4(a): ***(Syntax)*** Constants, Variables and Keywords

 

Keywords:

              Keywords are reserved words whose meaning is already known to the compiler and they can not be used elsewhere in the program for naming a variable or a function. 

==> List of C++ keywords is: https://en.cppreference.com/w/cpp/keyword

Constants:

Constants are very similar to a variable and they can also be of any data type. The only difference between a constant and a variable is that a constant’s value never changes.

Types of constants:

        There are three types of constants:

1.   Integer Constants ===================>    -1,    6,    7,    9  (all non-fraction numbers)
2.   Real Constants =====================>    -30.1,     2.5,    7.0   (all fraction numbers)
3.   Character Constants =================>      'a'     '$'     '@'     '+'    (letters or characters  enclosed within single inverted commas)

String Laterals (String Constants):

They are a sequence of characters enclosed in double quotation marks(" "). For example,  “This is a string literal!” is a string literal or just string. Escape sequences are also string literals.

If you write const before a variable, it will become constant for rest of the program and changing it will cause error.

#include <iostream>

using namespace std;

 

int main()

{

    const float PI = 3.14;      //PI becomes constant

    cout << "The value of PI is " << PI << endl;

    PI = 3.00;                //error, since changing a const variable is not allowed.

}

Thus output will be:

error: assignment of read-only variable 'PI'

C++ Variables:

Variables are containers for storing data values. For example:

                a = 3;           //  a is assigned "3"

                b = 4.7;       //   b is assigned "4.7"

                c = 'A' ;       //  c is assigned 'A'

Rules for naming a Variable:

1. A variable name can only contain alphabets, digits, and underscores( _ ).
2. No commas, blanks, or special characters other than underscore ( _ ) are allowed.
3. Variable names are case-sensitive ie Abc and abc are two different variables.

We must create meaningful variable names in our programs. This increases the readability of our program.

Types of Variables:

Various types of integers and their writing format in C are as follow:

1. Integer variables ======> int a = 3;                                           numbers without decimal
2. Real variables ========> float a = 7.7;    int a=7.7; is wrong   decimal number of low precision
3. Character variable ====> char a = 'B';        add ''                       single character i.e '+' or 'A'
4. Double variable =======> double a = 3.1412653589            decimal number of high precision   
5. Boolean variable ======> bool a = true                                  store true or false, 0 or 1
6. String variable ========> string a = "This is my name"

Declaration of Variable:

We cannot declare a variable without specifying its data type. The data type of a variable depends on what we want to store in the variable and how much space we want it to hold. The syntax for declaring a variable is simple:

data_type  variable_name = value;

For example:

    int a=3;

    float b=4.5;

    char c='A';

    double d=3.14159265;

    bool e=true;

Note:

Taking full line as input and printing it in 4(b) ie next post