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Keywords in C

A keyword is a word that has a predetermined meaning. The C compiler knows the purposes of the keywords. The objectives of these keywords cannot be modified. Keywords are the fundamental blocks to writing program statements. Keywords are also known as Reserved words.

Rules for the keywords

  • Using Lowercase letters to write keywords.
  • Keywords are predefined that cannot be used as a variable name.

List of Keywords available in C language

There are 32 keywords available in the C language; these keywords have their functionality. A list of keywords is given below:

auto		double		int		struct
break		else		long		switch
case		enum		register	typedef
char		extern		return		union
const		float		short		unsigned
continue	for		signed		void
default	        goto		sizeof		volatile
do		if		static		while

They can be further classified based on their purpose:

  • Keywords for datatypes
  • Keywords for modifiers
  • Keywords for loops
  • Keywords for storage classes
  • Keywords for user-defined datatypes
  • Keywords for decision
  • Keywords for loop control
  • Keywords for derived datatypes
  • Keywords for function
  • Other keywords

1) Keywords for datatypes

  • int
  • char
  • float
  • double

int keyword: The int keyword defines integer-type variables that hold integer values.

Example

int id=10;

Here, id is the variable name, and int is the keyword that defines id as an integer variable. The id variable stores the integer value 10 in the above example.

char keyword: The char keyword defines character variables that hold characters.

Example

char ch=’a’;

Here, ch is the variable name, and char is the keyword that defines ch as a character variable. The character value ‘a’ is stored in the ch variable in the above example.

float keyword: The float keyword defines floating point variables with single precision floating point values.

Example

float b=2.25;

Here, b is the variable name, and float is the keyword that defines b as a floating-point variable. The b variable stores the value 2.25 in the above example.

double keyword: The double keyword defines floating point variables with double precision floating point values.

Example:

double t=2.4535;

Here, t is the variable name, and double is the keyword that defines t as a floating-point variable. The t variable stores the value 2.4535 in the above example.

2) Keywords for modifiers

  • signed
  • unsigned
  • short
  • long

signed keyword: The signed keyword describes a variable that can contain positive and negative values.

Example:

signed int z;

Here, z is the variable name, and signed is the keyword that defines z as a variable that can contain both positive and negative values.

unsigned keyword: The unsigned keyword describes a variable that can contain only positive values.

Example:

unsigned int y;

Here, y is the variable name, and unsigned is the keyword that defines y as a variable that can contain only positive values.

Correct usage of unsigned and signed modifiers

  • unsigned int z;
  • signed int z;
  • unsigned short int z;
  • unsigned long int z;
  • signed char z;
  • unsigned char z;

Incorrect usage of unsigned and signed modifiers

  • unsigned float z;
  • unsigned double z;

short keyword: The short keyword is used to specify a variable that can contain small values.

Example:

short int z;

Here, z is the variable name, and short is the keyword that defines z as a variable that can contain small values. The value z ranges from -32768 to +32767 in the above example.

long keyword: The long keyword is used to specify a variable that can contain large values.

Example:

long int z;

Here, z is the variable name, and long is the keyword that defines z as a variable that can contain large values. The value z ranges from -2147483648 to +2147483647 in the above example.

Correct usage of long and short modifiers

  • short int z;
  • short z;
  • long z;
  • long int z;
  • long double z;

Incorrect usage of long and short modifiers

  • short char z;
  • short float z;
  • short double z;
  • long short z;
  • long char z;
  • long float z;

3) Keywords for loops

  • for
  • while
  • do

for keyword: The for keyword is used to implement loops in which the number of iterations is already known.

Syntax

for(initialization; condition; updation){
//block of code
}

while keyword: The while keyword is used to implement loops in which the number of iterations is unknown.

Syntax

while(condition){
//block of code
}

do keyword: The do keyword is used to implement loops in which the number of iterations is unknown, and the loop must execute at least once.

