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Decision Structures if, if/else conditions. Selection DECISION: determine which of 2 paths to follow (1 or more statements in each path)

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Presentation on theme: "Decision Structures if, if/else conditions. Selection DECISION: determine which of 2 paths to follow (1 or more statements in each path)"— Presentation transcript:

1 Decision Structures if, if/else conditions

2 Selection DECISION: determine which of 2 paths to follow (1 or more statements in each path)

3 Selection options ( in Java ) [selection, guard, decision, conditional] Single selection if Double selection if... else Multiple selection switch or if … else if … else if … else …

4 Plain if ’s (3 variations) if (condition true) action; if (condition true) { action; } | if (condition true) | { | action1; |... | actionN; | } |

5 if … else (3 variations) if (condition true) action1; else action2; if (condition true) {action1; } else {action2; } | if (condition true) | { action1A; |... | action7A; | } | else | { action1B; |... | action5B; | }

6 Each action could be: A simple action: Assignment statement with arithmetic expression or method call I/O from keyboard / file / window Call to another method Another selection statement: if or if...else or switch A while or do… while or for loop [or do absolutely NOTHING] ;

7 Conditions Comparison (equality, relational) operators: == != = NOTE: == compare for equality = the assignment operator Compare 2 operands, which can be: variables, constants, arithmetic expressions, returned values from a method call,... But NOT Strings (different methods to compare them)

8 Conditions are true or false (ageOfStudent < MI_DRINKING_AGE) (age != 25) (michiganResident) // a boolean variable ( (a + 2 * 3) <= ( (b - 4) % 3) ) ( (Math.PI * r * r) < maxSize ) NOTE: need ( ) around whole condition

9 Logic operators in conditions && (and) || (or) ! (not) ( !(a == 25) ) [same as (a != 25) ] ( (a < b) && (c < d) ) ( (a == 3) || (c == 1) ) [Note: use truth tables to determine results]

10 Order of precedence ? NOTE: need ( ) around whole condition ( (a == b) && (c > -14) ) typical (a == b) && (c > -14) WRONG ( a == b && c > -14 ) OK “All juniors and seniors with a gpa of at least a 3.0” (gpa >= 3.0 && classStatus == 3 || classStatus == 4) WRONG

11 Order of precedence of operators Unary operators - + ! Arithmetic operators * / % + - Relational operators = equality operators == != Logic operators && [AND] important: “and before or” || [OR] Assignment operator = but ( ) can over-ride these

12 Translation from English? “All juniors and seniors should get bonus points” if (classStatus == 3 && classStatus == 4) {... } WRONG classStatus is a variable that holds ONE value if (classStatus == 3 || classStatus == 4) {... } RIGHT

13 Actions total = total + exam; counter++; System.out.println(“blah blah”); num = keyboard.nextInt();...

14 “do nothing” ( ; )Action if (maritalStatus != ‘M’) numNotMarried = numNotMarried + 1; OK, but if it’s clearer (less likely to lead to bugs) to specify POSITIVE condition vs. NEGATIVE if (maritalStatus == ‘M’) ; // empty statement - do nothing else nNotMarried = nNotMarried + 1;

15 Caution with ; (it’s “Empty block of actions”) WRONG if (a < b) ; System.out.println(“a

16 Nested if/else if (a == 4) // note indent/align formatting if (b == 5) answer = 1; else answer = 2; else if (b == 5) answer = 3; else answer = 4; Trace this code using: a: 4, b: 5, answer >> a: 2, b: 5, answer >> a: 4, b: 2, answer >> a: 2, b: 2, answer >>

17 Empty statement if (a == 4) if (b == 5) ; // do nothing here, OK else answer = 2; else if (b == 5) answer = 3; else answer = 4;

18 Dangling else ? if (a == 4) if (b == 5) answer = 1; // WRONG ? Indentation suggests…, but... else // this else paired with if (b==5) if (b == 5) answer = 3; else answer = 4; // NOTE: compiler ignores formatting and does what instructions actually “say to do”

19 Prior example actually “says”: if (a == 4) if (b == 5) answer = 1; else // so b != 5 falls here if (b == 5) answer = 3; else answer = 4; Trace this code using: a: 4, b: 5, answer >> a: 2, b: 5, answer >> a: 4, b: 2, answer >> a: 2, b: 2, answer >>

20 Dangling else - the FIX if (a == 4) {if (b == 5) answer = 1; } else // else now applies to 1 st if if (b == 5) answer = 3; else answer = 4;

21 Nested if/else if/else... [works, but NOT TYPICAL FORMATTING] if (total >= 90) System.out.println(‘A’); else if (total >= 80) System.out.println(‘B’); else if (total >= 70) System.out.println(‘C’); else if (total >= 60) System.out.println(‘D’); else System.out.println(‘E’);

22 Nested… generally written as: if (total >= 90) System.out.println(‘A’); else if (total >= 80) System.out.println(‘B’); else if (total >= 70) System.out.println(‘C’); else if (total >= 60) System.out.println(‘D’); else System.out.println(‘E’);

23 Stacked ( vs. Nested ) if ’s int bonus = 0; if (attendancePercent >= 90) bonus = bonus + 5; if (labPoints >= 600) bonus = bonus + 35;

24 Stacked - WRONG if (total >= 90) System.out.println(‘A’); if (total >= 80) System.out.println(‘B’); if (total >= 70) System.out.println(‘C’); if (total >= 60) System.out.println(‘D’); System.out.println(‘E’);

25 Nested vs. Stacked NESTED if/else ’s control goes to ONLY 1 block (the 1 st condition that’s true), so ONLY 1 set of actions is done (or none) used for: –mutually exclusive categories- which state to use for tax –FIRST category that applies- grades example –~ GUI radio buttons STACKED if ’s control goes to ALL blocks (& checks all conditions), so ALL/many sets of actions MIGHT be done used for: –ALL categories that apply- cumulative bonus –~ GUI check boxes

26 Nested if/else ’s Since control goes to ONLY 1 (or 0) action block (the 1 st condition that applies) and none of the subsequent else if blocks (nor the final else block) (or, if there’s no final else maybe 0 actions occur) So the ORDER of conditions MAY be important –NO for mutually exclusive categories- state –YES for “use “1 st category that applies”- grade example (shown in earlier slide) NOTE: grades are really mutually exclusive, but example didn’t specify conditions in mutually exclusive way

27 Switch statement Equivalent to nested if/else Shown later


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