Lecture 3 – Variables and Constants


Variables and Constants
While writing a program you need some place to put your data on. For that we need variables and constants.

Variable

A variable is a named memory location, whose contents can be changed during the course of the program. For example a, x, y, k.
There are two types of variable in C.
1.      Numeric
a.       Integer
An integer variable is a variable which has NO fractional part. For example a=5, b=6 etc. We will be using two types of integer variable during our course. int for integer and short for short integer. Integer takes 2-bytes whereas Short takes 1-byte in memory.
·         Integer can handle values from -32768 to 32767
·         Short can handle values from -128 to 127
b.      Floating point
An floating point variable is a variable which has fractional part. For example a=5.6, b=6.134, PI=3.1416 etc. Float takes 4-bytes in memory.
2.      Character
Character variable is used to store characters in memory. A single character can store only one character in memory. If we need to store things like your name, we need to create a character array. Arrays are beyond the scope of our current course, they will not be discussed.

Constant

Constant is a literal which can be assigned a value during its declaration and its value cannot be changed. For example you’re Date of birth, the value of PI, speed of light etc.

Variable Declaration

Variable declaration is the process of reserving space, for your variable in the memory. You must declare a variable before you can use it. If you do not declare a variable and try to use it, it would be a syntax error. Declare variables’ at the start of main function. The general declaration syntax is

<datatype> <variablename>;    e.g. à  int a;      or            float b;                 or            char c;


We can declare multiple variables of same data type like
int a, b, c;                                            // a, b and c all are of integer data type
int x, y, z; float k;                              // x, y, z are of type integer, and k is a type of float

 

Rules for naming a variable

1.       Variable name must
a.       start with an alphabet or underscore
b.      be less than 31 characters
c.       be declared before it can be used
2.       Variable name cannot
a.       start with any digit
b.      contain any spaces
c.       contain any special character(s)
d.      be declared more than once
e.      be a reserved word
f.        be a keyword

 

Variable initialization

Assigning first value to a variable is called variable initialization. A variable can be assigned a value using the ‘=’ operator. For example
int a;      // variable declaration
a=5;       // variable initialization
We can use compound assignment operator to initialize multiple variables. E.g
a=b=c=5;                             // firstly c = 5 will be evaluated, then b=c and lastly a =b
We can declare and initialize a variable at the same time. E.g.
int z=0;
similarly multiple variables can be declared and initialized at the same time. E.g.
int a, b, c=5;        // only c will be initialized because no value is specified for a and b
int a=5, b=10, c=15;         // a will be initialized with 5, b with 10 and c with 15

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