1 Introduction to Computer Science with Fadi P. Deek CONTROL STRUCTURES: SEQUENTIAL, SELECTIVE, AND...

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Introduction to Computer Sciencewith Fadi P. Deek

CONTROL STRUCTURES:

SEQUENTIAL, SELECTIVE,AND REPETITIVE

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THREE CATEGORIES OF CONTROL STRUCTURES:

• SEQUENTIAL

• SELECTIVE

• REPETITIVE

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SEQUENTIAL

CONTROL FLOWS FROM STATEMENT_1 TO STATEMENT_2 AND SO ON TO STATEMENT_N.

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statement_2

statement_1

statement_n

. .

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SELECTIVE

CONTROL FOLLOWS ONE OF SEVERAL ALTERNATIVES OF LOGIC FLOW BASED ON A CONDITION.

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process process

logical expression

true false

ENTER

EXIT

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REPETITIVE

CONTROL ALLOWS FOR THE REPETITION OF A CERTAIN TASK BASED ON A CONDITION.

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processlogical

expression

false

true

ENTER

EXIT

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EXAMPLE:

PROBLEM:

DESIGN, DEVELOP AND IMPLEMENT A PAYROLL SYSTEM THAT COMPUTES AN EMPLOYEE'S GROSS PAY. THE SYSTEM SHOULD ALSO COMPUTE NET PAY USING THE FOLLOWING CRITERION TO DETERMINE THE AMOUNT TO BE DEDUCTED FROM THE EMPLOYEE'S GROSS SALARY FOR SOCIAL SECURITY TAX:

IF AN EMPLOYEE EARNS MORE THAN $100.00 IN A WEEK, DEDUCT A TAX OF $25.00; OTHERWISE, DEDUCT NO TAX.

PROCESSING SHOULD BE REPEATED, BASED ON THE USER’S REQUEST.

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ANALYSIS AND SPECIFICATIONS:

1. DISCUSSION

TO COMPUTE GROSS PAY (THE PROBLEM’S OUTPUT), WE MUST KNOW THE HOURS WORKED AND THE HOURLY RATE (THE PROBLEM’S INPUT). AFTER READING THESE DATA, WE CAN COMPUTE GROSS PAY BY FINDING THEIR PRODUCT. NEXT WE CAN COMPUTE THE EMPLOYEE'S NET PAY (THE PROBLEM’S OTHER OUTPUT) BY SUBTRACTING ANY TAX DEDUCTION FROM THE GROSS PAY.

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2. DATA REQUIREMENTS

a) INPUThours (float) // HOURS WORKEDrate (float) // HOURLY RATEfinal (int) // NUMBER OF REPETITIONS

b) OUTPUTgross (float) // GROSS PAYnet (float) // NET PAY

c) INTERMEDIATE DATAcounter (int) // LOOP CONTROL VARIABLE

d) NAMED CONSTANTStax_bracket = 100.00 // MAXIMUM SALARY WITHOUT

// A TAX DEDUCTIONtax = 25.00 // AMOUNT OF TAX WITHHELD

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3. RELEVANT FORMULAE

gross pay = hourly rate x hours worked

net pay = gross pay - deductions

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PAYROLLSYSTEM

COMPUTE GROSS PAY

GET DATA

DISPLAYRESULTS

LEVEL 0

LEVEL 1

DISPLAYINSTRUCTIONS

COMPUTENET PAY

DESIGN:

1. STRUCTURE CHART

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2. MODULE SPECIFICATION(WE INCLUDE ONLY ONE MODULE DESCRIPTION AS AN EXAMPLE):

AS AN ALTERNATIVE METHOD TO FURTHER AID IN UNDERSTANDING HOW DATA ARE TRANSMITTED, WE WILL INCLUDE MODULE SPECIFICATIONS FOR "GROSS PAY" MODULE.

MODULE SPECIFICATIONS FOR THE GROSS PAY ARE:

GROSS PAYDATA RECEIVED: hours, rateINFORMATION RETURNED: grossLOGIC: THE HOURS ARE MULTIPLIED BY THE PAY

RATE TO PRODUCE THE GROSS PAY.

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3. ALGORITHM:

1. DISPLAY USER INSTRUCTIONS2. WHILE COUNTER <= FINAL

2.1. GET DATA2.2. COMPUTE GROSS PAY2.3. COMPUTE NET PAY

2.3.1. IF GROSS PAY > TAXBRACKET THEN

DEDUCT TAXELSE

DEDUCT NO TAX2.4. DISPLAY RESULTS

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NEW CONCEPTS

LOGICAL EXPRESSIONS, SELECTION, AND REPETITION

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LOGICAL EXPRESSIONS

CONDITIONS ARE WRITTEN IN THE FORM OF LOGICAL EXPRESSIONS.

