2.1: Constants and Variables

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Overview

A constant is a value that cannot be altered by the program during normal execution, i.e., the value is constant. When associated with an identifier, a constant is said to be “named,” although the terms “constant” and “named constant” are often used interchangeably. This is contrasted with a variable, which is an identifier with a value that can be changed during normal execution, i.e., the value is variable.^[1]

Discussion

Understanding Constants

A constant is a data item whose value cannot change during the program’s execution. Thus, as its name implies – the value is constant.

A variable is a data item whose value can change during the program’s execution. Thus, as its name implies – the value can vary.

Constants are used in two ways. They are:

literal constant
defined constant

A literal constant is a value you type into your program wherever it is needed. Examples include the constants used for initializing a variable and constants used in lines of code:

21
12.34
'A'
"Hello world!"
false
null

In addition to literal constants, most textbooks refer to symbolic constants or named constants as a constant represented by a name. Many programming languages use ALL CAPS to define named constants.

Language	Example
C++	`#define PI 3.14159` or `const double PI = 3.14159;`
C#	`const double PI = 3.14159;`
Java	`const double PI = 3.14159;`
JavaScript	`const PI = 3.14159;`
Python	`PI = 3.14159`
Swift	`let pi = 3.14159`

Technically, Python does not support named constants, meaning that it is possible (but never good practice) to change the value of a constant later. There are workarounds for creating constants in Python, but they are beyond the scope of a first-semester textbook.

Defining Constants and Variables

Named constants must be assigned a value when they are defined. Variables do not have to be assigned initial values. Variables once defined may be assigned a value within the instructions of the program.

Language	Example
C++	`double value = 3;`
C#	`double value = 3;`
Java	`double value = 3;`
JavaScript	`var value = 3; let value = 3;`
Python	`value = 3`
Swift	`var value:Int = 3`

Key Terms

constant: A data item whose value cannot change during the program’s execution.

variable: A data item whose value can change during the program’s execution.

References

cnx.org: Programming Fundamentals – A Modular Structured Approach using C++

Wikipedia: Constant (computer programming) ↵