Part 2: Identifiers, Variables, and Basic Programs (90 mins)
Topic 1: Identifiers in C - The "Names" of Things
Definition: Identifiers are names used to identify variables, functions, arrays, structures, or any other user-defined items in a program. They uniquely identify program elements and allow us to refer to them later.
// Creating a variable with identifier 'val'
int val = 10;
// Creating a function with identifier 'func'
void func() {}
// Creating an array with identifier 'scores'
int scores[5];
Rules for Naming Identifiers in C
Identifiers must follow these strict rules:
- Can contain: uppercase (A-Z), lowercase (a-z), digits (0-9), and underscores (_)
- First character must be a letter or underscore (not a digit)
-
Case-sensitive:
age,Age, andAGEare different identifiers -
Cannot be C keywords (reserved words like
int,return,if) - No spaces or special characters (@, #, $, etc.)
Valid vs Invalid Identifiers
| Valid Identifiers | Invalid Identifiers | Reason |
|---|---|---|
student_name |
student name |
Space not allowed |
_count |
9students |
Starts with digit |
totalAmount |
int |
Keyword not allowed |
MAX_SIZE |
student@name |
Special character @ |
Examples of Identifiers in Action
Variable Identifier Example
#include <stdio.h>
int main() {
// Creating an integer variable with identifier 'var'
int var;
// Assigning value using the identifier
var = 10;
// Referring to the variable using its identifier
printf("%d", var);
return 0;
}
Output: 10
Function Identifier Example
#include <stdio.h>
// Function declaration with identifier 'sum'
int sum(int a, int b) {
return a + b;
}
int main() {
// Calling function using its identifier
printf("%d", sum(10, 20));
return 0;
}
Output: 30
Naming Conventions (Best Practices)
While not enforced by the compiler, following conventions makes code more readable:
| Element Type | Convention | Examples |
|---|---|---|
| Variables | camelCase or snake_case | studentCount, total_amount |
| Constants | UPPER_SNAKE_CASE | MAX_SIZE, PI_VALUE |
| Functions | camelCase (verb-based) | calculateTotal(), getUserName() |
| Structures | PascalCase | StudentInfo, CarDetails |
Keywords vs Identifiers
| Feature | Keywords | Identifiers |
|---|---|---|
| Definition | Reserved words with special meaning | User-defined names for program elements |
| Purpose | Define program structure and control flow | Name variables, functions, etc. |
| Examples |
int, if, while,
return
|
age, calculateTotal,
studentName
|
| Modification | Fixed by language (cannot be changed) | Created by programmer as needed |
Common Error: Using Keywords as Identifiers
#include <stdio.h>
int main() {
// ERROR: 'const' is a keyword, cannot be used as
identifier
int const = 90;
return 0;
}
Compilation Error: expected identifier or '(' before '=' token
Topic 2: Variables in Depth - Declaration, Definition, and Scope
Variable Declaration vs Definition
Declaration: Tells the compiler about the variable's existence and type
Definition: Actually allocates memory for the variable
// Declaration (informs compiler)
extern int globalVar;
// Definition (allocates memory)
int score = 95;
int age; // Also a definition (memory allocated)
Variable Scope - Where Variables are Accessible
1. Local Variables (Block Scope)
Declared inside a function or block, accessible only within that block
#include <stdio.h>
void demoFunction() {
int localVar = 10; // Local variable
printf("Inside function: %d\n", localVar);
}
int main() {
demoFunction();
// printf("%d", localVar); // ERROR: localVar not
accessible here
return 0;
}
2. Global Variables (File Scope)
Declared outside all functions, accessible throughout the file
#include <stdio.h>
// Global variable
int globalCount = 0;
void increment() {
globalCount++;
printf("Count: %d\n", globalCount);
}
int main() {
increment();
increment();
printf("Final count: %d\n", globalCount);
return 0;
}
Output:
Count: 1
Count: 2
Final count: 2
Topic 3: Practical Programming Examples
Example 1: Basic Arithmetic Calculator
#include <stdio.h>
int main() {
int num1, num2;
int sum, difference, product;
float quotient;
printf("Enter two integers: ");
scanf("%d %d", &num1, &num2);
// Calculations
sum = num1 + num2;
difference = num1 - num2;
product = num1 * num2;
quotient = (float)num1 / num2; // Type casting for
decimal result
printf("\nResults:\n");
printf("Sum: %d\n", sum);
printf("Difference: %d\n", difference);
printf("Product: %d\n", product);
printf("Quotient: %.2f\n", quotient);
return 0;
}
Example 2: Temperature Converter
#include <stdio.h>
int main() {
float celsius, fahrenheit;
int choice;
printf("Temperature Converter\n");
printf("1. Celsius to Fahrenheit\n");
printf("2. Fahrenheit to Celsius\n");
printf("Enter your choice (1 or 2): ");
scanf("%d", &choice);
if (choice == 1) {
printf("Enter temperature in Celsius: ");
scanf("%f", &celsius);
fahrenheit = (celsius * 9/5) + 32;
printf("%.2f°C = %.2f°F\n", celsius,
fahrenheit);
