Variables, expressions, and operators

Variables in C++ are used for storing values which are usually input, manipulated, and output. Variables must be explicitly declared as a specific data type (e.g. basic data types) before usage.

Identifiers

As in regular mathematics, variables have a name that we call an identifier in C++. Identifiers have various rules and conventions.

Identifier rules

Identifiers must adhere to the following rules:

• May only contain alphabetic characters, digits, and underscores
• May not contain any special characters or spaces
• May not start with a digit
• May not be a reserved word
• May not be identical to other identifiers in the same scope

Identifier conventions

Although the following are all legal, identifiers should typically:

• Not start with an underscore (these are frequently used for header files)
• Not be all capital letters, unless used as a constant
• Be descriptive about what is to be stored in them to help with code readability

Identifiers may, additionally, adhere to one of many established naming conventions.

Declaration

A variable can be declared as datatype identifier;

Multiple variables can be declared in one line, for example int x, y, z;

Initialization

Variables can be initialized upon declaration or later. Variables should always have some value placed in them (via input from the user and/or a file or via initialization) before they are used in any calculations or output, otherwise erroneous results may occur.

int age = 30; // Initialization upon declaration

double rate;
rate = 0.05;  // Initialization later

Assignment

Variables can be assigned values as such: x = expression;