling the function by its identifier
        return 0;
    }

3. Array Identifiers

An array identifier represents an array, which is a collection of variables of the same type. Arrays allow you to group related data items under a single identifier. In C, arrays are zero-indexed, meaning the first element has an index of `0`.

Example

int scores[5] = {90, 80, 85, 70, 95}; // 'scores' is the identifier for the array

printf("First score: %d\n", scores[0]); // Output: First score: 90

Also Read: Static Function in C: Definition, Examples & Real-World Applications

4. Constant Identifiers

A constant identifier is used to define a constant value that does not change during the program's execution. Constants are typically defined using the `#define` directive or the `const` keyword.

Example

#define PI 3.14 // 'PI' is a constant identifier

printf("Value of PI: %.2f\n", PI); // Output: Value of PI: 3.14

5. Type Identifiers

In C, the `typedef` keyword is used to create a new type alias. The name given to the new type is called a type identifier.

Example

typedef unsigned int uint; // 'uint' is a type identifier for unsigned int

uint population = 100000; // Using 'uint' to define the variable

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Scope of Identifiers in C

The scope of an identifier in C refers to the region of the program where the identifier (variable, function, etc.) can be accessed or used. The scope defines the visibility and lifetime of an identifier. Understanding the scope of identifiers is crucial because it helps you control where and how variables and functions are accessible, and ensures that they do not interfere with other parts of the program.

There are two main types of scope of identifiers in C:

1. Local Scope

2. Global Scope

Let’s take a closer look at each:

1. Local Scope

An identifier is said to have local scope if it is declared within a function or a block (e.g., within a loop, conditional statement, or a block of code enclosed in braces `{}`). Variables with local scope are only accessible within that function or block in which they are declared. Once the function or block exits, the identifier goes out of scope, and its memory is freed.

Characteristics of Local Scope:

• The identifier is visible and can only be used inside the function or block where it is declared.
• Local variables are created when the function or block is entered and destroyed when it is exited.
• They cannot be accessed outside the function or block they are declared in.
• Local variables can have the same name as global variables, but they will be treated as distinct.

Example of Local Scope:

#include <stdio.h>

void printAge() {
    int age = 25;  // 'age' has local scope within this function
    printf("Age: %d\n", age);
}

int main() {
    // The 'age' variable here does not affect the 'age' in printAge()
    printAge();  // Output: Age: 25
    // printf("Age: %d\n", age);  // Error: 'age' is not declared in main
    return 0;
}

Explanation

• The variable `age` is declared inside the `printAge` function. It is local to that function and cannot be accessed outside it.
• If you try to use `age` in `main()`, you will get an error because `age` is not within the scope of `main()`.

Also explore if-else statements in C programming for better understanding of identifiers in C in further code examples and concepts.

2. Global Scope

An identifier is said to have global scope if it is declared outside of all functions, typically at the top of the file. Global variables can be accessed by any function in the program, making them visible throughout the entire file.

Characteristics of Global Scope:

• Global variables are declared outside all functions (usually before the `main()` function).
• They are accessible from any function within the same file.
• Global variables have a long lifetime: they are created when the program starts and destroyed when the program ends.
• Global variables should be used with caution, as they can be modified by any part of the program, which can lead to unexpected behavior and bugs.

Example of Global Scope:

#include <stdio.h>
int age = 30;  // 'age' has global scope

void printAge() {
    printf("Global Age: %d\n", age);  // Accessing global variable 'age'
}

int main() {
    printAge();  // Output: Global Age: 30
    age = 35;  // Modifying the global variable
    printAge();  // Output: Global Age: 35
    return 0;
}

Explanation

• The variable `age` is declared outside of any function, so it has global scope.
• Both `main()` and `printAge()` functions can access and modify the global variable `age`.

3. Block Scope

Identifiers that are declared inside a block (i.e., within a set of curly braces `{}`), such as in loops or conditional statements, have **block scope**. These variables are only visible inside the block where they are defined.

Example of Block Scope:

#include <stdio.h>

int main() {
    if (1) {
        int blockVar = 100;  // 'blockVar' is local to this block
        printf("Inside block: %d\n", blockVar);  // Output: Inside block: 100
    }
    // printf("Outside block: %d\n", blockVar);  // Error: 'blockVar' is not visible here
    return 0;
}

Explanation

• The variable `blockVar` is declared inside the `if` block and can only be accessed within that block.
• Trying to access `blockVar` outside the block where it was declared would result in an error because its scope is limited to the block.