} else if (choice == 2) {
printf("Enter temperature in Fahrenheit:
");
scanf("%f", &fahrenheit);
celsius = (fahrenheit - 32) * 5/9;
printf("%.2f°F = %.2f°C\n", fahrenheit,
celsius);
} else {
printf("Invalid choice!\n");
}
return 0;
}
Example 3: Student Grade Calculator
#include <stdio.h>
int main() {
char studentName[50];
float marks[5];
float total = 0, average;
int i;
printf("Enter student name: ");
scanf("%s", studentName);
printf("Enter marks for 5 subjects:\n");
for(i = 0; i < 5; i++) {
printf("Subject %d: ", i+1);
scanf("%f", &marks[i]);
total += marks[i];
}
average = total / 5;
printf("\n--- Student Report ---\n");
printf("Name: %s\n", studentName);
printf("Total Marks: %.2f/500\n", total);
printf("Average: %.2f\n", average);
if(average >= 90) {
printf("Grade: A\n");
} else if(average >= 80) {
printf("Grade: B\n");
} else if(average >= 70) {
printf("Grade: C\n");
} else if(average >= 60) {
printf("Grade: D\n");
} else {
printf("Grade: F\n");
}
return 0;
}
Topic 4: Common Errors and Debugging
Error 1: Uninitialized Variables
#include <stdio.h>
int main() {
int uninitializedVar; // Contains garbage value
printf("Garbage value: %d\n", uninitializedVar); //
Unexpected output
return 0;
}
Error 2: Using Wrong Format Specifiers
#include <stdio.h>
int main() {
int age = 25;
// Wrong: using %f for integer
printf("Age: %f\n", age); // Undefined behavior
// Correct: using %d for integer
printf("Age: %d\n", age);
return 0;
}
Error 3: Case Sensitivity Issues
#include <stdio.h>
int main() {
int studentAge = 20;
// Wrong: different case
printf("Age: %d\n", StudentAge); // Compilation error
// Correct: same case
printf("Age: %d\n", studentAge);
return 0;
}
Part 5: Hands-On Practice Exercises
Exercise 1: Simple Interest Calculator
Problem: Write a program to calculate simple interest using the formula: SI = (P × R × T) / 100
Requirements: Take principal amount, rate of interest, and time as input from user
Exercise 2: Area Calculator
Problem: Write a program that calculates area of circle, rectangle, and triangle based on user choice
Formulas:
- Circle: πr²
- Rectangle: length × width
- Triangle: ½ × base × height
Exercise 3: BMI Calculator
Problem: Calculate Body Mass Index using formula: BMI = weight(kg) / (height(m))²
Categories:
- Underweight: BMI < 18.5
- Normal: 18.5 ≤ BMI < 25
- Overweight: 25 ≤ BMI < 30
- Obese: BMI >= 30
Part 6: Summary and Key Takeaways
What We Covered Today:
- Identifiers: Rules for naming variables, functions, and other program elements
- Variable Scope: Local vs global variables and their accessibility
- Practical Programming: Real-world examples with input/output operations
- Error Handling: Common mistakes and how to avoid them
Key Rules to Remember:
- Identifiers must start with letter/underscore, contain only letters/numbers/underscores
- Identifiers are case-sensitive
- Always initialize variables before use
- Use proper format specifiers in printf/scanf
- Follow naming conventions for readable code
Topic 3: Operators in C
Operators are symbols that tell the compiler to perform specific mathematical or logical operations on variables and values.
1. Arithmetic Operators
Used for basic mathematical operations: + - * / %
#include <stdio.h>
int main() {
int a = 10, b = 3;
printf("a + b = %d\n", a + b);
printf("a - b = %d\n", a - b);
printf("a * b = %d\n", a * b);
printf("a / b = %d\n", a / b); // integer division
printf("a %% b = %d\n", a % b); // remainder
return 0;
}
a + b = 13
a - b = 7
a * b = 30
a / b = 3
a % b = 1
2. Relational and Logical Operators
Relational: <, <=, >, >=, ==, !=
Logical: && (AND), || (OR), ! (NOT)
int x = 5, y = 10;
printf("x < y = %d\n", x < y);
printf("x == y = %d\n", x == y);
printf("(x < y) && (y > 0) = %d\n", (x < y) && (y >
0));
x < y = 1
x == y = 0
(x < y) && (y > 0) = 1
3. Assignment Operators
Used to assign values: =, +=, -=, *=, /=, %=
int a = 10;
a += 5; // a = a + 5 (now 15)
a *= 2; // a = a * 2 (now 30)
4. Increment / Decrement Operators
++ increases value by 1, -- decreases by 1.
They can be prefix (++x) or postfix (x++).
int x = 5;
printf("%d\n", ++x); // 6 (prefix: increment then use)
printf("%d\n", x++); // 6 (postfix: use then increment)
printf("%d\n", x); // 7
5. Bitwise Operators
Work at the binary level:
& (AND), | (OR), ^ (XOR), ~ (NOT), << (Left Shift),
>> (Right Shift)
int a = 5, b = 3;
printf("a & b = %d\n", a & b); // 0101 & 0011 = 0001
printf("a | b = %d\n", a | b); // 0101 | 0011 = 0111
printf("a ^ b = %d\n", a ^ b); // 0101 ^ 0011 = 0110
printf("~a = %d\n", ~a);
printf("a << 1 = %d\n", a << 1); // shift left
printf("b >> 1 = %d\n", b >> 1); // shift right
6. Ternary Operator
Shorthand for if-else:
condition ? expression1 : expression2
int age = 20;
printf("%s\n", (age >= 18) ? "Adult" : "Minor");
Adult
💡 Practice Tip
Try combining operators. For example, check if a number is even
using both modulo and ternary operator:
printf("%s", (num % 2 == 0) ? "Even" : "Odd");