Program

#include <stdio.h>
#define SIZE 100

int linearSearch( const int array[], int key, int size ); 

int main()
{   
   int a[ SIZE ]; 
   int x; 
   int searchKey; 
   int element; 

   for ( x = 0; x < SIZE; x++ ) { 
      a[ x ] = 2 * x;
   } 

   printf( "Enter integer search key:\n" );
   scanf( "%d", &searchKey );

   element = linearSearch( a, searchKey, SIZE );

   if ( element != -1 ) {
      printf( "Found value in element %d\n", element );
   } 
   else {
      printf( "Value not found\n" );
   } 

   return 0;

}

int linearSearch( const int array[], int key, int size )
{
   int n;
   for ( n = 0; n < size; ++n ) {
      if ( array[ n ] == key ) { 
         return n;
      } 
   } 
   return -1;

}

Result

Enter integer search key:2
Found value in element 1