Programming Fundamentals/Constants and Variables

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Overview[edit]

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[edit]

Understanding Constants[edit]

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:

  1. literal constant
  2. 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 3.14159or

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[edit]

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[edit]

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[edit]