Identifiers in C

01Why Do We Need Identifiers?

Think About Real Life...

Imagine if everyone in your class had no name. How would the teacher call on someone? "Hey you... no, not you, the other you!" It would be chaos!

Without names (chaos):

"Store 25 in... that thing... you know, the number holder"

With names (clear):

int age = 25;

What is an Identifier?

An identifier is simply a name you give to things in your program. Just like people have names, your variables, functions, and other program elements need names so you can refer to them!

Quick Rules Preview

a-z A-Z _ first

letters digits _

no spaces

no keywords

identifiers_example.c

1#include <stdio.h>
2
3int main() {
4    int age = 25;           // 'age' is an identifier
5    float salary = 50000.0; // 'salary' is an identifier
6    char grade = 'A';       // 'grade' is an identifier
7    
8    printf("Age: %d\n", age);
9    return 0;
10}
11// 'main', 'printf' are also identifiers!

Common Uses of Identifiers

Variables

int count, total, sum;

Functions

void calculate(), print();

Arrays

int numbers[10], grades[50];

Structures

struct Student, Person;

02Identifier Naming Rules

The 5 Rules You Must Follow

C has strict rules for naming identifiers. Break any of these, and your code won't compile!

Must start with a letter (a-z, A-Z) or underscore (_)

✓Valid

name, _value, Age

Invalid

1name, 2value, 9Age

Can contain letters, digits (0-9), and underscores only

✓Valid

age1, count_2, num_items

Invalid

my-var, count$, num@2

Cannot be a C keyword (reserved word)

✓Valid

myInt, forLoop, integer

Invalid

int, for, while, return

Case-sensitive: uppercase ≠ lowercase

age, Age, and AGE are three different identifiers!

No spaces allowed

✓Valid

firstName, first_name

Invalid

first name, my variable

Length Limit

While C allows very long identifiers, only the first 31 characters are guaranteed to be significant (C89/C90 standard). Modern compilers often support more.

03Valid vs Invalid Examples

Identifier	Valid?	Reason
studentName	✓Yes	Starts with letter, only letters
_count	✓Yes	Starts with underscore
num1	✓Yes	Digit after first character is OK
MAX_SIZE	✓Yes	Letters, underscore - all valid
1stPlace	No	Cannot start with digit
my-var	No	Hyphen not allowed
float	No	Reserved keyword
user name	No	Space not allowed
price$	No	Special characters not allowed

valid_invalid.c

1// ✓VALID Identifiers
2int age;
3float _temperature;
4char firstName;
5int student_count;
6int x1, x2, x3;
7double MAX_VALUE;
8
9// INVALID Identifiers (will cause errors!)
10// int 2fast;       // Cannot start with digit
11// float my-var;    // Hyphen not allowed
12// char int;        // 'int' is a keyword
13// int user name;   // Space not allowed

04Case Sensitivity

Important: C is Case-Sensitive!

age, Age, and AGE are THREE DIFFERENT variables!

case_sensitivity.c

1#include <stdio.h>
2
3int main() {
4    int age = 25;    // lowercase
5    int Age = 30;    // capitalized
6    int AGE = 35;    // uppercase
7    
8    // All three are DIFFERENT variables!
9    printf("age = %d\n", age);  // 25
10    printf("Age = %d\n", Age);  // 30
11    printf("AGE = %d\n", AGE);  // 35
12    
13    return 0;
14}

Output

age = 25

Age = 30

AGE = 35

05Naming Conventions

While rules are mandatory, conventions are best practices that make your code readable and professional.

Type	Convention	Example
Variables	camelCase or snake_case	studentAge, student_age
Constants	UPPER_SNAKE_CASE	MAX_SIZE, PI_VALUE
Functions	camelCase or snake_case	calculateSum, calculate_sum
Structs/Types	PascalCase	StudentRecord, Person
Macros	UPPER_SNAKE_CASE	MAX_BUFFER, DEBUG_MODE

naming_conventions.c

1#include <stdio.h>
2#define MAX_STUDENTS 100  // Constant: UPPER_CASE
3
4struct StudentRecord {    // Struct: PascalCase
5    int studentId;        // Variable: camelCase
6    char firstName[50];
7    float gradeAverage;
8};
9
10void printStudent(struct StudentRecord s) {  // Function: camelCase
11    printf("ID: %d, Name: %s\n", s.studentId, s.firstName);
12}
13
14int main() {
15    int studentCount = 0;  // Variable: camelCase
16    return 0;
17}

06Best Practices

✓Use Meaningful Names

int x; // What is x?

int studentCount; // Clear!

