freeCodeCamp/guide/english/cplusplus/tokens-variables/index.md

title
Tokens Part 1

What are tokens ?

Tokens are the smallest units of a program which are important to the compiler. There are different kinds of tokens:

• Keywords
• Operators
• Punctuators
• Literals
• Identifiers

• Combination of tokens form an expression

What are Variables ?

• Textbook definition : Variables are named memory locations whose data can be altered.

• But I would like you to think of a variable to be something like a box, something like this :

So, for example : I'm shifting to a new place and I need to arrange my stuff in boxes . Thus there come 2 things to my mind What kind of stuff will be stored in the box, so that the size off the box is known (the data type) and How do I identify the box ?(Naming the variable)
Hence , we know that a variable in C++ needs a name and a data type and that the value stored in them can be changed.

Data Types in C++ :

When declaring variables in c++ they must have a name to which you will refer later on, a value (constant or not) and a type. The type will tell the compiler the values that the variable can use, the possible operations and will save a certain space in memory. In c++ there are two types of data:

• Simple type
• Struct type

Simple data types

• Boolean -- bool Boolean variable can store either true or false.
• Character -- char Stores a single character.
• Integer -- int Stores an integer.
• Floating point -- float They can use decimals.
• Double floating point -- double Double precision of the float type.

Here you can see some examples:

bool GameRunning = true;
char a;
int x = 2;

These types can also be modified with modifiers such as:

signed unsigned short long

Struct data type

Identifiers.

• Identifiers are the names given to a variable or a class or a function or any user defined function.

Rules for naming a variable :

• Start naming with a letter from A-Z or a-z .
• Numbers can follow the first letter but we cannot start naming with numbers.
• NO use of spaces or special characters are allowed, instead, use an UNDERSCORE _ .

Declaring a variable :

The syntax is as follows <data type> <variable name>; or <data type> <variable name> = <value>; if we also want to initialize the variable.

For example :

int a ;    //declaring a variable named 'a' of type integer.
a=4;        //initializing a variable
int b = 5 ; //declaring and initializing a variable 'b' of type integer.

Examples of declaring a variable:

int a9;
char A;
double area_circle;
long l;

Wrong ways to declare variables-

int 9a;
char -a;
double area of circle;
long l!!;

• Variable names cannot start with a number
• Special characters are not allowed
• Spaces are not allowed

You can imagine different boxes of different sizes and storing different things as different variables.

NOTES :

1. The C++ compiler ignores white spaces and they are generally used for beautification of the code so that it is easy for any programmer to debug or understand the code.
2. If a variable is not initialized , it contains a garbage value. Let me give an example:

Scope of Variables

All the variables have their area of functioning, and out of that boundary they don't hold their value, this boundary is called scope of the variable. For most of the cases its between the curly braces,in which variable is declared that a variable exists, not outside it. We will study the storage classes later, but as of now, we can broadly divide variables into two main types,

*Global Variables.

*Local variables.

Global variables

Global variables are those, which ar once declared and can be used throughout the lifetime of the program by any class or any function. They must be declared outside the main() function. If only declared, they can be assigned different values at different time in program lifetime. But even if they are declared and initialized at the same time outside the main() function, then also they can be assigned any value at any point in the program.

Example : Only declared, not initialized.

#include <iostream>
using namespace std;
int x;                // Global variable declared
int main()
{
 x=10;                 // Initialized once
 cout <<"first value of x = "<< x;
 x=20;                 // Initialized again
 cout <<"Initialized again with value = "<< x;
}

Local Variables

Local variables are the variables which exist only between the curly braces, in which its declared. Outside that they are unavailable and leads to compile time error.

Example :

#include <iostream>
using namespace std;
int main()
{
 int i=10;
 if(i<20)        // if condition scope starts
  {
    int n=100;   // Local variable declared and initialized
  }              // if condition scope ends
 cout << n;      // Compile time error, n not available here
}

Constant Variables

Constant variable are the variables which cannot be changed. For example, if you needed "pi" in your code, you would not want to change it after initialization.

Example :

#include <iostream>
using namespace std;
const double PI = 3.14159253;
int main()
{
//Calculating the area of a circle, using user provided radius
double radius;
//input and output explained in other guide
cin>>radius;
//pi*r^2
double area = PI*radius*radius;
cout<<area<<endl;
}

Garbage Values in a Variable

If a variable is not initialized , it contains a garbage value. For example:

So in terms of boxes, you can imagine this as -

#include<iostream>
using namespace std;
int main()
{
   int a ;
   cout<<"Garbage value in a : "<<a<<endl; //declaring the variable named 'a' of type integer 
   a=5; //initializing variable.
   cout<<"New value in a "<<a<<endl;

}

The output is :

Garbage value in a : 0
New value in a :  5

As you can see, there is already a value stored in 'a' before we give it a value(here , it is 0 ). This should remain in the mind of every programmer so that when the variables are used they do not create a logical error and print garbage values.

Try the code yourself ! :)

Keywords :

Keywords are reserved words that convey a special meaning to the compiler. They CANNOT be used for naming in c++. Examples of Keywords : inline , operator, private int, double ,void , char, template ,using , virtual , break , case , switch , friend, etc.

Each of these keywords is used for a special function in C++.

Tokens part 1 is over. See you campers at Part 2 of Tokens :)

Good Luck to all of you

Happy Coding ! :)

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