03_identifiers_variables.md

Identifiers and Variables in C Programming

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Identifiers

Identifiers are names given to various program elements such as variables, functions, arrays, etc.
They help uniquely identify these elements in your code.

Rules for Identifiers

• Can contain letters (A-Z, a-z), digits (0-9), and underscores (_)
• Must begin with a letter or underscore
• Cannot be a reserved keyword (e.g., int, return)
• Case-sensitive (Count and count are different)

Examples of Valid Identifiers

• total
• _value
• sum1
• student_name

Examples of Invalid Identifiers

• 1value (cannot start with a digit)
• float (reserved keyword)
• my-value (hyphen not allowed)

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Variables

Variables are named memory locations used to store data that can change during program execution.

Types of Variables

1. Local Variables

   • Declared inside a function or block
   • Accessible only within that function/block
   • Example:

     void func() {
         int x = 10; // local variable
     }

2. Global Variables

   • Declared outside all functions
   • Accessible from any function in the file
   • Example:

     int count = 0; // global variable
     
     void func() {
         count++;
     }

3. Static Variables

   • Declared with static keyword
   • Retain their value between function calls
   • Example:

     void func() {
         static int num = 0;
         num++;
         printf("%d", num);
     }

4. Extern Variables

   • Declared with extern keyword
   • Used to access variables defined in other files
   • Example:

     extern int shared;

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Variables vs Constants

Variables can change their value during program execution, while constants have fixed values that do not change.

Declaring Constants

• Use the const keyword or #define preprocessor directive.

Examples

int age = 25;         // variable: value can change
age = 30;             // valid

const float PI = 3.14; // constant: value cannot change
// PI = 3.1415;        // invalid, will cause a compile error

#define MAX_SIZE 100   // constant using preprocessor

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Examples

#include <stdio.h>

int globalVar = 100; // global variable

void demo() {
    int localVar = 10; // local variable
    static int staticVar = 0; // static variable
    staticVar++;
    printf("Local: %d, Static: %d, Global: %d\n", localVar, staticVar, globalVar);
}

int main() {
    demo();
    demo();

    const int daysInWeek = 7; // constant
    int x = 5;                // variable
    x = 10;                   // valid
    // daysInWeek = 8;        // invalid, will cause error

    printf("Days in a week: %d\n", daysInWeek);
    printf("x: %d\n", x);

    return 0;
}

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Use Cases

• Use local variables for temporary data within functions.
• Use global variables for data shared across multiple functions.
• Use static variables to preserve state between function calls.
• Use extern variables for sharing data between different source files.
• Use constants for values that should not change (like mathematical constants, array sizes).

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Identifiers, variables, and constants are fundamental for organizing and managing data in your

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