✓Be Consistent

Pick one style (camelCase or snake_case) and stick with it throughout your project.

✓Keep It Short but Descriptive

int numberOfStudentsInClass;

int studentCount; // Better

Avoid Starting with Underscore

Names starting with _ or __ are reserved for the system and library functions. Use them only when necessary.

!Code Pitfalls: Common Mistakes & What to Watch For

These are the most common mistakes that trip up beginners. Study them carefully to avoid hours of debugging!

Starting with a Number

int 2ndPlace = 2; // ERROR!

Fix: int secondPlace = 2;

Using Keywords as Names

int return = 5; // ERROR!

Fix: int returnValue = 5;

Case Confusion

int Count = 5; printf("%d", count); // ERROR!

Count ≠ count (case-sensitive)

08Try It Yourself

Here's a complete program using good identifier names. Notice how readable it is!

good_identifiers.c

1#include <stdio.h>
2
3int main() {
4    // Good identifier names - clear and descriptive
5    int studentAge = 20;
6    float examScore = 85.5;
7    char studentGrade = 'B';
8    int totalStudents = 30;
9    
10    // Print with meaningful names
11    printf("Student Age: %d years\n", studentAge);
12    printf("Exam Score: %.1f%%\n", examScore);
13    printf("Grade: %c\n", studentGrade);
14    printf("Total Students: %d\n", totalStudents);
15    
16    return 0;
17}

Expected Output:

Student Age: 20 years
Exam Score: 85.5%
Grade: B
Total Students: 30

Watch Out When Copying Code!

Key Takeaways

•WHY: Identifiers give names to program elements so we can use them
•Must start with letter or underscore
•Can contain letters, digits, underscores only
•No keywords, no spaces, no special characters
•C is case-sensitive (age ≠ Age ≠ AGE)

11Naming Best Practices

Use Descriptive Names

Prefer student_count over sc,calculate_average over calcAvg. Code is read more often than written—make it self-documenting.

Follow a Consistent Convention

C traditionally uses snake_case for functions and variables.SCREAMING_CASE is reserved for macros and constants. Pick one style and stick to it throughout your project.

Use Prefixes for Related Items

Group related identifiers with prefixes: str_copy,str_length, str_compare. This makes code organization clear and helps avoid naming conflicts in large projects.

Avoid Abbreviations

Use customer_address not cust_addr. Modern IDEs have autocomplete—longer names cost nothing to type but make code much easier to read.

Be Consistent with Case

Pick one style and stick to it. C traditionally uses snake_case for variables and functions. Use SCREAMING_CASE for macros and constants. Consistent naming makes code predictable and easier to maintain.

01Why Do We Need Identifiers?

Think About Real Life...

Imagine if everyone in your class had no name. How would the teacher call on someone? "Hey you... no, not you, the other you!" It would be chaos!

Without names (chaos):

"Store 25 in... that thing... you know, the number holder"

With names (clear):

int age = 25;

What is an Identifier?

An identifier is simply a name you give to things in your program. Just like people have names, your variables, functions, and other program elements need names so you can refer to them!

Quick Rules Preview

a-z A-Z _ first

letters digits _

no spaces

no keywords

identifiers_example.c

1#include <stdio.h>
2
3int main() {
4    int age = 25;           // 'age' is an identifier
5    float salary = 50000.0; // 'salary' is an identifier
6    char grade = 'A';       // 'grade' is an identifier
7    
8    printf("Age: %d\n", age);
9    return 0;
10}
11// 'main', 'printf' are also identifiers!

Common Uses of Identifiers

Variables

int count, total, sum;

Functions

void calculate(), print();

Arrays

int numbers[10], grades[50];

Structures

struct Student, Person;

02Identifier Naming Rules

The 5 Rules You Must Follow

C has strict rules for naming identifiers. Break any of these, and your code won't compile!

