Quicksort

This is a discussion on Quicksort within the C++ Programming forums, part of the General Programming Boards category; I'm supposed to write a quicksort function but it won't work like it should. It's a recursive function and the ...

  1. #1
    Mad OnionKnight's Avatar
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    Quicksort

    I'm supposed to write a quicksort function but it won't work like it should. It's a recursive function and the problem seems to be that the function gets called so much that the application terminates. Here's the code:
    Code:
    #include <iostream>
    
    using namespace std;
    
    void quicksort (double* array, int slut, int start=0) 
    {
    	double k = array[start];
    	int kpos = 0;
    
    	if (slut - start > 1) {
    		for (int i = start; i < slut; i++) {
    			if (k > array[i]) {
    				for (int ix = i; ix > kpos; ix--) {
    					double temp = array[ix - 1];
    					array[ix - 1] = array[ix];
    					array[ix] = temp;
    				}
    				kpos++;
    			}
    		}
    		quicksort(array, kpos - 1);
    		quicksort(array, slut, kpos + 1);
    	}
    }
    
    int main()
    {
    	double tal[15] = {54.654,
    			-2.135435,
    			54.25,
    			76.345,
    			12.56,
    			54.124,
    			0.0,
    			-1.445,
    			54.455,
    			-234.4,
    			54.8,
    			23.6,
    			-645.5,
    			-3.0,
    			-45.5};
    	quicksort(tal, 15);
    	for (int i = 0; i < 15; i++)
    		cout << tal[i] << " ";
    	return 0;
    }
    No matter how much I look at it there seems to be nothing wrong. And slut = end.

  2. #2
    Registered User
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    Quicksort takes three arguements, and ur only passing two in main.

  3. #3
    Registered User
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    Quote Originally Posted by blindman858
    Quicksort takes three arguements, and ur only passing two in main.
    He has a default for the third, so thats ok.
    If any part of my post is incorrect, please correct me.

    This post is not guarantied to be correct, and is not to be taken as a matter of fact, but of opinion or a guess, unless otherwise noted.

  4. #4
    Registered User
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    Location
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    Your code logic is a slight bit messed up, so it never finishes sorting. It just keeps recursing, which causes a stack overflow and thus a crash.

    Here are some comments:
    Code:
    void quicksort (double* array, int slut, int start=0) 
    {
      double k = array[start];
    
      // Note that we don't wan't to start from the start of the array, but the start of the "chunk" we're currrently sorting
      // This was the error causing all the trouble
      int kpos = start;
    
      if (slut - start > 1)
      {
         // You included the start element here, which would make no sense; that is the one used as a base for comparisons...
         // It only results in one extra comparison, though...
         for (int i = start + 1; i < slut; i++)
         {
             if (k > array[i])
             {
               for (int ix = i; ix > kpos; ix--)
               {
                  double temp = array[ix - 1];
                  array[ix - 1] = array[ix];
                  array[ix] = temp;
               }
               ++kpos;
             }
         }
    
         quicksort(array, kpos-1);
         quicksort(array, slut, kpos+1);
       }
    }

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