GCD ( Greatest Common Divisor) program using C Programming

 #include   <stdio.h>

int  recursiveGreatestCommonDivisor( int  firstNumber,  int  secondNumber);

int  main() {

int  firstNumber, secondNumber, result;

printf( "Greatest Common Divisor (GCD) of 2 numbers: \n" );

printf( "\n" );

printf( "Enter two Whole Numbers \n" );

printf( "\n" );

printf( "first Number : " );

scanf( "%d" , &firstNumber);

printf( "second Number : " );

scanf( "%d" , &secondNumber);

result = recursiveGreatestCommonDivisor(firstNumber, secondNumber);

printf( "\n" );

if  (result == -1) {

printf( "Invalid Input \n" );

printf( "\n" );

}

else  {

printf( "Greatest Common Divisor (%d,%d) = %d \n" , firstNumber, secondNumber, result);

}

printf( "\n" );

return  0;

}

int  recursiveGreatestCommonDivisor( int   firstNumber ,  int   secondNumber )

{

if  (( firstNumber  < 0) || ( secondNumber  < 0)) {

return  -1;

}

if  (! firstNumber ) {

return   secondNumber ;

}

else   if  (! secondNumber ) {

return   firstNumber ;

}

if  (( secondNumber  <  firstNumber ) && ( firstNumber  %  secondNumber  == 0)) {

return   secondNumber ;

}

else   if  ( firstNumber  <  secondNumber ) {

return  recursiveGreatestCommonDivisor( secondNumber ,  firstNumber );

}

else  {

return  recursiveGreatestCommonDivisor( secondNumber , ( firstNumber  %  secondNumber ));

}

}