Must start with a letter (a-z, A-Z) or underscore (_)

✓Valid

name, _value, Age

Invalid

1name, 2value, 9Age

Can contain letters, digits (0-9), and underscores only

✓Valid

age1, count_2, num_items

Invalid

my-var, count$, num@2

Cannot be a C keyword (reserved word)

✓Valid

myInt, forLoop, integer

Invalid

int, for, while, return

Case-sensitive: uppercase ≠ lowercase

age, Age, and AGE are three different identifiers!

No spaces allowed

✓Valid

firstName, first_name

Invalid

first name, my variable

Length Limit

While C allows very long identifiers, only the first 31 characters are guaranteed to be significant (C89/C90 standard). Modern compilers often support more.

03Valid vs Invalid Examples

Identifier	Valid?	Reason
studentName	✓Yes	Starts with letter, only letters
_count	✓Yes	Starts with underscore
num1	✓Yes	Digit after first character is OK
MAX_SIZE	✓Yes	Letters, underscore - all valid
1stPlace	No	Cannot start with digit
my-var	No	Hyphen not allowed
float	No	Reserved keyword
user name	No	Space not allowed
price$	No	Special characters not allowed

valid_invalid.c

1// ✓VALID Identifiers
2int age;
3float _temperature;
4char firstName;
5int student_count;
6int x1, x2, x3;
7double MAX_VALUE;
8
9// INVALID Identifiers (will cause errors!)
10// int 2fast;       // Cannot start with digit
11// float my-var;    // Hyphen not allowed
12// char int;        // 'int' is a keyword
13// int user name;   // Space not allowed

04Case Sensitivity

Important: C is Case-Sensitive!

age, Age, and AGE are THREE DIFFERENT variables!

case_sensitivity.c

1#include <stdio.h>
2
3int main() {
4    int age = 25;    // lowercase
5    int Age = 30;    // capitalized
6    int AGE = 35;    // uppercase
7    
8    // All three are DIFFERENT variables!
9    printf("age = %d\n", age);  // 25
10    printf("Age = %d\n", Age);  // 30
11    printf("AGE = %d\n", AGE);  // 35
12    
13    return 0;
14}

Output

age = 25

Age = 30

AGE = 35

05Naming Conventions

While rules are mandatory, conventions are best practices that make your code readable and professional.

Type	Convention	Example
Variables	camelCase or snake_case	studentAge, student_age
Constants	UPPER_SNAKE_CASE	MAX_SIZE, PI_VALUE
Functions	camelCase or snake_case	calculateSum, calculate_sum
Structs/Types	PascalCase	StudentRecord, Person
Macros	UPPER_SNAKE_CASE	MAX_BUFFER, DEBUG_MODE

naming_conventions.c

1#include <stdio.h>
2#define MAX_STUDENTS 100  // Constant: UPPER_CASE
3
4struct StudentRecord {    // Struct: PascalCase
5    int studentId;        // Variable: camelCase
6    char firstName[50];
7    float gradeAverage;
8};
9
10void printStudent(struct StudentRecord s) {  // Function: camelCase
11    printf("ID: %d, Name: %s\n", s.studentId, s.firstName);
12}
13
14int main() {
15    int studentCount = 0;  // Variable: camelCase
16    return 0;
17}

06Best Practices

✓Use Meaningful Names

int x; // What is x?

int studentCount; // Clear!

✓Be Consistent

Pick one style (camelCase or snake_case) and stick with it throughout your project.

✓Keep It Short but Descriptive

int numberOfStudentsInClass;

int studentCount; // Better

Avoid Starting with Underscore

Names starting with _ or __ are reserved for the system and library functions. Use them only when necessary.

!Code Pitfalls: Common Mistakes & What to Watch For

These are the most common mistakes that trip up beginners. Study them carefully to avoid hours of debugging!

Starting with a Number

int 2ndPlace = 2; // ERROR!

Fix: int secondPlace = 2;

Using Keywords as Names

int return = 5; // ERROR!

