Lecture 9 - Control Structures pt. 1.pdf

8/14/2019 Lecture 9 - Control Structures pt. 1.pdf

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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 9

Control Structures: Selection (Chapter 3)

Dr. Eyhab Al-Masri

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Winter 2013 - UOIT

An algorithm is a sequence of steps for solving aproblem.

Engineering problem solutions to real worldproblems require complex algorithms.

Development of a good algorithm increases the

quality and maintainability of a solution, andreduces the overall time required to implementa correct solution.


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Top-down design begins with a "big picture"description of a problem solution in sequentialsteps.

The sequential steps are refined until the stepsare detailed enough to translate to languagestatements.

The refined steps, or algorithm, can be describedusing pseudo code or flowcharts.

ENGR 1200U

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Copyright 2012 Pearson Education,

Inc.


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A structured program is written using simplecontrol structures, including:

Sequence steps are performed one after another. Selection one set of statements is executed if a given

condition is true, a different set of statements, or nostatements at all, is executed if the condition is false.

Repetition A set of statements is executed repeatedlyas long as a given condition is true.

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Sequence

Selection

Repetition

?truefalse

?true

false

? => conditional

expression


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Assignment versus Equality In C++, = is an assignment operator

Example:

int c

c = 5; //assign value of 5 to c

The == operator denotes equality testing

Example:

int c

c = 5; //assign value of 5 to c

if ( c == 5) // test whether c equals 5

If (name == Harry) //compare strings

Use == inside testsUse = outside test

ENGR 1200U

Winter 2013 - UOITSyntax 3.2: C++ for Everyone by Cay Horstman


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Winter 2013 - UOITTable3..2:C++ forEveryone byCay Horstman

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Value of x Value of y Expression Result

12 5 x + 3 >= y * 3 true

10 1 x + 3 y * 10 true


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Simplified Syntaxif (boolean_expression)

statement;

if (boolean_expression){

statement1;

statement_n;}

Examples//statement executed if x>0if (x>0)

++k;

//statement block executed if x>0if (x>0)

{ x = sqrt(x);++k;}

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If we wish to execute several statements (or asequence structure) when the condition is true,we use a statement block


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ENGR 1200U

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Syntaxif (boolean_expression)

statement;[else

statement;]

if (boolean_expression){

}[else{

} ]

Example

if (x>0) { //statement block executed if x>0

} else { //statement block executed if x


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ENGR 1200U

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C++ allows a conditional operator to be used inplace of a simple if/else statement

Syntax: condition ? value1 : value2

if (age>65){

retired=true;}

else

{retired=false;

}

retired=age>65?true :false;

cout


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It is a common error to use the assignmentoperator (=) instead of the relational operator(==)

Consider the following program

int x(4),y(5);if (x=y){

cout


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We will model a person entering agrade by getting input from the user:

intgrade;coutgrade;

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If the user inputs any grade valuegreater than or equal to 50 The program must set the class status to 1

Otherwise We simply set the class status to 0

int class_status;

if (grade

>

49){class_status=1;}else

{class_status=0;}


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if (grade>49){

class_status=1;}

else

{class_status=0;

}

A condition that is true or false.Often uses relational operators

== != < >=Braces are not required if

the branch contains a

single statement, but it is

a good practice to include

them.

If the condition is true,

the statement(s) in this

branch are executed in

sequence; if the condition

is false, they are skipped.

If the condition is false,

the statement(s) in this

branch are executed in

sequence; if the condition

is true, they are skipped.

Omit the else

branch if there isnothing to do

Lining up braces isa good practice

Dont put a semicolon here

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grade > 49?

class_status = 1 class_status = 0

Condition


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#include

using namespace std;

int main()

{

int grade;

coutgrade;int class_status;

if (grade>49){

class_status=1;cout


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To avoid confusion and possible errors whenusing if/else statements, you should use {} toclearly define statement blocks.

Do not use == with real values Instead of x==3.14, use

fabs(x-3.14)str2)

cout


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String ordering rules: All uppercase letters come before the lowercase letters

Example: Z comes before a

Space character comes before all printable characters

Numbers come before letters

Punctuation marks are ordered

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Expression Value Result

word1 == word2 falseBoth words are not equal in thefirst character

word1 > word2 trueT comes after S inlexicographical ordering

word1 < Tremble FalseFifth characters do not match, andb comes before e

Word2 == Small True They are equal

stringword1,word2;word1="Tremendous";word2="Small";


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bool Data Type A built-in data type consisting of jus two values: true

or false

bool (a reserved C++ keyword) is short for Boolean Boolean data is used for testing conditions in a

program

Each variable of type bool can contain one of twovalues: true or false

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In C++, assertions take the form of logicalexpressions (also called Boolean expressions)Just as an arithmetic expression consists of

numeric values and operations, a logicalexpression is made up of logical values andoperations

Every logical expression has one of two values: true orfalse


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By combining logical operators with relationaloperators, we can make more complexassertions

Example:

grade >= 50.0 && grade


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Consider the following logical expression x == 1 && y > 2

Some programming languages use full evaluationof logical expressions Computer first evaluates both subexpressions (both x

== 1 and y > 2) before applying && operator toproduce the final result

In contrast, C++ uses short-circuit (orconditional evaluation

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Winter 2013 - UOIT

Evaluation proceeds from left to right, and thecomputer stops evaluating subexpressions assoon as possible (i.e. as it knows the Booleanvalue of the entire expression)

Examples E.g. if A is false, A && B is always false, regardless of the

value of B. E.g. if A is true, A || B is always true, regardless of the

value of B.

Lecture 9 - Control Structures pt. 1.pdf

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