Principles of Computer Implementing decisions using if statements - PDF document

Lecture Outline Principles of Computer  Implementing decisions using if statements Science I  Grouping statements into blocks  Comparing numbers, strings, and objects Prof. Nadeem Abdul Hamid  Using Boolean operators and variables CSC 120 – Fall 2005 Lecture Unit 6 - Decisions 1 2 CSC120 — Berry College — Fall 2005 Making Decisions if / else Statement  Does this work?  Computer programs often need to make decisions if ( amount <= balance )  Take different actions depending on some condition(s) balance = balance - amount;  Example: Can’t withdraw more money than in if ( amount > balance ) account balance balance = balance - OVERDRAFT_PENALTY;  “If amount-to-withdraw is less than available balance then deduct from balance; otherwise charge a penalty to the  How about this? balance.” if ( amount <= balance ) balance = balance - amount;  else if ( amount <= balance ) balance = balance - OVERDRAFT_PENALTY; balance = balance - amount; 3 4 Types of Statements Syntax: if Statement  Simple if ( condition ) statement balance = balance - amount;   Compound if ( condition ) statement1 else statement2 if ( amount <= balance ) balance = balance - amount;   Block Purpose: Purpose:  Groups multiple statements together To execute a statement(s) depending on whether  Can be used anywhere a single statement is used a condition is true or false { double newBalance = balance - amount; balance = newBalance; } 5 6 1

Brace Layout Syntax: Block Statement  Doesn’t matter to compiler – matters to human {  Two suggested styles – choose one and stick to it statement1 statement2 ... if ( amount <= balance ) } { double newBalance = balance - amount; balance = newBalance; } Purpose: Purpose:  or To group several statements together to form a if ( amount <= balance ) { single statement double newBalance = balance - amount; balance = newBalance; } 7 8 Indentation public class BankAccount { . . . Comparing Values public void withdraw( double amt ) {  Another very critical way to if ( amt <= balance ) { make programs readable  Relational operators double newBal = balance - amt; for humans balance = newBal; } Java Math Notation Description  Use spaces instead of tab } . . . } > > Greater than key >= ≥ Greater than or equal  2, 3, or 4 spaces are best public class BankAccount { < < Less than  Tips . . . <= ≤ Less than or equal public void withdraw( double amt ) Always type the beginning and {  == = Equal if ( amt <= balance ) ending braces first, then fill in { between != ≠ Not equal double newBal = balance - amt; balance = newBal; Put comment after closing  } // end if  == operator denotes equality testing brace to indicate what it } // end withdraw method matches . . . } // end BankAccount class a = 5; // Assign 5 to a if ( a == 5 ) . . . // Test whether a equals 5 9 10 Comparing Floating Point Comparing Floating Point (Correctly)  Test whether (absolute value of) the double r = Math.sqrt( 2 ); difference between two number is close to 0 double d = r * r -2; if ( d == 0 )  Threshold often referred to as ε – ‘epsilon’ System.out.println( "sqrt(2)squared minus 2 is 0” ); else System.out.println( "sqrt(2)squared minus 2 is not 0 but " + d ); x � y � � sqrt(2)squared minus 2 is not 0 but 4.440892098500626E-16  In Java:  Don’t compare floating point numbers for (exact) equality == final double EPSILON = 1E-14;  Doesn’t work because of roundoff errors . . .  Instead, check if they are close enough (up to a if ( Math.abs(x - y) <= EPSILON ) // x is approximately equal to y desired threshold) 11 12 2

