const
- C course
- Beginner
- Lesson 9/116
- 3 min
Learn to declare immutable variables with the const keyword.
The const keyword
The const keyword tells that a variable's value will not change during the program. After declaration, you cannot assign a new value:
const double PI = 3.14;
PI = 3.15; // errorConstants are named values that make code readable. Writing PI instead of 3.14 states your intent clearly. The compiler forbids accidental changes.
What is const for?
const serves three main purposes:
- Safety: prevents accidental changes
- Readability:
TAXis more meaningful than0.18 - Documentation: states clearly that the value is fixed
Use const for any value that should not change during the program. This conveys your intent to both the compiler and the reader.
Practice Compute with a constant
Define const double TAX = 0.18;. Set double price = 100.0; and print price * TAX with printf using %lf.
Starter code
// define const TAX, set price = 100.0, print with printfSample solution
#include <stdio.h>
int main(void) {
const double TAX = 0.18;
double price = 100.0;
printf("%lf", price * TAX);
return 0;
}Expected output
18.000000
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 After const double PI = 3.14;?
- You can assign any value to PI
- PI cannot be reassigned Correct answer
- PI is undefined
- PI automatically becomes 0
Explanation: A const value cannot be reassigned.
2 const type
const ____ PI = 3.14;const double PI = 3.14;Explanation: PI is a decimal value, so double is used.
3 Match the keywords
| Term | Meaning |
|---|---|
const |
Constant |
int |
Integer |
double |
Decimal |
char |
Character |