ITEC 120 2009spring ibarland nokie jmdymacek

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lect08a
booleans

• We've seen simple “boolean expressions” in things like “n >= 4” and “numTickets < roomCapacity” and “someName.equals("amy")”, eacho of which evaluate to a “boolean” value: either true or false.

• As a review in if-else, let's complete the following method, to make estimated grades for the registrar's office at midk-semester (or, any time during the semester). We'll use a rather stupid algorithm for the moment (although we know how to extend it, using multiple else-if clauses):

  /* If somebody's score is 85% or more of the points possible, * we'll return "A" * otherwise we return "B". */ String estimateGrade( int ptsEarned, int ptsPossible ) { // ...?? }

  After a bit of wrestling with int-to-double conversion, we seem to get our initial attempt to work: lect08a-soln.html

  So, our code incorporated into a web page that students can visit whenever they want, and the latest letter-grade-estimate gets incorporated into the page. This works great … until the first day of a (new) semester, when suddenly all my students report that the web page crashes! I look at the error log and see “division by zero” errors! Rats — if only my test cases would have included the edge-case of ptsPossible == 0, I would have averted a bunch of disgruntled students!

  How to fix? First, we need to decide what the correct answer should be: While it might be nice to return a String like "N/A", the web-page software which calls our method might require us to always return a letter grade. What does MLB do? At the start of a season, everybody's batting average is                 . We'll do the same: at the start of the semester, we want everybody's grade to be                 . That is, we want to return "A" if ptsPossible == 0 or the student's average is at least 85%: lect08a-soln.html

• Similarly, use || for “or”, and ! for “not”.
  What is the value of true || true && false? Remember or earler lesson on operator-precedence: use parentheses to make your code clear, if it's ever in doubt!

• In general, && looks at the boolean expressions on each side, and the entire && will evaluate to true if (and only if) the left side and the right side evaluate to true:

  expr1	expr2	(expr1&&expr2)
  false	false	false
  false	true	false
  true	false	false
  true	true	true

  (expr₁ and expr₂ can be any other boolean expression—including more-deeply-nested and/or expressions!)
  Complementing &&, an entire || expression evaluates to true if (and only if) one side or the other evaluates to true:

  expr1	expr2	(expr1||expr2)
  false	false	false
  false	true	true
  true	false	true
  true	true	true

  Really, you can think of && as a function with the signature boolean &&( boolean b1, boolean b2 ) (although, like arithmetic¹, we write them in infix notation, with the function-name coming inbetween its two inputs, rather than in-front-of-them-with-parentheses).

  Exercise: What is the value of:

  ( ((3 <= 7) || (4 == (2+3))) && (Math.sqrt(4) > 0.0) )

  This is just like solving an arithmetic problem, except that we are dealing with true and false instead of all the integers².

  It's worth a mention that in English, “or” is used in two different ways: The inclusive-or, which is what || means, means “one or the other or both”. For example, if the question on the tax form asks “are you over 18, or had income of more than $20,000”, and you meet both criteria, then the answer is true. (People might use “and/or” to emphasize they include the “and” part.) However, sometimes English uses “or” to mean one or the other but not both. For instance, “You can eat veal, or you can be morally responsible” is implying that one option precludes the other. (You may or may not agree with that presumption, of course.) Similarly, if you are told “Keep dating your ex, or date me!”, don't presume that the option of both is included in the “or”!

  While we're on the topic, there is one last boolean operator, ! (“not”):

  expr	!expr
  false	true
  true	false

  So (2+2 == 4) is true, but !(2+2 == 4) is false. This could also have been written directly using the not-equals operator: (2+2 != 4).

  Exercise: What is

  • (true || true) && false
  • true || (true && false)
  • true || true && false, without any parentheses? Yikes, while Java might have some rules for this, the real answer is: whenever there's the slightest doubt, add parentheses.

• We see from BlueJ's codepad, that answers have the type boolean. We can use this type wherever we use any type in Java: declare variables which hold a boolean, write methods which return a boolean, write methods with boolean parameters (inputs).

  • boolean variables:

    /** Return a message to be incorporated into the waiting-area display screen. * @param planeCapacity The number of passenger seats on the plane. * @param ticketsSold The number of tickets sold on this flight. * @return a message to be incorporated into the waiting-area display screen. */ String boardingRoomMessage( int ticketsSold, int planeCapacity ) { boolean areEnoughSeats; areEnoughSeats = (numTickets <= planeCapacity); roomIsOkay if (roomIsOkay == true) { return "Please board whan called."; } else { return "The flight is overbooked; panic!"; } }

    Note how the == true is redundant: Consider the value of each version when the variable roomIsOkay contains true, and when the variable roomIsOkay contains false.

  ---

  Reached here, 9am section

  • We'll see next time, that we can also write methods which return a boolean: boolean mustRegisterForSS( int age ).
  • We'll see next time, that we can also write methods with boolean parameters: boolean mustRegisterForSS( int age, boolean isMale ).

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©2009, Ian Barland, Radford University Last modified 2009.Mar.16 (Mon)

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