Basics of C++

Let us talk about some basics of the language. 

Identifiers

 All variables and functions and objects need a name - they need identifiers. Identifiers in C++ need to follow some simple rules

•      They need to start with either a-z/A-Z or _ (underline)
•      They can contain only alphabets, digits or underline
•      They need to be unique in their scope
•      They can not be keywords

Examples of some bad identifiers
   1var
   sum of numbers
   int

Data types : 

C++ has the following basic data types. We will talk about classes and objects later

•     int
•     float
•     char
•     double
•     bool

int

An int data type is used to store integer values.

e.g.
 int a=10; int b = -100; 

 float and double

 are used to store real numbers. double gives a better accuracy than float. float numbers are stored using 4 bytes and double are stored using 8 bytes.

e.g.
     float m = 1.9f;  double c = 22.3;

We specify f or F at the end of float literal (constant) to differentiate them from double literals .

char

char variables store a single character. They store h not the complete hello world. 

Character literals are enclosed in single quotes. 

Words - character strings are stored using array of characters or an object of string class.  

char ch = 'A';ch = ch+10;

The second statement above tells us that the character literals can be treated as numbers. How is that done? Internally a character is stored as its ASCII code - e..g. A is stored as number 65. B as 66 and so on.

bool 

data type is used to store boolean values true or false.

bool m = true;bool n = false;

bool data type is not present in C, but only present in C++.

Also remember that, bool values are read and displayed as int values 1 and 0.

Variables and their definitions

When you define a variable, you need to specify its name, its datatype and optional initializer. Undefined variables give compilation error. Variable names should be unique in their scope.

int m;
char ch;
float a,b,c;
double d =12;
float r = 1.2f;

(By now, you have noticed that each statement in C++ must end with a semicolon)

Prefix 0 and 0x

An integer literal starting with letter 0 is treated as octal number and a number starting with 0x is treated as hexadecimal integer.

int m = 033; //m is 27
int n = 0x12;//n is 18

Let us write a simple program to read two numbers and print their product.

#include<iostream>
using namespace std;
int main()
{
    int a,b;
    cin>>a>>b;
    cout<<"Product of "<<a<<"and "<<b<<" is"<<a*b<<endl;
    float c = 100;
    cout<<"The square of number is "<<c*c<<endl;
    bool d = false;
    cout<<"The boolean varaible is "<<d<<endl;
    char ch ='A';
    cout<<"ch is "<<ch<<endl;
    return 0;
}

Note : cin is used to read a value from console and cout is used to display a value on the console.

The output of this program after 11 and 22 are given as input is

Product of 11and 22 is242
The square of number is 10000
The boolean varaible is 0
ch is A