Fix: int returnValue = 5;

Case Confusion

int Count = 5; printf("%d", count); // ERROR!

Count ≠ count (case-sensitive)

08Try It Yourself

Here's a complete program using good identifier names. Notice how readable it is!

good_identifiers.c

1#include <stdio.h>
2
3int main() {
4    // Good identifier names - clear and descriptive
5    int studentAge = 20;
6    float examScore = 85.5;
7    char studentGrade = 'B';
8    int totalStudents = 30;
9    
10    // Print with meaningful names
11    printf("Student Age: %d years\n", studentAge);
12    printf("Exam Score: %.1f%%\n", examScore);
13    printf("Grade: %c\n", studentGrade);
14    printf("Total Students: %d\n", totalStudents);
15    
16    return 0;
17}

Expected Output:

Student Age: 20 years
Exam Score: 85.5%
Grade: B
Total Students: 30

Watch Out When Copying Code!

Key Takeaways

•WHY: Identifiers give names to program elements so we can use them
•Must start with letter or underscore
•Can contain letters, digits, underscores only
•No keywords, no spaces, no special characters
•C is case-sensitive (age ≠ Age ≠ AGE)

11Naming Best Practices

Use Descriptive Names

Prefer student_count over sc,calculate_average over calcAvg. Code is read more often than written—make it self-documenting.

Follow a Consistent Convention

C traditionally uses snake_case for functions and variables.SCREAMING_CASE is reserved for macros and constants. Pick one style and stick to it throughout your project.

Use Prefixes for Related Items

Group related identifiers with prefixes: str_copy,str_length, str_compare. This makes code organization clear and helps avoid naming conflicts in large projects.

Avoid Abbreviations

Use customer_address not cust_addr. Modern IDEs have autocomplete—longer names cost nothing to type but make code much easier to read.

What You Will Learn

01Why Do We Need Identifiers?

Think About Real Life...

What is an Identifier?

Common Uses of Identifiers

02Identifier Naming Rules

The 5 Rules You Must Follow

Must start with a letter (a-z, A-Z) or underscore (_)

Can contain letters, digits (0-9), and underscores only

Cannot be a C keyword (reserved word)

Case-sensitive: uppercase ≠ lowercase

No spaces allowed

Length Limit

03Valid vs Invalid Examples

04Case Sensitivity

Important: C is Case-Sensitive!

05Naming Conventions

06Best Practices

✓Use Meaningful Names

✓Be Consistent

✓Keep It Short but Descriptive

Avoid Starting with Underscore

!Code Pitfalls: Common Mistakes & What to Watch For

Starting with a Number

Using Keywords as Names

Case Confusion

08Try It Yourself

Expected Output:

Watch Out When Copying Code!

Key Takeaways

11Naming Best Practices

Use Descriptive Names

Follow a Consistent Convention

Use Prefixes for Related Items

Avoid Abbreviations

Be Consistent with Case

Test Your Knowledge

Related Tutorials

Keywords in C

Data Types & Variables

Basic Syntax in C

Have Feedback?

What You Will Learn

01Why Do We Need Identifiers?

Think About Real Life...

What is an Identifier?

Common Uses of Identifiers

02Identifier Naming Rules

The 5 Rules You Must Follow

Must start with a letter (a-z, A-Z) or underscore (_)

Can contain letters, digits (0-9), and underscores only

Cannot be a C keyword (reserved word)

Case-sensitive: uppercase ≠ lowercase

No spaces allowed

Length Limit

03Valid vs Invalid Examples

04Case Sensitivity

Important: C is Case-Sensitive!

05Naming Conventions

06Best Practices

✓Use Meaningful Names

✓Be Consistent

✓Keep It Short but Descriptive

Avoid Starting with Underscore

!Code Pitfalls: Common Mistakes & What to Watch For

Starting with a Number

Using Keywords as Names

Case Confusion

08Try It Yourself

Expected Output:

Watch Out When Copying Code!

Key Takeaways

11Naming Best Practices

Use Descriptive Names

Follow a Consistent Convention

Use Prefixes for Related Items

Avoid Abbreviations

Be Consistent with Case

Test Your Knowledge

Related Tutorials