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C++ Comments & Variables - Notes | Study C++ Programming for Beginners - Class 8

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C++ Comments

Comments can be used to explain C++ code, and to make it more readable. It can also be used to prevent execution when testing alternative code. Comments can be singled-lined or multi-lined.

Single-line Comments

  • Single-line comments start with two forward slashes (//).
  • Any text between // and the end of the line is ignored by the compiler (will not be executed).
  • This example uses a single-line comment before a line of code:

Example

// This is a comment

cout << "Hello World!";

This example uses a single-line comment at the end of a line of code:
Example

cout << "Hello World!"; // This is a comment

C++ Multi-line Comments

Multi-line comments start with /* and ends with */.

Any text between /* and */ will be ignored by the compiler:

Example

/* The code below will print the words Hello World!

to the screen, and it is amazing */

cout << "Hello World!";

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, specify the type and assign it a value:

Syntax

type variableName = value;

Where type is one of C++ types (such as int), and variableName 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;

You can also declare a variable without assigning the value, and assign the value later:
Example

int myNum;

myNum = 15;

cout << myNum;

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

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)

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.";

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;

C++ Declare Multiple Variables

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

Example

int x = 5, y = 6, z = 50;

cout << x + y + z;

One Value to Multiple Variables

You can also assign the same value to multiple variables in one line:

Example

int x, y, z;

x = y = z = 50;

cout << x + y + z;

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).

Note: It is recommended to use descriptive names in order to create understandable and maintainable code:

Example

// Good

int minutesPerHour = 60;


// OK, but not so easy to understand what m actually is

int m = 60;

The general rules for naming variables 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

C++ Constants

When you do not want others (or yourself) to override existing variable values, use the const keyword (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'

You should always declare the variable as constant when you have values that are unlikely to change:
Example

  • const int minutesPerHour = 60;
  • const float PI = 3.14;
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