4. Function Scope

Identifiers declared as function parameters have function scope. These variables are accessible only within the function and can’t be accessed outside.

Example of Function Scope:

#include <stdio.h>
void printAge(int age) {  // 'age' is a parameter with function scope
    printf("Age inside function: %d\n", age);  // Output: Age inside function: 25
}
int main() {
    int age = 30;  // 'age' in main()
    printAge(25);  // Passing value of '25' to the function
    // printf("Age inside main: %d\n", age);  // Output: Age inside main: 30
    return 0;
}

Explanation

• The parameter `age` inside `printAge()` is **local to that function** and has function scope. It can’t be accessed outside the function.
• The variable `age` in `main()` is a separate variable and does not conflict with the `age` in `printAge()`.

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5. Static Variables and Their Scope

C also supports static variables, which are declared using the `static` keyword. A static variable retains its value across function calls, but its scope is limited to the function or block where it is declared.

Characteristics of Static Scope:

Static variables are initialized only once and retain their values between function calls.

The variable is still visible only within the function or block it is declared in, but its lifetime is throughout the program execution.

Example of Static Variable Scope:

#include <stdio.h>
void counter() {
    static int count = 0;  // Static variable with function scope
    count++;
    printf("Count: %d\n", count);
}

int main() {
    counter();  // Output: Count: 1
    counter();  // Output: Count: 2
    counter();  // Output: Count: 3
    return 0;
}

Explanation:

• The `count` variable is static, so it maintains its value across multiple function calls, but its scope is still limited to the `counter()` function.
• The variable is initialized only once, and each call to `counter()` increments the value of `count`.

Rules for Valid Identifiers in C

To ensure your code runs without errors, it’s important to follow the rules for valid identifiers in C. Here are the key rules to keep in mind:

1. Start with a Letter or Underscore (`_`)

The first character of an identifier must be either a letter (uppercase or lowercase) or an underscore.

Valid:

int totalAmount; // Starts with a letter

float _rate; // Starts with an underscore

Invalid:

int 1totalAmount; // Starts with a number

2. Subsequent Characters Can Be Letters, Digits, or Underscores

After the first character, the rest of the identifier can include letters, digits, or underscores.

Valid:

int totalAmount1; // Includes a number after the first character

float _tax_rate; // Includes an underscore and lowercase letters

Invalid:

int total#Amount; // Includes a special character

3. Cannot Be a Reserved Keyword

C has reserved keywords like `int`, `return`, `for`, `while`, etc. These cannot be used as identifiers.

Valid:

int totalAmount; // 'totalAmount' is a valid identifier

Invalid:

int return = 5; // 'return' is a reserved keyword in C

4. Identifiers Are Case-Sensitive

C is a case-sensitive language, meaning that `TotalAmount`, `totalAmount`, and `TOTALAMOUNT` are all considered different identifiers.

Valid:

int TotalAmount; // Different from 'totalAmount'

int totalAmount; // Different from 'TotalAmount'

Invalid:

int totalAmount = 5; // 'totalAmount' is fine

int TOTALAMOUNT = 10; // 'TOTALAMOUNT' is considered a different identifier

5. Length Restrictions

While the C standard does not limit the length of identifiers, some compilers may only recognize the first 31 characters of an identifier. It’s best to keep identifiers descriptive yet concise.

Common Invalid Identifiers in C

Let’s look at a few examples of invalid identifiers in C, with explanations:

1. Starting with a Number:

int 1stPlace = 10; // Invalid: Identifiers cannot start with a number

2. Using Special Characters:

int total@Amount = 50; // Invalid: Special characters like @ are not allowed

3. Using Reserved Keywords:

int return = 100; // Invalid: 'return' is a reserved keyword in C

4. Using Spaces:

int my variable = 10; // Invalid: Spaces are not allowed in identifiers

Keywords vs Identifiers in C: A Detailed Comparison

It’s essential to understand the difference between keywords and identifiers in C. Although they both play a vital role in the language, they serve different purposes. Here’s a detailed comparison:

How to Name Identifiers in C: Best Practices

Naming identifiers correctly is crucial for writing clean and maintainable code. Here are some best practices to follow:

1. Use Descriptive Names

Choose names that describe the purpose of the variable or function. This makes the code more readable and intuitive.