Syntax

do{
//block of code
}while(condition)

4) Keywords for storage classes

  • auto
  • register
  • static
  • extern

auto keyword: The auto keyword defines variables as local variables. These variables are accessed within the function block only.

Variables are stored with garbage values unless explicitly initialized.

Syntax

auto <datatype> <variable_name>;

Example

auto int c;

Here, c is a variable defined as auto(storage class).

register keyword: The register keyword defines variables stored in the CPU registers instead of memory. Registers allow quick access to the register variables. Variables are stored with garbage values unless explicitly initialized.

Syntax

register <datatype> <variable_name>;

Example

register int x;

Here, x is a variable defined as register(storage class).

static keyword: The static keyword defines variables whose scope persists between multiple function calls. static variables are declared only once. The default value stored in these variables is zero.

Syntax

static <datatype> <variable_name>;

Example

static int c;

Here, c is a variable defined as static(storage class).

extern keyword: The extern keyword defines variables as global variables. These variables have their scope throughout the program.

The default value stored in these variables is zero.

Syntax

extern <datatype> <variable_name>;

Example

extern int b;

Here, b is a variable defined as extern(storage class).

5) Keywords for user-defined datatypes

  • enum
  • typedef

enum keyword: The enum keyword (Enumeration type) helps users define new datatypes built using existing datatypes. enum helps in creating customized datatypes. The user can determine what values are stored in the variable.

Syntax

enum <enum_variable> {list of constants};

Example

enum cars {Honda, Benz, Audi};

typedef keyword: The typedef keyword is an acronym for type definition. typedef keyword helps the user to give another name to an existing datatype. typedef allows users to access the variables faster, and it helps in better code readability.

Syntax

typedef <existing type> <alias name>

Example

typedef unsigned char name;
name a,b;

Here, variables a and b are defined with an alias name.

6) Keywords for decision

  • if
  • else
  • switch
  • case
  • default

if keyword: The if keyword is used to make a decision. It is used to check a particular condition, and if the condition is satisfied, it performs some operations.

Syntax

if(condition){
//block of code
}

Example

#include<stdio.h>
int main(){
int number=5;
if(number==5){
printf("%d ",number);
}
return 0;
}

Output

5

else keyword: The else keyword is used to make a decision. It is used when the if condition is not satisfied; else block cannot be used without the if block. if statement is extended to the if-else statement where two operations are performed for a single condition.

Syntax

if(condition){
//block of code
}
else{
//block of code
}

Example

#include<stdio.h>
int main(){
int number=4;
if(number==5){
printf("%d ",number);
}
else{
printf(“0”);
}
return 0;
}

Output

0

switch keyword: The switch keyword is used as an alternative for the if-else statement. It becomes hard to perform multiple operations using if-else statements. The switch keyword helps the user to perform one particular operation from all other operations based on the expression provided.

Syntax

switch(expression){
case a:
      //block of code
     break;
case b:
      //block of code
     break;
}

Here, the switch keyword is followed by an expression.

case keyword: The switch statement compares the expression with a list of values. These values are referred to as cases. case keyword is used to represent these values. A code block is executed if the expression matches the value represented by the case keyword.

Syntax

switch(expression){
case a:
      //block of code
     break;
case b:
      //block of code
     break;
}

Here, the case keyword represents the a and b values. These values are compared with the expression provided in the switch statement.

default keyword: In the switch statement, the default keyword can be used to perform a particular task when no cases are matched with the expression provided. It is an optional statement that runs when no case is checked.

Syntax

switch(expression){
case a:
      //block of code
     break;
case b:
      //block of code
     break;
default:
      //block of code
}

7) Keywords for loop control

  • break
  • continue
  • goto

break keyword: The break keyword is used for loop control. It terminates the execution of the loop. The break keyword breaks out from the nearest enclosing loop statements such as do, while, for, and switch. The control is passed to the following statement after the terminated statement.