THERE ARE TWO POSSIBLE VALUES FOR SUCH AN EXPRESSION: 1 (REPRESENTS TRUE), OR 0

(REPRESENTS FALSE).

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LOGICAL EXPRESSIONS MAY BE FORMED BY USING COMBINATIONS OF TWO KINDS OF OPERATORS:

RELATIONAL AND LOGICAL.

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RELATIONAL OPERATORS

OPERATOR MEANING

== EQUAL TO

!= NOT EQUAL TO < LESS THAN <= LESS THAN OR EQUAL TO > GREATER THAN >= GREATER THAN OR EQUAL

TO

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LOGICAL OPERATORS

OPERATOR MEANING

&& AND

|| OR ! NOT

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LOGICAL EXPRESSIONS ARE MADE UP OF

VARIABLES, CONSTANTS AND OPERATORS.

operand operator operand

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EXAMPLES:

(counter <= FINAL)

(class_average < 70.0)

total_grade >= class_average

hours_worked <= 35.0

divisor == 0

(angle1 + angle2 + angle3) != 180

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(midterm_grade >= 90) && (final_grade >= 90)

(side_1 == side_2) || (side_2 == side_3)

! (class_average >= 70.0)

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operand operator operand outcome

1(true) && 1(true) 1(true)

1(true) && 0(false) 0(false)

0(false) && 1(true) 0(false)

0(false) && 0(false) 0(false)

&& TRUTH TABLE

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|| TRUTH TABLE

operand operator operand outcome

1(true) || 1(true) 1(true)

1(true) || 0(false) 1(true)

0(false) || 1(true) 1(true)

0(false) || 0(false) 0(false)

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! TRUTH TABLE

operator operand outcome

! 1(true) 0(false)

! 0(false) 1(true)

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PRECEDENCE OF OPERATORS (REVISITED)

!* / %+ -

< <= >= >== !=&&||=

HIGHEST PRECEDENCE

LOWEST PRECEDENCE

EXPRESSION EVALUATION IS LEFT TO RIGHT.

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THE if STATEMENT

if STATEMENT INSTRUCTIONS ARE EXECUTED IN A

LOGICAL ORDER THAT DIFFERS FROM THEIR PHYSICAL

ORDER.

A CONDITION IS EVALUATED, AND AN ACTION IS TAKEN

DEPENDING ON THE TRUTH VALUE OF THE CONDITION

(THE ANSWER CAN BE EITHER 1 (TRUE) OR 0 (FALSE).

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ONE ALTERNATIVE

if (logical_expression)

statement_1;

if (total_grade >= class_average) cout << "passing!" << endl;

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LOGIC FLOW OF if WITHONE ALTERNATIVE

statement_1

ENTER

EXIT

true

false

logical expression

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TWO ALTERNATIVESif (logical expression)

statement_1a;

else

statement_1b;

if (total_grade >= class_average) cout << "passing!" << endl;else cout << "in trouble!!!" << endl;

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LOGIC FLOW OF if WITHTWO ALTERNATIVES

statement_1a statement_1b

ENTER

EXIT

true falselogical expression

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COMPOUND STATEMENTSAND THE if

THE “TASK” TO BE CARRIED OUT WITHIN THE if STATEMENT MAY BE SIMPLE OR COMPOUND.

ASSIGNMENT STATEMENTS, FUNCTION CALLS, INPUT/OUTPUT STATEMENTS, OTHER if STATEMENTS, ETC. MAY ALSO BE INCLUDED IN ANY OF THE if ALTERNATIVES.

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EXAMPLE:

if (total_grade >= class_average){ passing_grade = passing_grade + 1; cout << "passing!" << endl;}else{ weak_grade = weak_grade + 1; cout << "in trouble!!!" << endl;}

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MULTIPLE ALTERNATIVESif (logical expression_1)

statement_1;

else if (logical expression_2)

statement_2;

.

.

else if (logical expression_n)

statement_n;

else

statement_n+1;

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if (total_grade >= 90)

letter_grade = ‘A’;

else if (total_grade >= 80)

letter_grade = ‘B’;


letter_grade = ‘C’;


letter_grade = ‘D’;

else

letter_grade = ‘F’;

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statement_1

ENTER

EXIT

true false

LOGIC FLOW OF if WITHMULTIPLE ALTERNATIVES

logical expression_1


falselogical expression_2

true

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false

statement_2

ENTER

EXIT

true false

LOGIC FLOW OF if WITHMULTIPLE ALTERNATIVES



true

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SEQUENTIAL if VERSUSNESTED if

STATEMENTSif (number > 0)

positive_num = positive_num + 1;

if (number < 0)

negative_num = negative_num + 1;

if (number == 0)

zero_num = zero_num + 1;

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if (total_grade >= 90)

letter_grade = ‘A’;


letter_grade = ‘B’;


letter_grade = ‘C’;


letter_grade = ‘D’;

else

letter_grade = ‘F’;

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.

