// Strings keep an index of each character, starting at 0.
// The substr(num1, num2) method takes two integer arguments:
// num1 is the index of the starting character of the requested substring;
// num2 is number of characters that are sought;

// string_name.substr(0, 2)  returns the first two characters of the string


// if num2 is omitted, the rest of the string starting at num1 is shown;
// likewise if num2 extends beyond the string length

// if num1 is negative, counting starts from the end of the string



function showSubString(startAt, endAt, field, fieldToSet)
  {
    if (endAt < 0)
    {
      // avoid an error in case number of characters is negative
      endAt = endAt * -1;
    }
    
    if ((startAt < 0) && (startAt * -1) > field.value.length)
    {
      alert("String is too short for request.\nShowing complete string.");
      fieldToSet.value = field.value;
    }
    else if ((startAt < 0) && (startAt * -1) <= field.value.length && endAt > 0)
    {
      alert("Showing string starting from character index " + (field.value.length + parseInt(startAt)) 
      + "\nto character index " + (field.value.length + parseInt(startAt) + parseInt(endAt)) + "." );
      fieldToSet.value = field.value.substr(startAt, endAt);
    }
    else if ((startAt < 0) && ((startAt * -1) <= field.value.length) && (endAt ==  "" || endAt == 0))
    {
      alert("Showing remaining string starting at character index " + (field.value.length + parseInt(startAt)) + ".");
      fieldToSet.value = field.value.substr(startAt);
    }
    else if (endAt ==  "" || endAt == "0")
    {
      alert("Showing complete string starting an character index " + startAt + "." );
      fieldToSet.value = field.value;
    }
    else
    {
      fieldToSet.value = field.value.substr(startAt, endAt);
    }
  }