Constants in Programming

What is a Constant?

A constant is a value that cannot be changed after it is assigned.

Constants are useful when you have a value that should stay the same while the program runs, like the value of PI, the number of days in a week, or a fixed URL.

Unlike variables, which can be updated, constants keep the same value once they are created:

PI = 3.14159
print(PI)

const PI = 3.14159;
console.log(PI);

final double PI = 3.14159;
System.out.println(PI);

const double PI = 3.14159;
cout << PI;

Why Use Constants?

Using constants makes your code easier to understand and maintain:

• Readability: Names like PI or DAYS_IN_WEEK explain what the number means.
• Safety: Constants protect important values from being accidentally changed.
• Easy updates: If you need to change a constant (like a tax rate), you only update it in one place.

Changing a Constant

Once a constant is assigned, trying to change it will usually cause an error or be ignored, depending on the language.

PI = 3.14159
PI = 3   # This works in Python, but it is not recommended
print(PI)

const PI = 3.14159;
PI = 3;   // Error: Assignment to constant variable

final double PI = 3.14159;
PI = 3;   // Error: cannot assign a value to final variable

const double PI = 3.14159;
PI = 3;   // Error: assignment of read-only variable

Constants in Practice

Here are some common examples of constants:

• PI = 3.14159   (The mathematical constant pi)
• DAYS_IN_WEEK = 7
• URL = "https://example.com"

DAYS_IN_WEEK = 7
print("A week has", DAYS_IN_WEEK, "days")

const DAYS_IN_WEEK = 7;
console.log("A week has " + DAYS_IN_WEEK + " days");

final int DAYS_IN_WEEK = 7;
System.out.println("A week has " + DAYS_IN_WEEK + " days");

const int DAYS_IN_WEEK = 7;
cout << "A week has " << DAYS_IN_WEEK << " days";