What is a Constant?

A constant is a named value that should not change during the execution of a program.

In simple words, a constant is like a fixed value stored with a name. Once the value is assigned, it is expected to remain the same throughout the program.

For example, the number of days in a week is always 7. If a program needs this value, we can store it as a constant named DAYS_IN_WEEK.

Easy Real-Life Example

A constant works in a similar way. It stores a fixed value, and the program should not change that value later.

Why are Constants Important?

Constants are important because some values in a program should remain fixed. If such values are accidentally changed, the program may produce incorrect results.

Simple Example of a Constant

The following pseudocode shows how a constant can be used.

CONSTANT DAYS_IN_WEEK = 7

DISPLAY DAYS_IN_WEEK

Expected Output

7

Here, DAYS_IN_WEEK is a constant because the number of days in a week should not change during program execution.

Variable vs Constant

Variables and constants both store values, but the major difference is whether the value can change.

Constants Store Fixed Data

Constants are commonly used to store values that have a fixed meaning in the program.

CONSTANT PASS_MARK = 35
CONSTANT DAYS_IN_WEEK = 7
CONSTANT PI = 3.14159
CONSTANT MAX_LOGIN_ATTEMPTS = 3

These values are important program rules. If they are changed accidentally, the program may behave incorrectly.

Common Examples of Constants

Constants are useful in many real-world programs.

Constant Declaration

Constant declaration means creating a constant and assigning its fixed value.

Different programming languages use different syntax for constants. In this course, we will use language-neutral pseudocode.

CONSTANT CONSTANT_NAME = value

Example

CONSTANT PASS_MARK = 35
CONSTANT COURSE_NAME = "Programming Mastery"

Here, PASS_MARK and COURSE_NAME are constants.

Constants Should Not Be Changed

Once a constant is defined, its value should not be changed later in the program.

Correct Usage

CONSTANT PASS_MARK = 35

IF marks >= PASS_MARK THEN
    DISPLAY "Pass"
ELSE
    DISPLAY "Fail"
END IF

Incorrect Idea

CONSTANT PASS_MARK = 35

SET PASS_MARK = 40

This is not correct because a constant should not be reassigned after it is defined.

Constants Make Code More Readable

Constants help explain the meaning of fixed values. This makes the program easier to understand.

Without Constant

IF marks >= 35 THEN
    DISPLAY "Pass"
END IF

Here, the value 35 is used directly. A beginner may not immediately know what 35 means.

With Constant

CONSTANT PASS_MARK = 35

IF marks >= PASS_MARK THEN
    DISPLAY "Pass"
END IF

This version is easier to understand because PASS_MARK clearly explains the purpose of the value.

Magic Numbers and Constants

A magic number is a number used directly in code without explaining its meaning.

Magic numbers can make code confusing. Constants help remove magic numbers by giving meaningful names to fixed values.

Constants Make Maintenance Easier

If a fixed value is used many times in a program, constants make updates easier.

Without Constant

SET tax1 = price1 * 0.18
SET tax2 = price2 * 0.18
SET tax3 = price3 * 0.18

If the tax rate changes, the programmer must update every place where 0.18 is used.

With Constant

CONSTANT TAX_RATE = 0.18

SET tax1 = price1 * TAX_RATE
SET tax2 = price2 * TAX_RATE
SET tax3 = price3 * TAX_RATE

Now the tax rate is defined in one place. If it changes, the programmer updates only the constant value.

Constants Improve Safety

Constants protect important values from accidental modification.

For example, if MAX_LOGIN_ATTEMPTS is accidentally changed during program execution, the login security logic may behave incorrectly.

CONSTANT MAX_LOGIN_ATTEMPTS = 3

IF attempts > MAX_LOGIN_ATTEMPTS THEN
    DISPLAY "Account locked"
END IF

Using a constant makes it clear that MAX_LOGIN_ATTEMPTS is a fixed rule.

Constants Can Store Different Data Types

Constants are not limited to numbers. They can store different kinds of fixed values.

CONSTANT PASS_MARK = 35
CONSTANT PI = 3.14159
CONSTANT COURSE_NAME = "Programming Mastery"
CONSTANT IS_PUBLIC_COURSE = true
CONSTANT DEFAULT_LANGUAGE = "English"

Naming Constants

Constants should have clear and meaningful names. Many programmers write constant names in uppercase letters to show that the value should not change.

Types of Constants by Purpose

Constants can be grouped based on how they are used in a program.

Complete Example Using Constants

The following language-neutral example shows constants used in a student result program.

/*
This program checks whether a student passed or failed.
*/

CONSTANT PASS_MARK = 35
CONSTANT SUBJECT_COUNT = 3

ENTRY POINT
    INPUT studentName
    INPUT mathMarks
    INPUT scienceMarks
    INPUT englishMarks

    SET totalMarks = mathMarks + scienceMarks + englishMarks
    SET averageMarks = totalMarks / SUBJECT_COUNT

    IF averageMarks >= PASS_MARK THEN
        SET result = "Pass"
    ELSE
        SET result = "Fail"
    END IF

    DISPLAY studentName
    DISPLAY totalMarks
    DISPLAY averageMarks
    DISPLAY result
END ENTRY POINT

Constant Breakdown

How Constants Help Debugging

Constants help debugging because fixed values are named clearly and defined in one place.

Best Practices for Constants

Good constant usage makes programs cleaner, safer, and easier to update.

Common Beginner Mistakes

Prerequisites Before Learning Constants

To understand constants properly, students should know a few basic programming concepts.

Practice Activity: Identify Constants

This activity helps students identify constants and understand why they are useful.

Task

CONSTANT TAX_RATE = 0.18
CONSTANT DELIVERY_CHARGE = 50

ENTRY POINT
    INPUT productPrice

    SET taxAmount = productPrice * TAX_RATE
    SET finalAmount = productPrice + taxAmount + DELIVERY_CHARGE

    DISPLAY finalAmount
END ENTRY POINT

Sample Answer

Mini Quiz

Interview Questions on Constants

Quick Summary