Good: `int totalAmount;`

Bad: `int x;`

2. Consistent Naming Conventions

Use a consistent naming style throughout your code. This makes it easier to understand and maintain:

CamelCase: Often used for variables and functions (`totalAmount`, `calculateTotal`).

Snake_case: Common in C for variables (`total_amount`, `calculate_total`).

UPPERCASE: For constants (`MAX_SIZE`, `PI`).

3. Avoid Single Character Identifiers

Avoid using single-letter identifiers except in specific cases like loop counters. Single letters don’t provide useful information about the variable’s purpose.

Good: `int userAge;`

Bad: `int x;`

4. Start with a Letter or Underscore

An identifier must start with a letter or an underscore, but avoid using an underscore at the beginning unless necessary (e.g., for system-level identifiers).

Good: `int studentScore;`, `float _temp;`

Bad: `int 123score;`

5. Avoid Reserved Keywords

Never use reserved keywords as identifiers. These are predefined and have a specific function in C.

Bad: `int return = 5;` (This will cause a compilation error)

Conclusion:

Understanding Identifiers in C is about more than just following rules—it's about writing code that is clear, readable, and professional. These names are the fundamental labels you give to your variables and functions, and choosing them wisely is the first step to creating maintainable software.

You now have a complete answer to the question, "what are identifiers in C?" They are the backbone of well-structured code. By applying the best practices and naming conventions from this guide, you are not just writing code that works, but code that is a pleasure to read and easy to debug.

FAQS

1. What are the fundamental rules for creating valid identifiers in C?

To create valid Identifiers in C, you must follow a few strict rules. An identifier can only be composed of letters (both uppercase A-Z and lowercase a-z), digits (0-9), and the underscore character (_). The first character of an identifier must be either a letter or an underscore; it cannot be a digit. Finally, you cannot use any of C's reserved keywords (like int or while) as an identifier. Adhering to these rules is mandatory for the compiler to understand your code.

2. Are identifiers in C case-sensitive?

Yes, Identifiers in C are case-sensitive. This means that the compiler treats uppercase and lowercase letters as distinct characters. For example, totalAmount, TotalAmount, and totalamount would be treated as three completely different and separate identifiers. This is a crucial rule to remember, as a small mistake in capitalization can lead to "undeclared identifier" compilation errors.

3. What is the difference between an identifier and a keyword in C?

A keyword is a reserved word that has a special, predefined meaning in the C language, such as if, else, for, or return. You cannot use these keywords for any other purpose. An identifier, on the other hand, is a unique, user-defined name that you create to refer to a variable, function, or other entity in your program. In short, keywords are the built-in vocabulary of the language, while identifiers are the names you create.

4. What happens if I accidentally use a reserved keyword as an identifier?

If you attempt to use a reserved keyword as a name for a variable or function (e.g., int while = 5;), your program will fail to compile. The C compiler will immediately flag this as a syntax error because it recognizes the keyword as having a specific purpose that cannot be overridden. This is a fundamental rule in understanding what are identifiers in C versus the language's built-in components.

5. What is the difference between the declaration and the definition of an identifier?

A declaration of an identifier introduces its name and type to the compiler but does not allocate any memory for it. For example, extern int userCount; declares that an integer variable named userCount exists somewhere else. A definition, on the other hand, not only declares the identifier but also allocates storage for it. int userCount; is both a declaration and a definition. A single identifier can be declared multiple times, but it can only be defined once in a program.

6. What is the "scope" of an identifier?

The scope of an identifier refers to the region of the program where that identifier is visible and can be accessed. In C, there are mainly four types of scopes: block scope (visible only within a block of code like a for loop or a function), function scope (visible throughout a function), file scope (visible from the point of declaration to the end of the source file), and prototype scope (for parameter names in a function prototype). Understanding scope is key to managing Identifiers in C.

7. What happens if I declare two variables with the same name in different scopes?

If you declare two variables with the same name in different, nested scopes (for example, a global variable and a local variable inside a function), they are treated as two distinct and separate entities. The variable in the inner scope will "shadow" or hide the variable in the outer scope. This means that within the inner scope, any reference to that name will refer to the locally defined variable, not the global one.

8. Can I use an identifier in a nested scope with the same name as one in an outer scope?

Yes, C allows an identifier to be reused in a nested scope, a concept known as shadowing. A variable declared within an inner block (like inside a for loop or an if statement) can have the same name as a variable in an outer scope (like the main body of the function). Within that inner block, the inner variable takes precedence and is the only one that can be accessed by that name. Once the block is exited, the outer variable becomes visible again.

