This is pretty much the answer( or at least mine ) to the typical CS question : Find the nth element of an array without sorting or creating a new array. I have a very nice version using templates and all the fancies , and then an Arduino version that is float specific.

nth_largest.cpp

#include <iostream>

using namespace std;

/* This function uses a trick with templates 
to determine the size of the array  */
template <class T,size_t size>
size_t arr_size( T(&)[size] ){

	return size;
}

/* gets the highest value under the specified roof */
template <class T>
T get_max( T array[], size_t size, T min, T roof = NULL ){
	
	T max = min;
	for ( int i = 0; i < size; i++ ){

		if ( roof != (float)NULL ){
			if ( array[i] >= roof ) /* Ignore the roof and continue with the loop */
				continue;
		}

		if( array[i] > max ) max = array[i];
	}

	return max;
} 

/*
	Pass through the array, the size of the array , 
	and the value 
*/
template <class T>
T get_nth_num( T array[], size_t size, int n ){
	if ( n < 1 || (size_t)n > size ){

		cout << "get_nth_num(), n cannot be less than 1 or larger than the array";
		return -1;
	}

	T ignore = NULL;
	for ( int i = 0; i < n; i ++ ){

		ignore = get_max(array, size, (T)0, ignore);
	}

	return ignore;
}

/* program entry point */
int main( void ){

	float array[] = {6,5,2,7,3,8,32,1,43,87,45};

	//find the max
	cout << "nth max is " << get_nth_num( array, arr_size(array) , 4 ) << endl;
	

	return 0;
}

nth_largest_arduino.cpp

#include <iostream>

using namespace std;

/* This function uses a trick with templates 
to determine the size of the array  */

/* gets the highest value under the specified roof */
float get_max( float array[], size_t size, float min, float roof = NULL ){
	
	float max = min;
	for ( int i = 0; i < size; i++ ){

		if ( roof != (float)NULL ){
			if ( array[i] >= roof ) /* Ignore the roof and continue with the loop */
				continue;
		}

		if( array[i] > max ) max = array[i];
	}

	return max;
} 

/*
	Pass through the array, the size of the array , 
	and the value 
*/
float get_nth_num( float array[], size_t size, int n ){
	if ( n < 1 || (size_t)n > size ){

		Serial.println( "get_nth_num(), n cannot be less than 1 or larger than the array" );
		return -1;
	}

	float ignore = NULL;
	for ( int i = 0; i < n; i ++ ){

		ignore = get_max(array, size, (float)0, ignore);
	}

	return ignore;
}

/* program entry point */
int main( void ){

	float array[] = {6,5,2,7,3,8,32,1,43,87,45};

	//find the max
	Serial.println( "nth max is :");
        Serial.println( get_nth_num( array, sizeof(array)/sizeof(*array) , 4 )   );
	

	return 0;
}