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TK 1914 : C++ Programming

Control Structures I (Selection)

OBJECTIVES

In this chapter you will: • Learn about control structures • Examine relational and logical operators • Explore how to form and evaluate logical (Boolean) expressions • Discover how to use the selection control structures if , if ...

else , and switch in a program FTSM :: TK1914, 20112012 2

INTRODUCTION

• A computer can process: – In sequence – Selectively (branch) - making a choice – Repetitively (iteratively) - looping • Some statements are executed only if certain conditions are met • A condition is represented by a logical (Boolean) expression that can be true or false • A condition is met if it evaluates to true FTSM :: TK1914, 20112012 3

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• We have seen sequencing in C++. • How do we translate selection and repetition structures into C++?

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SELECTION STRUCTURES IN C++

• Selection structures can be translated into C++ using

if

statements.

• Four forms of selection structures will be looked at: – One-Way – Two-Way – Nested – Switch FTSM :: TK1914, 20112012 6

if STATEMENT : ONE-WAY SELECTION

• The syntax of one-way selection is:

if

(expression) statement • statement expression is executed only if the value of is

true

• expression is usually a logical expression.

if

is a reserved word • statement must be a single C++ statement.

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if STATEMENT : ONE-WAY SELECTION

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CLASS ACTIVITY 3.1

• Refer prog03.1.cpp

.

– Examine the source code. Focus on the

if

statements. – What do you think the output would be for the following input?

• Price per unit: RM25 Quantity: 4 • Price per unit: RM5.50 Quantity: 5 – Run the program with the above input. Is the output generated the same as expected?

– Draw a flowchart representing the algorithm implemented in the program.

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if-else STATEMENT: TWO WAY SELECTION

• Two-way selection takes the form:

if

(expression) statement1

else

statement2 • If expression is

true

, statement1 Otherwise, statement2 is executed •

else

is a reserved word is executed. • statement1 and statement2 single C++ statements.

must both be FTSM :: TK1914, 20112012 11

if-else STATEMENT: TWO WAY SELECTION

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CLASS ACTIVITY 3.2

• Refer prog03.2.cpp

.

– Examine the source code. Focus on the

if

statements. – What do you think the output would be for the following input?

• Number of adults: 2 Number of children: 2 Ticket type: First class • Number of adults: 2 Number of children: 2 Ticket type: Economy – Run the program with the above input. Is the output generated the same as expected? – Draw a flowchart representing the algorithm implemented in the program.

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CLASS ACTIVITY 3.2

• Observe that prog03.2.cpp

includes the following declarations:

const float const float

PRICE_FIRST_CLASS = 30.00; PRICE_ECONOMY = 5.00; The identifiers PRICE_FIRST_CLASS PRICE_ECONOMY and are called constants. Their values cannot be changed during program execution.

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COMPOUND STATEMENTS

• Consider the following structure:

input num num = 0?

false true

print warning num ← 10 result ← sum / num

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COMPOUND STATEMENTS

• Note that there are two statements to be executed if num is equal to 0. • Is it correct to translate the above to C++ as follows?

cin >> num; if (num == 0) cout << "Invalid value!"; num = 10; result = sum / num; • Answer: No. According to C++ syntax, the statement num = 10; is actually not part of the

if

statement .

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COMPOUND STATEMENTS

• In this case, we need to use a

compound statement

to group those statements into a single statement.

• A compound statement is simply a set of single statements enclosed within curly brackets.

{ statement_1; … statement_n; } • Remember that a compound statement

is a single

C++ statement.

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• Example: cin >> num;

if (num == 0) { cout << "Invalid value!"; num = 10; }

result = sum / num; compound statement FTSM :: TK1914, 20112012 19

NESTED IF

• Consider the syntax of a one-way

if

statement:

if

(expression) statement • You might ask: Could statement be another

if

statement? Example:

if

(weight > 100)

if

(weight <= 1000) price = 0.20 * weight; What does it mean?

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NESTED IF

• Example:

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NESTED IF

• Can also be written as follows (remember that the compiler ignores unnecessary whitespace characters): FTSM :: TK1914, 20112012 22

CLASS ACTIVITY 3.3

• Examine the following code: How is this code different from that in the next slide?

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CLASS ACTIVITY 3.4

• The following bands are defined: – Band 1: 75 and above – Band 2: 51 - 74 – Band 3: 50 and below • Refer prog03.3a.cpp

.

– Examine the source code. Focus on the

if

statements. – What do you think the output would be for the following input?

• 80 • 60 • 40 – Run the program with the above input. Is the output generated as expected?

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CLASS ACTIVITY 3.4 (CONT)

• Refer prog03.3b.cpp

.

– Examine the source code. How is it different from the previous program?

– What do you think the output would be for the following input?

• 80 • 60 • 40 – Run the program with the above input. Is the output generated as expected?

– Draw a flowchart representing the algorithm implemented in the program.

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CLASS ACTIVITY 3.4 (CONT)

• Refer prog03.3c.cpp

.

– Examine the source code. How is it different from the first program?

– What do you think the output would be for the following input?

• 80 • 60 • 40 – Run the program with the above input. Compare the output with that of the previous program.

– Draw a flowchart representing the algorithm implemented in the program.

