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Logic Error

Posted on 2004-04-24
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Last Modified: 2010-03-31
I think I have a logic error in here but I can't figure out what it is, anyone have a clue?

The method is to sort an array of double values.
  /** The method for sorting the numbers */
  static void selectionSort(double[] list) {
    for (int i = list.length - 2; i >= 1; i--) {
      // Find the maximum in the list[0..i]
      double currentMax = list[0];
      int currentMaxIndex = 0;

      for (int j = 1; j <= i; j++) {
        if (currentMax < list[j]) {
          currentMax = list[j];
          currentMaxIndex = j;
        }
      }

      // Swap list[i] with list[currentMaxIndex] if necessary;
      if (currentMaxIndex != i) {
        list[currentMaxIndex] = list[i];
        list[i] = currentMax;
      }
    }
  }
}
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Question by:Job4Suprmn
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5 Comments
 
LVL 92

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by:
objects earned 0 total points
ID: 10909801
>  list.length - 2

at 1st glance it would appear that should be  list.length - 1
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by:GrandSchtroumpf
GrandSchtroumpf earned 25 total points
ID: 10909830
And your swap needs to be inside the for loop:

  static void selectionSort(double[] list) {
    for (int i = list.length - 1; i >= 1; i--) {
      // Find the maximum in the list[0..i]
      double currentMax = list[0];
      int currentMaxIndex = 0;

      for (int j = 1; j <= i; j++) {
        if (currentMax < list[j]) {
          currentMax = list[j];
          currentMaxIndex = j;
        }

        // Swap list[i] with list[currentMaxIndex] if necessary;
        if (currentMaxIndex != i) {
          list[currentMaxIndex] = list[i];
          list[i] = currentMax;
        }
      }
    }
  }
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Expert Comment

by:rama_krishna580
ID: 10911076
try this program logic...seudo code..

void sortArray()
    {
        for(i=0; i<MAX; i++)
        {
            for(j=i+1;j<MAX;j++)
            {
                sort(number[i],number[j]);

                number[i]=a;
                number[j]=b;
            }
        }
    }

void shuffle()
    {
        for(i=0; i<MAX; i++)
        {
            j=(int)(Math.random()*MAX);

            if(i!=j)
            {
                int temp=number[i];
                number[i]=number[j];
                number[j]=temp;
            }
        }
    }


R.K

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LVL 30

Assisted Solution

by:Mayank S
Mayank S earned 25 total points
ID: 10911219
>>  sort(number[i],number[j]);

BTW, where is this method in your code?

>> for(i=0; i<MAX; i++)

could be:

for ( i = 0 ; i < MAX - 1 ; i ++ )

Also, Job4Suprmn, your method is called selectionSort () but it doesn't actually implement selection-sort. The correct algo would be:

for ( int i = 0 ; i < array.length - 1 ; i ++ )
  for ( int j = i + 1 ; j < array.length ; j ++ )
    if ( array[i] > array[j] )
      // swap them
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Expert Comment

by:GrandSchtroumpf
ID: 10913833
:°)
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