#else

The #else preprocessor directive evaluates the expression or condition if condition of #if is false. It can be used with #if, #elif, #ifdef and #ifndef directives.
Syntax:
#if expression//if code #else//else code #endif
Syntax with #elif:
#if expression//if code #elif expression//elif code #else//else code #endif

#else example

see a simple example to use #else preprocessor directive. 
#include <stdio.h>    
#define NUMBER 5  
void main() {  
#if NUMBER==0  
printf("Value of Number is equal: %d",NUMBER);  
#else  
printf("Value of Number is not equal");  
#endif           
  }  
Output:
Value of Number is not equal

#error

The #error preprocessor directive indicates error. The compiler gives fatal error if #error directive is found and skips further compilation process.
#error example:
see a simple example to use #error preprocessor directive. 

#include <stdio.h>      
#ifndef __MATH_H  
#error First execute then compile it 
#else  
void main(){  
float c;  
c=sqrt(7);  
printf("%f",c);  
  }  
#endif  
Output:
compile Time Error: First execute then compile it
But, if you include math.h, it does not gives error. 
#include <stdio.h>    
#include <math.h>
#ifdef __MATH_H  
#error First execute then compile it
#else  
void main(){  
float c;  
c=sqrt(7);  
printf("%f",c);  
   }  
#endif  
Output:
2.645751

#pragma

The #pragma preprocessor directive is used to provide additional information to the compiler. The #pragma directive is used by the compiler to offer machine or operating-system feature.
Syntax:

#pragma token
Different compilers can provide different usage of #pragma directive.