please help with reverse

eidalina20 · #1 31-Aug-2007, 23:17

Hello everyone... i just wrote a simple quicksort program using C++ ..
it works fine sorting it and all..
but what i am trying to do now is to reverse the sort results...
so for instance if you type in: 5 7 3 9 6
it should give: 9 7 6 5 3 instead of 3 5 6 7 9

my code is:

CPP / C++ / C Code:

/* 
Assignment #1 - Quicksort
Alina Eideman
CS 575
*/

#include <iostream>
using namespace std;

#define ARRAY_SIZE 5                                  

void QuickSort(int* array, int left, int right);
int Partition(int* array, int pivotValue, int left, int right);
void swap(int &a, int &b);
void PrintArray(int* array, int n);


// This function does the quicksort

void QuickSort(int* array, int left, int right)
{
    int pivot = array[left];                    
    int sl; // split point
    
    if(left < right)                         
    {
        sl = Partition(array, pivot, left, right);
                                                     
        array[sl] = pivot;
        QuickSort(array, left, sl-1);   
        QuickSort(array, sl+1, right);     
    }
}


// This function splits the array around the pivot

int Partition(int* array, int pivot, int left, int right)
{
    int lb = left; //left boundary
    int rb = right; //right boundary
    
    while(lb < rb)               
    {
         while( pivot < array[rb]&& rb > lb)      
         {
              rb--;                        
         }
         swap(array[lb], array[rb]);
         
         while( pivot >= array[lb]&& lb < rb)      
         {
              lb++;                         
         }
         swap(array[rb], array[lb]);
    }
	return lb; 
}


// This function swaps two numbers

void swap(int &a, int &b)
{
    int temp;
    temp = a;
    a = b;
    b = temp;
}


// This function prints an array.

void PrintArray(int* array, int n)
{
    int i;
     
    for( i = 0; i < n; i++) cout<<array[i]<<'\t';
}


int main(void)
{
    int array[ARRAY_SIZE];
    int i;
    
    for( i = 0; i < ARRAY_SIZE; i++)                
    {
         cout<<"Enter an integer : ";
         cin>>array[i];
    }
    
    cout<<endl<<"The list you input is : "<<endl;
    PrintArray(array, ARRAY_SIZE);
    QuickSort(array,0,ARRAY_SIZE - 1);                
    cout<<endl<<"The list has been sorted, now it is : "<<endl;
    PrintArray(array, ARRAY_SIZE);
    
    cin.get();
    cin.get();
    return 0;
}

i am pretty sure the changes are minor... but not sure what needs to be changed... please anyone help me..

thanks!!!!

#2 01-Sep-2007, 09:54

Since it appears that you don't want to use the std::algorithm reverse operation, maybe this will work for you?

CPP / C++ / C Code:

#include <iostream>

using namespace std;

void swap( int* p1, int* p2 )
{
    int tmp = *p1;
    *p1 = *p2;
    *p2 = tmp;
}

void reverse( int* sorted, const int len )
{
    int* p_front = &sorted[0];
    int* p_back  = &sorted[len -1];

    while( p_front < p_back )
    {
        swap( p_front++, p_back-- );
    }
}

int main()
{
    int sorted[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 };
    reverse( sorted, sizeof( sorted ) / sizeof( int ) );
    for( int i = 0; i < (int)(sizeof( sorted ) / sizeof( int )); i++ )
    {
        cout << sorted[i] << endl;
    }

    return 0;
}

Output:

Code:

Also, why do:

CPP / C++ / C Code:

// you use
int* name

// but use
int &name

// and not
int& name

:davis:

eidalina20 · #3 01-Sep-2007, 10:32

hey,... ummmmm is there a way not to use reverse function.... maybe just change the "<" to">" somewhere.. ?

i mean i understand the use of reverse function but not sure how to apply to the code that i provided...

can anyone modify what i have?

sunboy · #4 01-Sep-2007, 22:27

i don't know whether i understand you or not.
just change two places;

CPP / C++ / C Code:

/* 
Assignment #1 - Quicksort
Alina Eideman
CS 575
*/

#include <iostream>
using namespace std;

#define ARRAY_SIZE 5                                  

void QuickSort(int* array, int left, int right);
int Partition(int* array, int pivotValue, int left, int right);
void swap(int &a, int &b);
void PrintArray(int* array, int n);


// This function does the quicksort

void QuickSort(int* array, int left, int right)
{
    int pivot = array[left];                    
    int sl; // split point
    
    if(left < right)                         
    {
        sl = Partition(array, pivot, left, right);
                                                     
        array[sl] = pivot;
        QuickSort(array, left, sl-1);   
        QuickSort(array, sl+1, right);     
    }
}


// This function splits the array around the pivot

int Partition(int* array, int pivot, int left, int right)
{
    int lb = left; //left boundary
    int rb = right; //right boundary
    
    while(lb < rb)               
    {
     while( pivot > array[rb]&& rb > lb)   //change from   pivot < array[rb]
         {
              rb--;                        
         }
         swap(array[lb], array[rb]);
         
         while( pivot <= array[lb]&& lb < rb)  //change from   pivot >= array[lb]
  
         {
              lb++;                         
         }
         swap(array[rb], array[lb]);
    }
	return lb; 
}


// This function swaps two numbers

void swap(int &a, int &b)
{
    int temp;
    temp = a;
    a = b;
    b = temp;
}


// This function prints an array.

void PrintArray(int* array, int n)
{
    int i;
     
    for( i = 0; i < n; i++) cout<<array[i]<<'\t';
}


int main(void)
{
    int array[ARRAY_SIZE];
    int i;
    
    for( i = 0; i < ARRAY_SIZE; i++)                
    {
         cout<<"Enter an integer : ";
         cin>>array[i];
    }
    
    cout<<endl<<"The list you input is : "<<endl;
    PrintArray(array, ARRAY_SIZE);
    QuickSort(array,0,ARRAY_SIZE - 1);                
    cout<<endl<<"The list has been sorted, now it is : "<<endl;
    PrintArray(array, ARRAY_SIZE);
    
    cin.get();
    cin.get();
    return 0;
}

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please help with reverse

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