CS 117 Spring 2002

Decision Making

Hanly Chapter 3

Friedman-Koffman Chapter 4

Flow control

Three types of program flow• sequential (what we’ve done so far)• selection (Chapter 3)

– if - else– switch

• repetition (Chapter 4)– while– do - while– for

Boolean variables

• bool type - variables that can be either true or false

• For regular variables, a zero value is considered to be false, anything else is true

Boolean operators

• unary! logical

NOT

• binary operators&& logical and|| logical or

p !p

T F

F T

Truth Table

p q p && q p || q

T T T T

T F F T

F T F T

F F F F

Short-circuit evaluation

• -if the result is uniquely determined by the value of the first operand, the second won’t be evaluated.

– p && q has to be false if p is false

– p || q is always true if p is true

Boolean Operators

• comparison operators < less than<= less than or equal>= greater than or equal> greater than== equal!= not equal

bitwise operators

• (we won’t use these)^ complement

& and

| or

Precedence revisited

function calls( )use to force the desired order! unary - (negation) * / %+ - (binary)< <= >= >== !=&&||= (also += -= *= /= etc)

Selection Statements

• These provide a way to select different paths through the code– if - else– switch

Examples of Boolean Expressions

p

!p

p || q

!(p && q) && p||q

x < y

0 <=x && x <= 100

a + b == c

Cautions

• a==b is different from a=b

• using == with double variables is not recommended - test the magnitude of the difference

• 0 <= x <= 100 is not what you'd expect from math

When do you need selection

• some operations need only be done under certain conditions – you can't withdraw more money than your bank account

holds - it doesn’t make sense to have a negative balance

• sometimes things are done differently in different ranges of a variable – you are taxed differently in different ranges of income– piecewise functions are calculated differently in different

regions

• a program with a menu has to do different things depending on what is selected

execute code sometimesdo if clausecontinue after if

check if

condition

FalseTrue

if statement

if (condition)thenDoThis;

thisAlwaysDone;

• condition is a boolean expression• First statement after if is done if condition is true• to execute multiple statements, surround them

with { }

different code at different timesexecutebody of if

continue after if

check if condition

FalseTrueexecutebody of else

if..else

if (condition)thenDoThis;

elsedoThat;

thisAlwaysDone;

• body of if and else one statement unless { } used

Nested if statements

if (condition) {if (condition2)

thenDoThis;else

doThat;}

elsedoTheOther;

thisAlwaysDone;

Nested if caution

• in the absence of { }, an else always goes with the nearest if.if (cond1)

if (cond2)

thenDoThis;

else

doTheOther; // done if cond1 true && cond2 false

thisAlwaysDone;

Multiple if statements

• Sometimes there are more than two cases. if (condition)

thenDoThis;else if (condition2)

doThat;else if (condition3)

…else

doInAllOtherCases;thisAlwaysDone;

switch statement

• another selection statement

• allows you to select between several discrete values of a variable

• Use this only with enumerable types (int, char)

format of switch

switch (variable) {case value1:

action1:break;

case value2:action2;break;

default: // if no other case holdsdefault action;

}

switch statement

• The first case for which variable has the value given in the case is executed

• break forces exit from the switch statement; without it, execution falls through to next case

if vs switch

• ranges of values

• any type– boolean expressions

– double

– char

– int

• discrete values

• enumerable types– int

– char