Variable in Programming C

Variables

In programming, a variable is a container (storage area) to hold data.

To indicate the storage area, each variable should be given a unique name (identifier). Variable names are just the symbolic representation of a memory location. For example:

int abc = 35;

Here, abc is a variable of int type. Here, the variable is assigned an integer value 35.

The value of a variable can be changed, hence the name variable.

char ch = 'a';
// some code
ch = 'l';

Rules for naming a variable

  1. A variable name can only have letters (both uppercase and lowercase letters), digits and underscore.
  2. The first letter of a variable should be either a letter or an underscore.
  3. There is no rule on how long a variable name (identifier) can be. However, you may run into problems in some compilers if the variable name is longer than 31 characters.

Note: You should always try to give meaningful names to variables. For example: firstName is a better variable name than fn.

C is a strongly typed language. This means that the variable type cannot be changed once it is declared. For example:

int number = 5;      // integer variable
number = 5.5;        // error
double number;       // error

Here, the type of number variable is int. You cannot assign a floating-point (decimal) value 5.5 to this variable. Also, you cannot redefine the data type of the variable to double. By the way, to store the decimal values in C, you need to declare its type to either double or float.

Literals

Literals are data used for representing fixed values. They can be used directly in the code. For example: 1, 2.5, ‘c’ etc.

Here, 1, 2.5 and ‘c’ are literals. Why? You cannot assign different values to these terms.

1. Integers

An integer is a numeric literal(associated with numbers) without any fractional or exponential part. There are three types of integer literals in C programming:

  • decimal (base 10)
  • octal (base 8)
  • hexadecimal (base 16)

For example:

Decimal: 0, -9, 22 etc
Octal: 021, 077, 033 etc
Hexadecimal: 0x7f, 0x2a, 0x521 etc

In C programming, octal starts with a 0, and hexadecimal starts with a 0x.

2. Floating-point Literals

A floating-point literal is a numeric literal that has either a fractional form or an exponent form. For example:

-2.0
0.0000234
-0.22E-5

Note: E-5 = 10-5

3. Characters

A character literal is created by enclosing a single character inside single quotation marks. For example: ‘a’, ‘m’, ‘F’, ‘2’, ‘}’ etc.

4. Escape Sequences

Sometimes, it is necessary to use characters that cannot be typed or has special meaning in C programming. For example: newline(enter), tab, question mark etc.

In order to use these characters, escape sequences are used.

Escape SequencesCharacter
\bBackspace
\fForm feed
\nNewline
\rReturn
\tHorizontal tab
\vVertical tab
\\Backslash
\'Single quotation mark
\"Double quotation mark
\?Question mark
\0Null character

For example: \n is used for a newline. The backslash \ causes escape from the normal way the characters are handled by the compiler.

5. String Literals

A string literal is a sequence of characters enclosed in double-quote marks. For example:

"good"                  //string constant
""                     //null string constant
"      "               //string constant of six white space
"x"                    //string constant having a single character.
"Earth is round\n"         //prints string with a newline
Constants

If you want to define a variable whose value cannot be changed, you can use the const keyword. This will create a constant. For example,

const double PI = 3.14;

Notice, we have added keyword const.

Here, PI is a symbolic constant; its value cannot be changed.

const double PI = 3.14;
PI = 2.9; //Error

Types of Variables in C

There are many types of variables in c:

  1. local variable
  2. global variable
  3. static variable
  4. automatic variable
  5. external variable
Local Variable

A variable that is declared inside the function or block is called a local variable.

It must be declared at the start of the block.

void shishir(){
int x=10;                      //local variable
}

You must have to initialize the local variable before it is used.

Global Variable

A variable that is declared outside the function or block is called a global variable. Any function can change the value of the global variable. It is available to all the functions.

It must be declared at the start of the block.

int value=20;   //global variable
void shishir(){
int x=10;    //local variable
}
Static Variable

A variable that is declared with the static keyword is called static variable.

It retains its value between multiple function calls.

void shishir(){
int x=10;      //local variable
static int y=10;   //static variable
x=x+1;
y=y+1;
printf("%d,%d",x,y);
}

If you call this function many times, the local variable will print the same value for each function call, e.g, 11,11,11 and so on. But the static variable will print the incremented value in each function call, e.g. 11, 12, 13 and so on.

Automatic Variable

All variables in C that are declared inside the block, are automatic variables by default. We can explicitly declare an automatic variable using auto keyword.

void main(){
int x=10;       //local variable (also automatic)
auto int y=20;  //automatic variable
}
External Variable

We can share a variable in multiple C source files by using an external variable. To declare an external variable, you need to use extern keyword.

myfile.h

extern int x=10;   //external variable (also global)

program1.c

#include "myfile.h"
#include
void printValue(){
printf("Global variable: %d", global_variable);
}
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