Skip to main content
Engineering LibreTexts

2.1: Constants and Variables

  • Page ID
    10616
  • 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:

    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 3.14159
    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.