How to count comparisons and swaps in insertion sort? (JAVA)

问题

public class Sorting
{
   public static int numOfComps = 0,
                     numOfSwaps = 0;

   public static void insertionSort(int[] array)
   {
      int unsortedValue;  // The first unsorted value
      int scan;           // Used to scan the array

      // The outer loop steps the index variable through 
      // each subscript in the array, starting at 1. The portion of
      // the array containing element 0  by itself is already sorted.
      for (int index = 1; index < array.length; index++)
      {
         // The first element outside the sorted portion is
         // array[index]. Store the value of this element
         // in unsortedValue.
         unsortedValue = array[index];

         // Start scan at the subscript of the first element
         // outside the sorted part.
         scan = index;

         // Move the first element in the still unsorted part
         // into its proper position within the sorted part.
         while (scan > 0 && array[scan-1] > unsortedValue)
         {
            array[scan] = array[scan - 1];
            scan--;

                // Counts the number of values swaps
                numOfSwaps ++;
         }

         // Insert the unsorted value in its proper position
         // within the sorted subset.
         array[scan] = unsortedValue;

         // Counts the number of values comparisons
        numOfComps ++;
      }
        System.out.println("\n\nNumber of comps = " + numOfComps);
       System.out.println("Number of swaps = " + numOfSwaps);       
   }
}

Newbie here again. How do I code this insertion sort program in Java to count the number of comparisons and the number of swaps? I have inserted comparison and swap codes into the program but not sure they're in the correct place. I have posted the program. Thanks for any help.

回答1:

The number of comparisons is the number of times array[scan-1] > unsortedValue is executed. That's not what you are counting.

Tips:

while (EXPRESSION) { STATEMENTS } can be rewritten as while (true) { if (!(EXPRESSION)) { break; } STATEMENTS }
!(EXPRESSION1 && EXPRESSION2) can be rewritten as !(EXPRESSION1) || !(EXPRESSION2).
if (EXPRESSION1 || EXPRESSION2) { break; } can be rewritten as if (EXPRESSION1) { break; } if (EXPRESSION2) { break; }.

The algorithm doesn't swap the value of pairs of variables. However, there is a form of multi-variable swap that occurs (A⇒B, B⇒C, C⇒D, D⇒A). The number of times this occurs is the number of times array[scan] = unsortedValue is executed when scan is different than index. That's not what you are counting.

Notes:

Your code is buggy. scan can be -1 when you reach array[scan] = unsortedValue;. This will happen when sorting 2, 1.
Note that this is a poor implementation of insertion sort. A binary search should be used instead of a linear search. This will reduce the maximum number of comparisons from N * N to N * log N.

来源：https://stackoverflow.com/questions/29127062/how-to-count-comparisons-and-swaps-in-insertion-sort-java

标签

java

insertion-sort