ITEC 120 2009spring ibarland nokie jmdymacek

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lab12b
cleaning up main

The previous lab had three phases: read input, print the average, and then have a user repeatedly probe the array interactively.

Today's task: rewrite your program so that main is only three lines long (but it still behaves as before):

    public static void main( String[] args ) {
        double[] myNums = readArrayFromKbd(5);
        System.out.println( "The average value is " + avg(myNums) );
        letUserQueryArray(myNums);
    }

(Make those methods static, since their answer doesn't depend on which object is being called.)

Discussion: is this code easier to write and understand, or more difficult? Is writing the helper methods easier, when they are only in charge of one small task, independent of the rest of program?
In general, humans can only cope with large software systems by breaking them down into small, independent methods (and classes) which call each other. Large programs will have literally thousands of methods. (And often, each individual method will be quite short.)

Reading input

Note: We have relaxed lab12a's problem statement, so that we can assume the user only types integers, and never a non-integer; we stop when we read a number not in the expected range.

Here is a high-level description of how to handle the input-gathering, when we can assume that only integers are entered:

  n ← prompt-and-read-a-number
  while n is in 1..5, begin:
    print the requested item from our array
    n ← prompt-and-read-a-number
  endwhile

For the curious, If you really wanted to handle the case which includes non-integers being typed:

• one thing to note is “I wish Scanner had a method “hasNextIntInRange(int,int)” because then my own code would be easy”:

    print "Please enter a number: "
    while (s.hasNextIntInRange(1,5)) {
      n ← s.nextInt()
      print the (n-1)st item from our array
      print "Please enter a number: "
    end
  

  However, Scanner doesn't quite have such a method, and I can't write that as a method myself, using Scanner.
• The problem would also be easy if I allow myself to set n some dummy, sentinel value (perhaps -1), to indicate "no valid integer was read". However, this may not scale to other problems where any int is a valid input, so there is no possible sentinel value for n.

•     boolean amDone ← false;
      print "Please enter a number:"
      while (not amDone, and s.hasNextInt()), do:
          n ← nextInt();
          if n is in 1..5, then:
              print the (n-1)st item from our array
              print "Please enter a number: "
          else
              amDone ← true
          endif
      endwhile
  

  One surprising fact: the order of the two tests in the “and” are important: if you test s.hasNextInt() first, that method will wait for something to be typed, even if amDone has been set to true.
  (This subtle bug(?) is due to the fact that hasNextInt violates one of our design goals: it returns a value, and changes state — the state of the input buffer. However, the answer it (finally) gives is never incorrect.)

• Finally, a longer (but more methodical) approach would use two two variables, for the input: a boolean numberWasTyped along with an int theNumberTyped; the latter's value is garbage if numberWasTyped isn't true:

      print "Please enter a number:"
      if (s.hasNextInt()) {
          numberWasTyped ← true;
          numberTyped ← nextInt();
      }
      else {
          numberWasTyped ← false;
          numberTyped ← -99;  // doesn't matter.
      }
  
      while (numberWasTyped && numberTyped is in 1..5), do:
          print the (n-1)st item from our array
  
          print "Please enter a number:"
          if (s.hasNextInt()), then:
              numberWasTyped ← true;
              numberTyped ← nextInt();
          else
              numberWasTyped ← false;
              numberTyped ← -99;  // doesn't matter.
          endif
      endwhile
  

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©2009, Ian Barland, Radford University Last modified 2009.Apr.15 (Wed)

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