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C++ Variables


C++ Variables

Variables are containers for storing data values.

In C++, there are different types of variables (defined with different keywords), for example:

  • int - stores integers (whole numbers), without decimals, such as 123 or -123
  • double - stores floating point numbers, with decimals, such as 19.99 or -19.99
  • char - stores single characters, such as 'a' or 'B'. Char values are surrounded by single quotes
  • string - stores text, such as "Hello World". String values are surrounded by double quotes
  • bool - stores values with two states: true or false

Declaring (Creating) Variables

To create a variable, you must specify the type and assign it a value:

Syntax

type variable = value;

Where type is one of C++ types (such as int), and variable is the name of the variable (such as x or myName). The equal sign is used to assign values to the variable.

To create a variable that should store a number, look at the following example:

Example

Create a variable called myNum of type int and assign it the value 15:

int myNum = 15;
cout << myNum;
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You can also declare a variable without assigning the value, and assign the value later:

Example

int myNum;
myNum = 15;
cout << myNum;
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Note that if you assign a new value to an existing variable, it will overwrite the previous value:

Example

int myNum = 15;  // myNum is 15
myNum = 10;  // Now myNum is 10
cout << myNum;  // Outputs 10
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However, you can add the const keyword if you don't want others (or yourself) to override existing values (this will declare the variable as "constant", which means unchangeable and read-only):

Example

const int myNum = 15;  // myNum will always be 15
myNum = 10;  // error: assignment of read-only variable 'myNum'
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Other Types

A demonstration of other data types:

Example

int myNum = 5;               // Integer (whole number without decimals)
double myFloatNum = 5.99;    // Floating point number (with decimals)
char myLetter = 'D';         // Character
string myText = "Hello";     // String (text)
bool myBoolean = true;       // Boolean (true or false)

You will learn more about the individual types in the Data Types chapter.



Display Variables

The cout object is used together with the << operator to display variables.

To combine both text and a variable, separate them with the << operator:

Example

int myAge = 35;
cout << "I am " << myAge << " years old.";
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Add Variables Together

To add a variable to another variable, you can use the + operator:

Example

int x = 5;
int y = 6;
int sum = x + y;
cout << sum;
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Declare Many Variables

To declare more than one variable of the same type, you can use a comma-separated list:

Example

int x = 5, y = 6, z = 50;
cout << x + y + z;
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C++ Identifiers

All C++ variables must be identified with unique names.

These unique names are called identifiers.

Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).

The general rules for constructing names for variables (unique identifiers) are:

  • Names can contain letters, digits and underscores
  • Names must begin with a letter or an underscore (_)
  • Names are case sensitive ("myVar" and "myvar" are different variables)
  • Names cannot contain whitespaces or special characters like !, #, %, etc.
  • Reserved words (like C++ keywords, such as int) cannot be used as names