ITP © Ron Poet Lecture 5 1 Branching

ITP © Ron Poet Lecture 5 2 CookTime After Midnight  We want to improve our program so that we can cook meals after midnight.  We detect the problem when we get a negative startTotalMins.  Easy to fix, just add total number of minutes in a day if startTotalMins is negative.  There are 24 * 60 = 1440 minutes in a day.  Assume cooking time is less than 24 hours!

ITP © Ron Poet Lecture 5 3 Tests In Programs  We can test values in our programs.  The program will take two different branches.  Depending on the actual values tested when the program runs.  startTotalMins can be positive most time, but occasionally negative.  If it is negative then we add 1440 and carry on.

ITP © Ron Poet Lecture 5 4 Simple Branch  The simplest form of branching structure is to do some additional work if a test is true.  Then carry on with the main scenario. Test Carry On Additional Work true false

ITP © Ron Poet Lecture 5 5 Refine Sequence of Activities  We take the simple activity:  Calculate cooking time.  and provide more details.  Calculate cookTime.  Calculate end time in total minutes  Calculate start time in total minutes  if start time is negative  add 1440 to start time  Calculate start hours and minutes.

ITP © Ron Poet Lecture 5 6 Simple Branch in Java  Java uses the word if for a simple branch. if (test is true) statement  The statement can also be a compound statement if we wanted to do several things before returning to the main path.  If the test is false then the statement is not done.

ITP © Ron Poet Lecture 5 7 Numerical Tests  There are 6 possible tests involving numerical values.  The answer is either true or false.  x == y true if x equals y  x != y true if x does not equal y  x > y true if x is bigger than y  x >= y true if x is bigger than or equal to y  x < y true if x is less than y  x <= y true if x is less than or equal to y

ITP © Ron Poet Lecture 5 8 Modifying the start time  We want a test that is true if the start time is negative.  Use a numerical test with <. if (startTotalMins < 0) startTotalMins = startTotalMins ;

ITP © Ron Poet Lecture 5 9 New calculate cook time int cookTime = (int) (minsPerKg * weight + extraMins); int endHours = time / 100; int endMins = time % 100; int endTotalMins = 60 * endHours + endMins; int startTotalMins = endTotalMins - cookTime; if (startTotalMins < 0) startTotalMins = startTotalMins ; int startHours = startTotalMins / 60; int startMins = startTotalMins % 60;

ITP © Ron Poet Lecture 5 10 Indentation  Note how the inside of the if is indented.  That makes it easier to follow the structure of the branch.  The code inside the if is only executed if the test is true.  The code before and after the if is always executed.

ITP © Ron Poet Lecture 5 11 Two Things Inside the if  Let us print a message on System.err as well as add 1440 if startTotalMins < 0.  The 'inside' of the if is a single statement  Which can be a compound statement. if (startTotalMins < 0) { startTotalMins = startTotalMins ; System.err.println("Midnight Feast!"); }

ITP © Ron Poet Lecture 5 12 Indentation Can Be Deceiving  Let us accidentally 'forget' the {}. if (startTotalMins < 0) startTotalMins = startTotalMins ; System.err.println("Midnight Feast!");  The print statement is not inside the if.  It will always print "Midnight Feast" even for meals during the day.  Only the first statement is inside the if.  The indentation has mislead us.

ITP © Ron Poet Lecture 5 13 Are Two Strings Equal?  Beware! Do not compare Strings with ==. String a = "Fred", b = "Fred"; if (a == b)...  is legal Java but does not do what you think!  In this case the test is false!

ITP © Ron Poet Lecture 5 14 Proper Equality Test For Strings  If a and b are Strings then the equality test is a.equals(b).  This is clumsy, but we must use it. if (a.equals(b))...  A test that is true if the Strings are not equal is even worse! if (!a.equals(b))...

ITP © Ron Poet Lecture 5 15 An Example With Strings  The following silly example asks the user to type their name.  If their name is "Ron" they get a special welcome before the general message. con.print("Who are you: "); String name = con.readWord(); if (name.equals("Ron")) con.println("Why, Hello Ron"); con.println("Now lets get down to work.");

ITP © Ron Poet Lecture 5 16 Alternate Paths  This is a more complicated branch.  We do one alternative or another before returning to the main path. Test Activity 1 Activity 2 Carry On true false

ITP © Ron Poet Lecture 5 17 One Or The Other  We either do Activity 1 or Activity 2  but not both.  Both branches join up to Carry On.  It s easy to see in our two dimensional diagram.  But harder to express as a linear sequence.

ITP © Ron Poet Lecture 5 18 The else Part of the if  In Java we write the part to do if the test is true as before.  Then write the word else and the part to do if the test is false. if (Test) statement_if_true; else statement_if_false;  Naturally we can use compound statements in each part.

ITP © Ron Poet Lecture 5 19 Example  We add a greeting if the person is not Ron. con.print("Who are you: "); String name = con.readWord(); if (name.equals("Ron")) con.println("Why, Hello Ron"); else con.println("I don\'t think we have met"); con.println("Now let\'s get down to work.");

ITP © Ron Poet Lecture 5 20 Calculating the Maximum  The following code assigns the maximum value of two variables a, b to max.  We must define max before the if because of scope. con.print("Type two integers: "); int a = con.readInt(); int b = con.readInt(); int max; if (a > b) max = a; else max = b;

ITP © Ron Poet Lecture 5 21 Chaining if Statements  We often want to make a series of tests on the same value one after the other.  The statement in the else part can be another if statement.  We must make sure we do the tests in the correct order!  The following example prints a message based on the strength of an earthquake.  Note the layout and indentation convention.

ITP © Ron Poet Lecture 5 22 Richter Scale // start with double richter == strength of earthquake if (richter >= 8.0) con.println("Most structures fall"); else if (richter >= 7.0) con.println("Many buildings destroyed"); else if (richter >= 6.0) con.println("Some buildings collapse"); else if (richter >= 4.5) con.println("Damage to poorly built buildings"); else if (richter >= 3.5) con.println("Felt but no destruction"; else con.println("Not noticed";

ITP © Ron Poet Lecture 5 23 Chaining Explained  Note that we must stop eventually.  There is a final else without another if.  Note that the first test only catches the very big earthquakes.  The tests are progressively easier.  What would happen if the first test caught them all?  The next example shows what happens if we omit the else words.  An earthquake of strength 6.3 would print 3 messages.

ITP © Ron Poet Lecture 5 24 Richter Scale, but WRONG // start with double richter == strength of earthquake if (richter >= 8.0) con.println("Most structures fall"); if (richter >= 7.0) con.println("Many buildings destroyed"); if (richter >= 6.0) con.println("Some buildings collapse"); if (richter >= 4.5) con.println("Damage to poorly built buildings"); if (richter >= 3.5) con.println("Felt but no destruction"; else con.println("Not noticed";