Decision Structures

if, if/else conditions

Selection

DECISION:

determine which of 2 paths

to follow

(1 or more statements in each path)

Selection options (in Java)

[selection, guard, decision, conditional]

Single selection if

Double selection if ... else

Multiple selectionswitch

or

if … else if … else if … else …

Plain if’s (3 variations)

if (condition true)

action;

------------------

if (condition true){

action;}

| if (condition true)| {| action1;| . . .| actionN;| }|||

if … else (3 variations)

if (condition true)action1;

elseaction2;

------------------if (condition true){ action1;}else{ action2;}

| if (condition true)| { action1A;| . . .| action7A;| }| else| { action1B;| . . .| action5B;| }

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] ;

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)

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

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]

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

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

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

Actions

total = total + exam;

counter++;

System.out.println(“blah blah”);

num = keyboard.nextInt();

. . .

“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

elsenNotMarried = nNotMarried + 1;

Caution with ;(it’s “Empty block of actions”)

WRONG

if (a < b);System.out.println(“a<b”);// println will ALWAYS happens; not related to

if

RIGHTif (a < b) // no ; here

System.out.println(“a<b”);// println MAY happen, depending on if

condition

Nested if/else

if (a == 4) // note indent/align formattingif (b == 5)

answer = 1;else

answer = 2;else

if (b == 5)answer = 3;

elseanswer = 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 >>

Empty statement

if (a == 4)if (b == 5)

; // do nothing here, OK

elseanswer = 2;

elseif (b == 5)

answer = 3;else

answer = 4;

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 formattingand does what instructions actually “say to do”

Prior example actually “says”:

if (a == 4)if (b == 5)

answer = 1;else // so b != 5 falls

hereif (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 >>

Dangling else - the FIX

if (a == 4){ if (b == 5)

answer = 1;}else // else now applies to 1st if

if (b == 5)answer = 3;

elseanswer = 4;

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’);

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’);

Stacked (vs. Nested) if’s

int bonus = 0;

if (attendancePercent >= 90)

bonus = bonus + 5;

if (labPoints >= 600)

bonus = bonus + 35;

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’);

Nested vs. Stacked

NESTED if/else’scontrol goes to ONLY 1 block (the 1st 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’scontrol 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

Nested if/else’s

Since control goes to ONLY 1 (or 0) action block(the 1st 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 “1st 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

Switch statement

Equivalent to nested if/else

Shown later