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1992-2012 by Pearson Education, Inc. & John Wiley & SonsSome portions are adopted from C++ f or Everyone by Horstmann

ENGR 1200U Introduction to Programming

Lecture 10

Control Structures: Selection (Chapter 3) (contd)

Dr. Eyhab Al-Masri

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What is a top down design?

Begin with a big picture description of a problem solutionin sequential steps.

Sequential steps are refined until the steps are detailed

enough to translate to language statements

Refined steps can be described using pseudocode orflowcharts

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What is a structured program?

A program that is written using simple control structuresincluding:

Sequence Selection

Repetition

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What is a ConditionalExpression?

A Boolean expression that evaluates to true or false

Example

if (x>y)

{...

}

conditional expression

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What is the differencebetween = and == operators?

= is called an assignment operator. It is used to assignvalues to identifiers.

== denotes equality testing (tests whether an identifier isequal to a given value or expression)

intc(10);

if (c

=

5){...}

intc(10);

if (c

==

5){...}

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What does the following expression mean

(a > 5) && (y < 10)

This is an example of logical expressions. They are used tomake assertions. Also called Boolean expressions.

Three logical operators: ! && ||

A B A&&B A||B A Bfalse false false false true true

false true false true true false

true false false true false true

true true true true false false

Short-circuit evaluation?

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Can we combinemultiple if

statements toevaluate complex

decisions?

Yes, we can.

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Consider the following UOIT GPA Grade Scale

How manybranches

do wehave?

5 or 6? For simplicity and to minimize number of branches inthis example, we will assume that

grade < 73 is passing but marginal

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Grade>= 90

Grade>= 85

Grade>= 80

Grade>= 77

Grade>= 73

A+(4.3 Grade Points)

A(4.0 Grade Points)

A-(3.7 Grade Points)

B+(3.3 Grade Points)

B(3.0 Grade Points)

Pass, Marginal

true

true

true

true

true

false

false

false

false

false

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if (grade>=90)

{

cout

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if (grade>=90)

{

cout=73)

{

cout

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Nested if statements: if statements inside other ifstatements

Exampleif (grade>=80)

{

if (grade>=90)

{

cout

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Exampledoubleship_rate=5.00;//standardshippingacrossCanada

if (Country==Canada)

if (Province==PEI)

ship_rate=9.00;

else

ship_rate=30.00;

This is actually how thecode is executed

This is not what we want.This is thedangling else problem

Sowhatisthesolution?

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Exampledoubleship_rate=5.00;//standardshippingacrossCanada

if (Country==Canada)

{

if (Province==PEI)

ship_rate=9.00;

}

elseship_rate=30.00;

Solution:Simple,putonestatementinablock{}

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The switch statement is used for multipleselection decision making

Exampleint digit;

...

if (digit==1){digit_name="one";}

else if (digit==2){digit_name="two";}

else if (digit==3){digit_name="three";}

else if (digit==4){digit_name="four";}

else if (digit==5){digit_name="five";}

else if (digit==6){digit_name="six";}

else if (digit==7){digit_name="seven";}

else if (digit

==

8)

{

digit_name

=

"eight";

}else if (digit==9){digit_name="nine";}

else {digit_name="";}

Thisisabitofamess!

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Exampleint digit;

...

switch (digit)

{

case 1:digit_name="one";break;

case 2:digit_name="two";break;

case 3:digit_name="three";break;

case 4:digit_name="four";break;

case 5:digit_name="five";break;

case 6:digit_name="six";break;

case 7:

digit_name

=

"seven";

break;case 8:digit_name="eight";break;

case 9:digit_name="nine";break;

default:digit_name="";break;

}

Moreorganized

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Syntaxswitch (control_expression) {

case constant:statement(s);break;

[ case constant:statement(s);break;

[] ][ default:

statement(s); ]

}

Control expression must

be an integral type (e.g.

char, int, etc)

Break statements are not

required; without a break

statement execution runs

through other case labels.

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Every branch of the switch must be terminated by abreak statement

If the break is missing, execution falls through to thenext branch, and so on, until finally a break or the

end of the switch is reached

If you accidentally forget the break statement, your

program compiles but executes unwanted code

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Activity TemperatureSwimming Temperature > 85

Tennis 70 < temperature