.

if (side_1 == side_2) && (side_2 == side_3)

cout << "Triangle is Equilateral" << endl;

else if (side_1 == side_2) || (side_2 == side_3) ||

(side_1 == side_3)

cout << "Triangle is Isosceles" << endl;

else

cout << "Triangle is Scalene" << endl;

.

.

COMPOUND if STATEMENTS

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THE REPETITION OF TASKS IN A PROGRAM IS CALLED

LOOPING.

WITH THE while LOOP, IF A CONDITION EXISTS, TASK IS

EXECUTED. AFTER THE EXECUTION OF THE TASK, THE

CONDITION IS TESTED AGAIN. THIS CONTINUES AS

LONG AS THE CONDITION REMAINS TRUE.

THE while STATEMENT

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while (logical_expression)

statement;

total = 0;counter = 1;

// the following is a loop

while (counter <= 10){ cout << "Enter grade: "; cin >> grade; total = total + grade; counter = counter + 1;}

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WHAT IF THE LOGICAL EXPRESSION IS FALSE TO

BEGIN WITH?

TASK NEVER EXECUTES, AND FLOW OF CONTROL

CONTINUES WITH THE FIRST STATEMENT

FOLLOWING THE LOOP.

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IMPLEMENTATION:// FILE: Payroll.cpp// Computes and prints gross pay and net pay given an // hourly rate and number of hours worked. Deducts a tax // of $25 if gross salary exceeds $100; otherwise, deducts// no tax.

#include <iostream.h> // needed for cin and cout

void main(){ // FUNCTIONS USED: // Display user instructions void Instruct_user (const float, // IN: max salary for no deductions const float); // IN: tax amount

// Find the gross salary float Compute_gross (float, // IN: number of hours worked float); // IN: hourly payrate

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// Find the net salary float Compute_net (float, // IN: gross salary const float, // IN: max salary for no deductions const float); // IN: tax amount

// LOCAL DATA: const float tax_bracket = 100.00; // max salary for // no deductions const float tax = 25.00; // tax amount

float hours; // input: hours worked float rate; // input: hourly payrate int final; // input: number of repetitions float gross; // output: gross pay float net; // output: net pay int counter; // intermediate: loop control variable

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// Display user instructions Instruct_user (tax_bracket, tax);

// Enter number of repetitions cout << "Number of repetitions : ”; cin >> final;

counter = 1; while (counter <= final) { // Enter hours and rate cout << "Hours worked: "; cin >> hours; cout << "Hourly rate: "; cin >> rate;

// Compute gross salary gross = Compute_gross (hours, rate);

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// Compute net salary net = Compute_net (gross, tax_bracket, tax);

// Print gross and net cout << "Gross salary is $" << gross << endl; cout << "Net salary is $" << net << endl; // Increment the counter counter ++; } // end of the while loop return;} // end of main ()

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// Display user instructionsvoid Instruct_user (const float tax_bracket, // IN: maximum salary // for no deduction const float tax) // IN: tax amount{ cout << "This program computes gross and net salary." << endl; cout << "A tax amount of $" << tax << " is deducted for" << endl; cout << "an employee who earns more than $" << tax_bracket << endl << endl; cout << "First, enter how many times do you want to " << "repeat the calculations." << endl << endl; cout << "Then, enter hours worked and hourly rate" << endl; cout << "on separate lines after the prompts." << endl; cout << "Press <RETURN> after typing each number." << endl << endl; return;} // end of Instruct_user

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// Find the gross salaryfloat Compute_gross (float hours, // IN: number of hours worked float rate) // IN: hourly payrate{ // Compute gross pay return hours * rate;} // end of Compute_gross

// Find the net salaryfloat Compute_net (float gross, // IN: gross salary const float tax_bracket, // IN: max salary // for no deductions const float tax) // IN: tax amount{ // Compute net salary if (gross > tax_bracket) return gross - tax; // deduct a tax amount else return gross; // deduct no tax} // end of Compute_net

1 Introduction to Computer Science with Fadi P. Deek CONTROL STRUCTURES: SEQUENTIAL, SELECTIVE, AND...

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