📌 Constants and Variables in C
🔹 1. What is a Variable?
A variable is like a labeled container in your computer’s memory that can store data which can change while the program runs.
- Variables have a name.
- Variables have a data type (like int, float, char).
- Variables can be changed or updated during program execution.
int age = 20; // 'age' is a variable of type int
float pi = 3.14; // 'pi' is a variable of type float
char grade = 'A'; // 'grade' is a variable of type char
You can change the value:
age = 25; // Value of 'age' updated to 25
🔹 2. What is a Constant?
A constant is a container whose value cannot be changed once assigned. It's useful when you want a value to stay the same throughout the program.
Types of Constants in C:
- Literal Constants: Direct values like 10, 'A', 3.14
- Defined Constants: Using
#define - Constant Variables: Using the keyword
const
Literal constant:
printf("The number is %d\n", 100); // 100 is a constant literal
Defined constant:
#define PI 3.14159
printf("PI = %f\n", PI);
Constant variable:
const int MAX_AGE = 100;
// MAX_AGE cannot be changed later in the code
💡 Why Should You Use Constants in C?
- Avoid Mistakes:- Using constants helps protect important values from being changed by accident in your code. Once a constant is defined, it cannot be changed later — this keeps your program safe from unexpected bugs.
- Make Code Clear:- Constants give meaningful names to values, which makes your code easier to read. Instead of seeing a random number like 3.14159, you can use a named constant like PI — and instantly know what it means.
- Easier to Update:- If you ever need to change the value, you only have to do it in one place. For example, if you define a tax rate as a constant, you can update it once and the whole program will use the new value automatically.
#include <stdio.h>
#define PI 3.14159
int main() {
float radius = 5.0;
float area = PI * radius * radius;
printf("Area of the circle: %.2f\n", area);
return 0;
}
🔹 4. Difference Between Variables and Constants
| Feature | Variable | Constant |
|---|---|---|
| Can value change? | Yes | No |
| Syntax | int age = 20; |
const int MAX = 50; or #define MAX 50 |
| Usage | Store data that changes | Store fixed values |
| Memory | Allocated storage | Fixed at compile time |
🔹 5. Example Program Using Both
#include <stdio.h>
#define PI 3.14159
int main() {
const int maxStudents = 30;
int currentStudents = 25;
printf("Maximum students allowed: %d\n", maxStudents);
printf("Current students enrolled: %d\n", currentStudents);
currentStudents = 28; // Allowed, variable can change
// maxStudents = 35; // Error! Cannot change a constant
printf("Updated current students: %d\n", currentStudents);
printf("Value of PI is: %f\n", PI);
return 0;
}
🔑 Summary
- Variables store data that can change.
- Constants store data that never changes.
- Use
constkeyword or#defineto create constants. - Using constants makes your program safer and easier to understand.
Frequently Asked Questions – Constants and Variables in C
Q: What is a variable in C?
A: A variable is like a box that stores data which can change while the program runs.
Q: What is a constant in C?
A: A constant is a fixed value that cannot be changed after it is set.
Q: How do I declare a variable in C?
A: You declare it using a data type and a name. Example: int age = 20;
Q: How do I declare a constant in C?
A: Use const keyword or #define. Example: const int MAX = 100; or #define PI 3.14
Q: Can I change the value of a variable?
A: Yes! Variables are meant to store data that can be updated anytime.
Q: Can I change the value of a constant?
A: No. Constants cannot be changed after they are initialized.
Q: What's the difference between const and #define?
A: const has a data type and is checked by the compiler. #define is a preprocessor macro and does not have a data type.
Q: Which is better to use: const or #define?
A: Use const for type safety and #define for simple constants like PI or limits.
Q: Can I use variables in a #define statement?
A: No. #define cannot use variables since it’s processed before the actual code runs.
Q: What happens if I try to change a constant?
A: The compiler will throw an error, preventing the change.
Q: Why should I use constants?
A: To avoid accidental changes, improve readability, and make code maintenance easier.
Q: Can I declare a constant without assigning it a value?
A: No. Constants must be initialized when declared. Example: const int MAX = 100;
Q: Can I use constants in math operations?
A: Yes. Constants work just like normal values in calculations.
Q: Can I declare a constant inside a function?
A: Yes. Constants can be declared inside functions just like variables.
Q: What is a literal constant?
A: It’s a value written directly in the code like 10, 'A', or 3.14.
Q: Are constants faster than variables?
A: Slightly, since their value is fixed at compile time, but in most cases, the difference is minor.
Q: Can constants be arrays?
A: Yes. You can use const with arrays. Example: const int arr[] = {1, 2, 3};
Q: Can I create global constants?
A: Yes. Constants can be declared outside main() to be used globally.
Q: What happens if I use an uninitialized variable?
A: It may cause unexpected results. Always initialize your variables before using them.
Q: What are common mistakes with constants and variables?
A: Forgetting initialization, trying to change constants, using wrong data types, or confusing #define with const.