Comparing Strings Comparing Order of Strings  Use the compareTo method  Don’t use == for strings either! s.compareTo(t) < 0 means s comes before t if (input == "Y") // WRONG!!!  s.compareTo(t) > 0 means s comes after t  Use the equals method  s.compareTo(t) == 0 means s and t are equal  if (input.equals("Y")) . . .  Java’s ‘dictionary’ order is according to Unicode  == tests identity; equals tests equal contents  ‘car’ comes before ‘cargo’  Will see this again in ‘Comparing Objects’ slides  All uppercase letters come before lowercase  ‘Hello’ comes before ‘car’  To test equality ignoring upper/lowercase (‘Y’ or ‘y’)  Numbers come before letters  ‘1’ comes before ‘a’ if (input.equalsIgnoreCase("Y")) . . .  See Appendix B in textbook 13 14 Comparing Objects Object References Rectangle box1 = new Rectangle(5, 10, 20, 30);  Like strings, == tests identity; equals tests contents Rectangle box2 = box1; Rectangle box3 = new Rectangle(5, 10, 20, 30); Rectangle box1 = new Rectangle(5, 10, 20, 30); Rectangle box2 = box1; Rectangle box3 = new Rectangle(5, 10, 20, 30);  box1 != box3 but box1.equals( box3 )  box1 == box2  Warning: equals method must be defined properly by the class before you can use it 15 16 Testing for null Strings and null  Object variable may be set to null  Empty string is ""  Indicates ‘no object’  Valid string of length 0 String middleInitial = null; // Not set if ( . . . )  null indicates a string variable does not middleInitial = middleName.substring(0, 1); refer to anything, not even an empty string  Can be used as a condition (use == ): if (middleInitial == null) System.out.println(firstName + " " + lastName);  Always test for null using == not the equals else method System.out.println(firstName + " " + middleInitial + ". " + lastName); 17 18 3

Conditions with Side Effects Multiple Alternatives  Sequences of comparisons  Avoid in if statements! if ( condition1 ) statement1 ;  Bad programming practice else if ( condition2 ) statement2 ;  Side effects: assignment, increment, decrement . . . else statementN ; Earthquake.java if ( ( d = b * b - 4 * a *c ) >= 0 ) r = Math.sqrt( d ); EarthquakeTester.java  The first matching condition is executed if ( n-- > 0 ) . . .  Order matters! if ( richter >= 0 ) // always passes r = "Generally not felt by people";  Can occasionally be useful to simplify loops else if ( richter >= 3.5 ) // not tested r = "Felt by many people, no destruction. . .  Next chapter 19 20 if vs. if/else Nested Branches  Consider carefully which one is appropriate to use  One if statement inside another if ( richter >= 8.0 ) if ( condition1 ) { r = "Most structures fall"; if ( condition1A ) if ( richter >= 7.0 ) statement1A ; r = "Many buildings destroyed"; if ( richter >= 6.0 ) else r = "Many buildings considerably damaged, some collapse"; statement1B ; if ( richter >= 4.5 ) } else r = "Damage to poorly constructed buildings"; statement2 ; if ( richter >= 3.5 ) r = "Felt by many people, no destruction"; if ( richter >= 0 ) r = "Generally not felt by people"; return r; 21 22 Example: Computing Taxes Taxes Flowchart If your filing status is single If your filing status is married Tax Bracket Percentage Tax Bracket Percentage $0 … $21,450 15% $0 … $35,800 15% Amount over $21,451, 28% Amount over 28% up to $51,900 $35,800, up to $86,500 Amount over $51,900 31% Amount over $86,500 31% 23 24 4

Tax Program Preparing Test Cases  Test cases should achieve complete  TaxReturn.java coverage of input possibilities  TaxReturnTester.java  Tax program  2 filing possibilities  3 tax brackets  = 6 possible combinations  To test the program, select 6 valid inputs  Beware ‘Dangling else’: pg 210 and at least 1 invalid input (negative income) 25 26 Selection Operator switch Statement condition ? value1 : value2  Replaces sequence of if / else / else comparing single integer value against constant alternatives  Combines values to yield another value depending on condition int digit; switch ( digit ) { . . . case 1: System.out.print( "one" );  if construct combines statements if ( digit == 1 ) break; System.out.print( "one" ); case 2: System.out.print( "two" ); else if ( digit == 2 ) break; System.out.print( "two" ); case 3: System.out.print( "three" ); if ( x >= 0 ) y = x; else y = -x; else if ( digit == 3 ) break; System.out.print( "three" ); . . . y = x >= 0 ? x : -x; . . . case 9: System.out.print( "nine" ); else if ( digit == 9 ) break; System.out.print( "nine" ); default: System.out.print( "error" ); else break; System.out.print( "error" ); } 27 28 switch Statement (cont.) The boolean Type  Case values must be constants and must be  George Boole (1815-1864): pioneer in the integers, characters, or enumerated study of logic constants  Cannot be used with floating point, string, or  Value of an expression like amount < 100 is objects either true or false  Without break statements, execution ‘falls  boolean type: one of these two truth values through’ to the next case until the end  Sometimes referred to as 0 and 1 double amount = 0; boolean b = amount < 1000; System.out.println( b ); 29 30 5