function changeNumericSequence (objSelectChosen)
{

  // ------------------------------------------------------------------------------

  // Created:  Jeffrey B. Berthiaume
  // Date:     12 December 2000
  // Version:  1.0

  // Description: changeNumericSequence manipulates numbers in a series of dropdown
  //   select lists.  If there are three dropdown select lists (each with the options
  //   of "1", "2", and "3") in a numerical sequence, this function will change the
  //   other select lists as appropriate.

  //   Each select list in the html form should have the name "cat_" + a common group
  //   name for the group of select lists + a unique identifier.

  //   for example:

  //     <select name="cat_1_item_32" onChange="changeNumericSequence (this);">

  // Parameter:
  //   objSelectChosen (should be the select object that was just changed)

  // Usage example: 
  //   <select name="cat_1_item_32" onChange="changeNumericSequence (this);">

  // ------------------------------------------------------------------------------

  // find out which group of select lists (category) are being manipulated
  strSelectChosenName = objSelectChosen.name;
  arrSelectChosenName = strSelectChosenName.split ("_");

  //   strCategoryName contains the name of the category (i.e. the n in "cat_n")
  strCategoryName = arrSelectChosenName [1];

  // find total number of items to manipulate
  intNumItems = objSelectChosen.length;

  // store the changed number (new number chosen for this dropdown)
  intChangedItemNumber = objSelectChosen.selectedIndex;

  arrItemNumbers = new Array (intNumItems);
  arrSelectNames = new Array (intNumItems);


  // go through and build array of all numbers in select list

  // increment through all select lists in the parent form of objSelectChosen
  // if their name begins with "cat_" + strCategoryName then they belong to this group of categories

  intTotalFormElements = objSelectChosen.form.elements.length
  intArrTotal = 0;
  for (i = 0; i < intTotalFormElements; i++)
  {
    strElementName = objSelectChosen.form.elements [i].name;
    arrSelectName = strElementName.split ("_");
    if (strCategoryName == arrSelectName [1])
    {

      if (strElementName == strSelectChosenName)
      {
        intChangedItemIndex = intArrTotal;
      }

      arrSelectNames [intArrTotal] = strElementName;
      arrItemNumbers [intArrTotal] = objSelectChosen.form.elements [i].selectedIndex;
      intArrTotal = intArrTotal + 1;
    }
  
  }

  // calculate the number that is missing
  intMissingNum = -1;
  i = 0;
  while (intMissingNum == -1)
  {
    boolNumFound = -1;

    for (i2 = 0; i2 < intNumItems; i2++)
    {

      if (arrItemNumbers [i2] == i)
      {
        boolNumFound = 0;
      }

    }

    if (boolNumFound == -1)
    {
      intMissingNum = i;
    }

    i++;

  }
  
  // re-order the numbers

  // the current item was intMissingNum
  // but now it's intChangedItemNumber


  if (intMissingNum < intChangedItemNumber)
  {

    // find all numbers between (intMissingNum + 1) and intChangedItemNumber 

    for (i = 0; i < intNumItems; i++)
    {

      num = arrItemNumbers [i];

      if ((i != intChangedItemIndex) && (num >= (intMissingNum + 1)) && (num <= intChangedItemNumber))
      {

        // decrement their indices by 1
        arrItemNumbers [i] = arrItemNumbers [i] - 1;

      }
    }


  }
  else
  {

    // find all numbers between (intChangedItemNumber + 1) and intMissingNum

    for (i = 0; i < intNumItems; i++)
    {

      num = arrItemNumbers [i];

      if ((i != intChangedItemIndex) && (num >= intChangedItemNumber) && (num < intMissingNum))
      {

        // increment their indices by 1
        arrItemNumbers [i] = arrItemNumbers [i] + 1;

      }
    }

  }

  // change the selection drop boxes to be the newly ordered numbers

  for (i = 0; i < intNumItems; i++)
  {
    eval ("document." + objSelectChosen.form.name + "." + arrSelectNames [i] + ".selectedIndex = " + arrItemNumbers [i] + ";")
  }


}

When you have a list of items to sort (say, three items, each with a drop-down from 1-3), this script will (when one of the item drop-downs is changed) automatically change the others to have appropriate numbers.  For example, if your list is numbered: 1, 2, 3...and you change the second item to 3, the list will then be numbered 1, 3, 2.

Auto-resort numbers in selectable numbered list

JavaScript