Syntax

// inside switch case or loop
break;

Example

#include<stdio.h>
int main(){
int p; 
for (p = 0; p< 5; p++) 
{ 
if (p== 1) 
{ 
break; 
} 
printf("%d\n", p); 
}
return 0;
}

Output

0

In the above example, the break statement occurred when p==1. Therefore, the loop has terminated.

continue keyword: The continue keyword is used for loop control. It passes the control to the next iteration. The code inside the loop that follows the continue keyword will not be executed, and the next iteration of the current loop will be performed.

Syntax

continue;

Example

#include <stdio.h>
int main()
{
   for (int q=0; q<=3; q++)
   {
      if (q==1)
      {
	    continue;
       }
       printf("%d ", q);
   }
   return 0;
}

Output

0 2 3

In the above example, the continue statement occurred when q==1. Therefore, the current iteration is skipped and moved on to the next iteration.

goto keyword: The goto keyword is used for loop control. It is also referred to as the jump statement in C. goto statement transfers the program control to a statement related to a particular label. The goto keyword helps in the repetition of a specific code.

The goto keyword is not recommended in writing the code, making the program unreliable, unreadable, and hard to debug.

Syntax

label:   
//code block;   
goto label;  

Example

#include <stdio.h>
int main()
{
   int sum=0;
   for(int r = 0; r<=5; r++){
	sum = sum+r;
	if(r==3){
	   goto add;
	}
   }
   add:
   printf("%d", sum);


   return 0;
}

Output

6

In the above example, add is the label, and the goto statement is activated when r is equal to 3(r==3). The sum displays the sum of numbers till three even though the loop has the condition r<=5; this is because of the goto statement.

8) Keywords for derived datatypes

  • struct
  • union

struct keyword: The struct keyword is used to represent Structures. struct helps to store a group of several related variables in one place. A variable in a structure is referred to as a member of the Structure.

A structure can hold variables of different data types such as int, float, char, etc. Each member of the Structure is allocated a unique memory location.

Syntax

struct Structure_Name {
  datatype member1;
  datatype member2; 
  datatype member3   //any datatype
  …
};

Example

struct Vehicle{
  char car[50];
  int carNo;
  float cost;
};

union keyword: The union keyword is a datatype in C that allows storing different data types at the same memory location.

union keyword helps define many members, of which only one member can hold a value at a particular time. The union data members use the same storage space occupied by the largest data member.

Syntax

union Union_Name {
  datatype member1;
  datatype member2; 
  datatype member3   //any datatype
  …
};

Example

union Employee{
  char name[50];
  int id;
  float salary;
};

9) Keywords for function

  • void
  • return

void keyword: The void keyword is used to represent a datatype with no data. The dictionary meaning of void is blank or empty.

The void keyword can be used in 3 ways depending on its context.

  • No return type
  • No parameters
  • Pointer with no specific type

Example

#include <stdio.h>


void main()
{
   printf(“hello”);
 }

Output:

hello

return keyword: The return keyword is used to end the execution of a function. The control returns to the calling function by the return keyword.

The return statement may or may not have conditional statements. The control resumes in the calling function. A void function does not have any return value, but a non-void function must have a return value.

Syntax

 return (expression);

Example

#include <stdio.h> 
void display() // void method 
{ 
printf("hi"); 
} 
int main() // main method 
{ 
display(); // Calling display
return 0; 
}

Output

hi

10) Keywords for function

  • Const
  • Volatile
  • sizeof

const keyword: The const keyword is used to declare constant values.

Example

const int c=10;

volatile keyword: The volatile keyword represents values that can be changed due to interference from an outside code. The volatile keyword prevents the compiler from making changes to the volatile objects.

Syntax

volatile int a;

Here, ‘a’ is a volatile variable.

sizeof keyword: The sizeof keyword is used to represent the size of the data variable or a constant.

Example:

#include <stdio.h>
int main()
{
    printf("%d bytes.",sizeof(int));
}

Output

4 bytes.



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