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Macros Vs Functions

Macros Vs Functions


Macros


It contains few lines of code
A macro is preprocessed
A macro is expanded in the  code
The program size increases
Execution of macros is  faster since code is replaced
No type checking is done for macros
Usually used for small code  which appear many times

Functions

It contains several lines of code
A function is compiled
Function code is not inserted in  the code
The program size does not  increase
Functions execute slower
Strict type checking is done for  functions.
Usually used for large code that  needs to be executed many times.

Conditional Compilation Directives


These directives allow specific parts of the program to be included or  discarded based on certain condition. It causes the preprocessor to  conditionally suppress the compilation of portions of source code. 

#if


Syntax: #if constant-expression 
It checks whether the constant expression is true(non-zero). 

#ifdef 


Syntax: #ifdef identifier 
It checks whether the identifier is currently defined using #define  directive. 

 #ifndef 

 Syntax: #ifndef identifier 
 It checks if the identifier is not currently defined.

#else 

Syntax: #else 
It is used with #if, #ifdef, #ifndef to delimit alternative source text  to be compiled if the condition is false. It is optional. 

#elif  

Syntax: #elif constant-expression 
It is used with #if, #ifdef, #ifndef or another #elif to indicate  alternative code if the condition is false. It is optional. It must be  followed by a condition.

#endif 

Syntax : #endif newline 
This directive ends the scope of the #if, #ifdef, #ifndef, #else, #elif  directive.

Example
: Check if macro NUM has an even value & display appropriate message.

#if (NUM%2==0) 
printf( “NUM is even”); 
#endif 

Check if macro NUM has an even or odd value & display appropriate  message.

#if (NUM%2==0) 
printf( “NUM is even”); 
#else 
printf( “NUM is odd”); 
#endif

Check if macro NUM has 0, positive or negative value and display  appropriate message.

#if (NUM==0) 
printf( “\n Number is zero”); 
#elif(NUM > 0) 
printf( “\n Number is positive”); 
#else  
printf( “\n Number is negative”); 
#endif 

If macro NUM is defined, then define macro MAX having value 20 
#ifdef NUM 
#define MAX 20 
#endif

If macro NUM is not defined, then define macro NUM having value 20 
#ifndef NUM 
#define NUM 20 
#endif 

If macro NUM is defined, then redefine it with value 75. If it is not  defined, define it with value 100. 
#ifdef NUM 
#undef NUM 
#define NUM 75 
#else 
#define NUM 100 
#endif

Predifined macros


LINE


Integer value representing the current line in the source  code file

FILE


A string literal containing a C string constant which is  the name of the source file being compiled


DATE


A string literal in the form “mmm dd yyyy” containing  the date on which the preprocessor is being run

TIME


A string literal in the form “hh:mm:ss” containing the  time at which the compilation process began

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