Variable Declaration and Initialization in C: A Comprehensive Guide

C variables are names used for storing a data value to locations in memory. The value stored in the c variables may be changed during program execution.

C is a strongly typed language. Every variable must be declared, indicating its data type before it can be used. The declaration can also involve explicit initialization, giving the variable a value; a variable that is declared but not explicitly initialized is of uncertain value (and should be regarded as dangerous until it is initialized). In K&R C, declarations must precede all other statements, but in modern versions of C, this rule is relaxed so that you don't have to declare a variable until just before you start using it:

The basic form of a variable declaration is shown here:

type identifier [ = value][, identifier [= value] ...] ; 

• The type is one of C's data types like int, chat, double etc.
• The identifier is the name of the variable. You can initialize the variable by specifying an equal sign and a value. Keep in mind
• You can initialize the variable by specifying an equal sign and a value. Keep in mind that the initialization expression must result in a value of the same (or compatible) type as that specified for the variable.
• To declare more than one variable of the specified type, use a comma-separated list.

Declaration of Variable

Declaration of variable in c can be done using following syntax:

data_type variable_name;
or
data_type variable1, variable2,…,variablen;

where data_type is any valid c data type and variable_name is any valid identifier.

For example,

int a;
float variable;
float a, b;

Initialization of Variable

C variables declared can be initialized with the help of assignment operator ‘=’.

Syntax

data_type variable_name=constant/literal/expression;
or
variable_name=constant/literal/expression;

Example:

 int a=10;
 int a=b+c;
 a=10;
 a=b+c;

Multiple variables can be initialized in a single statement by single value, for example, a=b=c=d=e=10;

NOTE: C variables must be declared before they are used in the c program. Also, since c is a case-sensitive programming language, therefore the c variables, abc, Abc and ABC are all different.

int Variable Declaration and Variable Initialization in two steps:

 <span class="pln">
</span><span class="com">/*
  C program to demonstrate Declaration and initialization
  C program to demonstrate int data type
*/</span><span class="pln">

</span><span class="com">#include</span><span class="str">&lt;stdio.h&gt;</span><span class="pln">
</span><span class="kwd">void</span><span class="pln"> main</span><span class="pun">()</span><span class="pln">
</span><span class="pun">{</span><span class="pln">
   </span><span class="kwd">int</span><span class="pln"> age</span><span class="pun">;</span><span class="pln">  </span><span class="com">// Declaration of variable age</span><span class="pln">
   age </span><span class="pun">=</span><span class="pln"> </span><span class="lit">20</span><span class="pun">;</span><span class="pln">  </span><span class="com">// initialization of the variable age</span><span class="pln">
   printf</span><span class="pun">(</span><span class="str">"%d \n"</span><span class="pun">,</span><span class="pln">age</span><span class="pun">);</span><span class="pln">
</span><span class="pun">}</span>

20
Press any key to continue . . .

int Variable Declaration and Variable Initialization in one line:

<span class="pln">
</span><span class="com">/*
  C program to demonstrate Declaration and initialization
  C program to demonstrate int data type
*/</span><span class="pln">

</span><span class="com">#include</span><span class="str">&lt;stdio.h&gt;</span><span class="pln">
</span><span class="kwd">void</span><span class="pln"> main</span><span class="pun">()</span><span class="pln">
</span><span class="pun">{</span><span class="pln">
   </span><span class="kwd">int</span><span class="pln"> age</span><span class="pun">=</span><span class="lit">20</span><span class="pun">;</span><span class="pln">  </span><span class="com">// Declaration and initialization of variable age     </span><span class="pln">
   printf</span><span class="pun">(</span><span class="str">"%d \n"</span><span class="pun">,</span><span class="pln">age</span><span class="pun">);</span><span class="pln">
</span><span class="pun">}</span>

20
Press any key to continue . . .

Multiple variables can be initialized in a single statement by single value

 a = b = c = d = e = 10;

<span class="pln">
</span><span class="com">/*
  C program to demonstrate Declaration and initialization
  C program to demonstrate int data type
*/</span><span class="pln">

</span><span class="com">#include</span><span class="str">&lt;stdio.h&gt;</span><span class="pln">
</span><span class="kwd">void</span><span class="pln"> main</span><span class="pun">()</span><span class="pln">
</span><span class="pun">{</span><span class="pln">
      
   </span><span class="kwd">int</span><span class="pln"> a</span><span class="pun">,</span><span class="pln"> b</span><span class="pun">,</span><span class="pln"> c</span><span class="pun">,</span><span class="pln"> d</span><span class="pun">,</span><span class="pln"> e</span><span class="pun">;</span><span class="pln"> </span><span class="com">// Declaration of variable  </span><span class="pln">
    a</span><span class="pun">=</span><span class="pln">b</span><span class="pun">=</span><span class="pln">c</span><span class="pun">=</span><span class="pln">d</span><span class="pun">=</span><span class="pln">e</span><span class="pun">=</span><span class="lit">10</span><span class="pun">;</span><span class="pln"> </span><span class="com">// initialization of variable  </span><span class="pln">
   printf</span><span class="pun">(</span><span class="str">"%d \n"</span><span class="pun">,</span><span class="pln">a</span><span class="pun">);</span><span class="pln">
   printf</span><span class="pun">(</span><span class="str">"%d \n"</span><span class="pun">,</span><span class="pln">b</span><span class="pun">);</span><span class="pln">
   printf</span><span class="pun">(</span><span class="str">"%d \n"</span><span class="pun">,</span><span class="pln">c</span><span class="pun">);</span><span class="pln">
   printf</span><span class="pun">(</span><span class="str">"%d \n"</span><span class="pun">,</span><span class="pln">d</span><span class="pun">);</span><span class="pln">
   printf</span><span class="pun">(</span><span class="str">"%d \n"</span><span class="pun">,</span><span class="pln">e</span><span class="pun">);</span><span class="pln">
</span><span class="pun">}</span>

10
10
10
10
10
Press any key to continue . . .

Point To remember

• Variables should be declared in the C program before to use.
• Memory space is not allocated for a variable while declaration. It happens only on the variable definition.
• Variable initialization means assigning a value to the variable.