9. What are the different types of identifiers in C?

In C, an identifier can be used to name several different kinds of program entities. The main types of Identifiers in C include variables (which store data values), functions (which represent blocks of executable code), user-defined data types (like structs, unions, and enums), and labels (used as targets for goto statements). Each of these uses an identifier to give a unique name to a specific part of your program.

10. What are some common naming conventions for identifiers in C?

While C does not enforce a specific naming style, several conventions are widely followed to improve readability. Camel case (e.g., totalAmount) and snake case (e.g., total_amount) are the two most popular styles for variable and function names. Constants defined with #define are typically written in all uppercase with underscores (e.g., MAX_SIZE). Following a consistent convention is crucial for writing maintainable code.

11. What is the difference between an identifier and a constant in C?

While a named constant is a type of identifier, the key difference is mutability. A variable identifier refers to a memory location whose value can be changed during program execution. A constant identifier, created using #define or the const keyword, is a name for a value that is fixed and cannot be changed once it is defined. For example, totalAmount is a variable, while MAX_LIMIT is a constant.

12. What is a reserved identifier in C?

A reserved identifier is a name that you should not use because it is either currently used by the C standard library for its internal functions or is reserved for future use. You should always avoid creating Identifiers in C that begin with an underscore followed by an uppercase letter (e.g., _MyVar) or that start with two underscores (e.g., __myVar), as these are reserved for the implementation.

13. How long can an identifier be in C?

The C standard itself does not impose a specific limit on the length of an identifier. However, it does specify the number of significant characters that a compiler or linker must consider. For an internal identifier (one with block or function scope), at least the first 63 characters must be significant. For an external identifier (one that is shared across files), at least the first 31 characters must be significant. It is a good practice to keep your identifiers concise yet descriptive.

14. Are there any restrictions on the length of an identifier in C?

While the standard dictates the minimum number of significant characters, a specific compiler might have its own, larger limit on the total length of an identifier. However, for maximum portability and to avoid any unexpected behavior, it is advisable to keep your Identifiers in C within the standard's limits (31 characters for external and 63 for internal identifiers), as some older or less common compilers might truncate longer names.

15. Can an identifier contain special characters like @ or #?

No, Identifiers in C are very strict about the characters they can contain. They are limited to uppercase and lowercase letters (A-Z, a-z), digits (0-9), and the underscore character (_). Any other special character, such as @, #, $, %, or *, is not allowed and will result in a compilation error. For example, total$Amount would be an invalid identifier.

16. How can I avoid name conflicts between identifiers in C?

To avoid name conflicts, you should limit the use of global variables as much as possible, as they have a wide scope and are more likely to cause issues. Use static for functions and global variables that do not need to be accessed from other files. Adopting a clear and consistent naming convention can also help. For example, some developers prefix global variables with g_ (e.g., g_userCount) to easily distinguish them from local variables.

17. What is the purpose of the typedef keyword in relation to identifiers in C?

The typedef keyword is a powerful feature that allows you to create a custom alias, or a new name, for an existing data type. This new name is a user-defined identifier. It is commonly used to simplify complex data type declarations, such as for structs or function pointers, making the code much more readable and easier to maintain. For example, after typedef unsigned long ulong;, ulong becomes a valid type identifier.

18. Can I use the main function as an identifier in C?

While main is a function name, it has a special role as the entry point of every C program. You technically can use main as an identifier for a variable (e.g., int main = 100;), but this is extremely poor practice. It will cause confusion for anyone reading your code and may even conflict with the linker. For all practical purposes, you should treat main as a reserved name and not use it for anything other than the main function.

19. How can I learn more about identifiers and best practices in C?

The best way to learn is through a combination of structured education and hands-on practice. A comprehensive program, like the software development courses offered by upGrad, can provide a strong foundation in C and its syntax rules. You should then apply this knowledge by writing your own programs and paying close attention to naming conventions and scope. Reading the source code of well-written open-source projects is also an excellent way to learn best practices for using Identifiers in C.

20. So, what is the key takeaway about what are identifiers in C?

The key takeaway to the question, "what are identifiers in C?", is that they are the fundamental building blocks for creating readable and maintainable code. They are not just random names; they are the labels you attach to your program's logic. By following the language's rules, adopting a clear naming convention, and understanding the concept of scope, you move from simply writing code that works to engineering software that is robust, professional, and easy for others to understand.

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