1. Decision Structures if, if/else conditions
CS Kaminski

2. selection DECISION: determine which of 2 paths to follow
(1+ statements in each path)
(ELSE path optional)

3. selection options (in Java)
[“selection”, “guard”, “decision”, “conditional”]
Single selection if
Double selection if ... else
Multiple selection (set of cases)
switch or
if … else if … else if … else …

4. if (3 variations) 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
do NOTHING ;

7. the condition comparison (equality & relational) operators:
== != < > <= >=
IMPORTANT: == compare for equality
= assignment operator
compare 2 operands - operands can be:
variables, constants, arithmetic expressions,
returned values from a method call,
but NOT Strings
(use different methods to compare them)

8. a condition is true or false
(ageOfStudent < MI_DRINKING_AGE)
(age != 21)
(michiganResident) // a boolean variable
( (a + 2 * 3) <= ( (b - 4) % 3) )
( (Math.PI * r * r) < maxSize )
NOTE: need ( ) around whole condition

9. logic operators && (and) || (or) ! (not)
( !(age == 25) ) [same as (age != 25)]
( (a < b) && (c < d) )
( (firstInitial == ‘W’) || (gender == ‘F’) )
[Note: use truth tables to determine results]

10. truth table
c1 c2 c1 && c2 c1 || c2 !c1 !c2 F F F F T T F T F T T F T F F T F T T T T T F F e.g., c1: (age >= 21) c2: (classYear = 1)

11. order of precedence NOTE: need ( ) around whole condition
( (a == b) && (c > -14) ) typical
(a == b) && (c > -14) WRONG
( a == b && c > -14 ) OK
1st rd nd
“All juniors and seniors with a gpa of at least a 3.0”
(gpa >= 3.0 && classYear == || classYear == 4) WRONG

12. order of precedence unary operators - + ! arithmetic operators * / %
+ -
relational operators < > <= >=
equality operators == !=
logic operators && [and]
NOTE: “and” before “or” || [or]
assignment operator =
but ( ) can over-ride these

13. translate from English?
“All juniors and seniors should get bonus points”
if (status == 3 && status == 4) {
. . .
} WRONG
NOTE: status variable holds ONE value
if (status == 3 || status == 4) {
} RIGHT

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

15. “do nothing” ( ; ) as the action
if (maritalStatus != ‘M’)
numNotMarried = numNotMarried + 1;
OK, but usually clearer to specify POSITIVE condition
vs. NEGATIVE condition
if (maritalStatus == ‘M’)
; // empty statement - do nothing
else

16. caution with ; (it means “empty block of actions”)
WRONG
if (a < b);
System.out.println(“a < b”);
// println will ALWAYS happen; not related to if
RIGHT
if (a < b) // no ; here
// println MAY happen, depending on if condition

17. nested if/else if (a == 4) // indent/align to show code logic
if (b == 5) // - helps human reader find bugs
answer = 1; // - computer ignores it
else
answer = 2;
if (b == 5)
answer = 3;
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 >>

18. empty statement if (a == 4) if (b == 0) ; // do nothing here; it’s OK
else
answer = 2;
if (b == 2)
answer = 3;
answer = 4;

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

20. prior example behaves as. . .
if (a == 4)
if (b == 5)
answer = 1;
else // so b != 5 falls here
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 >>

21. the FIX for dangling else
if (a == 4) {
if (b == 5)
answer = 1; }
else // else now applies to if (a == 4)
answer = 3;
else
answer = 4;

22. nested if/else if/else . . .
[works, but NOT TYPICAL FORMATTING
for category selection]
if (total >= 90)
System.out.println(‘A’);
else
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’);

23. common 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’);

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

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

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

27. caution on Nested Since control goes to ONLY 1 (or 0) action block
(i.e., block of 1st condition that applies)
and no subsequent else if blocks
nor the final else block (if any)
so ORDER of conditions MAY be important
NO for mutually exclusive categories - state
YES for “use 1st category that applies” - grade example
NOTE: grades are really mutually exclusive, but example didn’t specify ALL necessary conditions for a category

28. switch statement
Equivalent to nested if/else
Shown later