• C++ always associates an

else

unpaired

if

. FTSM :: TK1914, 20112012 with the closest 27

switch STRUCTURE

• switch structure: alternate to if-else .

• Example: Execute the statements inside the switch structure starting from the case which matches the value of the expression

switch

(code) {

case

1: cout << "one";

case

2: First, evaluate the switch expression cout << "two";

default

: cout << "Error"; } If there is no match, start from the default case FTSM :: TK1914, 20112012 28

CLASS ACTIVITY 3.5

• Refer prog03.4.cpp

.

– Examine the source code. Focus on the

switch

statement. – Observe that there are

return

statements in the

switch

statement. • What do you think will happen if they are executed?

• Why do you think they are there for? – What do you think the output would be for the following input?

• 2 adults, 2 children, normal first-class ticket • 2 adults, 2 children, economy ticket FTSM :: TK1914, 20112012 29

CLASS ACTIVITY 3.5 (CONT)

• Refer prog03.4.cpp

.

– Run the program with the input data given above. Is the output as expected? Explain.

– Draw a flowchart representing the algorithm implemented in the program.

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switch STRUCTURE

• Expression value can be only integral • Its value determines which statement is selected for execution • A particular case value should appear only once • One or more statements may follow a case label • Braces are not needed to turn multiple statements into a single compound statement.

• The

default

statement. case is optional in a

switch

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THE break STATEMENT

• The

break

a

switch

statement may be used to jump out of statement.

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CLASS ACTIVITY 3.6

• Refer prog03.5.cpp

.

– Examine the source code. Focus on the use of

break

statements in the

switch

statement.

– What do you think the output would be for the following input?

• 2 adults, 2 children, normal first-class ticket • 2 adults, 2 children, economy ticket – Run the program with the input data given above. Is the output as expected?

– Draw a flowchart representing the algorithm implemented in the program.

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switch Statement Rules

• When value of the expression is matched against a case value, – Statements execute until break found or the end of switch statement is structure is reached • If value of the expression does not match any of the case values – Statements following the default label execute If no default switch label, and if no match, the entire statement is skipped • A break statement causes an immediate exit from the switch structure FTSM :: TK1914, 20112012 35

RELATIONAL OPERATOR

• Relational operators: – Allow comparisons – Require two operands (binary) – Return 1 if expression is

true

, 0 otherwise • Comparing values of different data types may produce unpredictable results – For example, 8 < '5' should not be done • Any nonzero value is treated as

true

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CLASS ACTIVITY 3.7

• Refer prog03.6a.cpp

– Examine the source code. – What do you think the output would be for the following input?

• 47 • 67 – Compile the program. Are there any syntax errors?

– Run the program. Is the output as expected? Explain.

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CLASS ACTIVITY 3.7 (CONT.)

• Refer prog03.6b.cpp

– Examine the source code. Compare it with the previous program. – Run the program with the same input data as for the previous program. Is the output as expected? FTSM :: TK1914, 20112012 40

COMPARING string TYPES

• Relational operators can be applied to strings • Strings are compared character by character, starting with the first character • Comparison continues until either a mismatch is found or all characters are found equal • If two strings of different lengths are compared and the comparison is equal to the last character of the shorter string – The shorter string is less than the larger string FTSM :: TK1914, 20112012 41

string COMPARISON EXAMPLE

• Suppose we have the following declarations: string str1 = "Hello"; string str2 = "Hi"; string str3 = "Air"; string str4 = "Bill"; FTSM :: TK1914, 20112012 42

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CLASS ACTIVITY 3.8

• Refer prog03.7.cpp

– Examine the source code. What do you think the program does?

– What do you think the output would be for the following input?

• Jamal Omar Selamat Ahmad Ali Wali – Run the program. Is the output what you expected? FTSM :: TK1914, 20112012 46

LOGICAL (BOOLEAN) OPERATORS

• Logical (Boolean) operators enable you to combine logical expressions • Three logical (Boolean) operators: !

(not) && (and) || (or) • Logical operators take logical values as operands and yield logical values as results • !

is unary whereas && its value and || are binary operators • Putting !

in front of a logical expression reverses FTSM :: TK1914, 20112012 47

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CLASS ACTIVITY 3.9

• Problem: Substance A put in container E1 Substance B put in container E3 ) [ Substance C put in container E4 ) [ put in container E1 put in container E1 ] ( put in container E2 ] ( put in container E2 -100 FTSM :: TK1914, 20112012 50 52

CLASS ACTIVITY 3.9 (CONT)

• Refer prog03.8a.cpp

– Examine the source code. What do you think the program does?

– Focus on the

if

statement. Are all cases covered correctly? Test the program by running it with various input data.

• Refer prog03.8b.cpp

– Examine the source code. Compare the

if

statement with that in the previous program.

– Test the program by running it with the same input data as for the previous program.

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Precedence of Operators

• Relational and logical operators are evaluated from left to right • The associativity is left to right • Parentheses can override precedence FTSM :: TK1914, 20112012 54

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LOGICAL (BOOLEAN) EXPRESSIONS

• The

bool

Data Type and Logical (Boolean) Expressions – The data type

bool true

and

false

has logical (Boolean) values –

bool

,

true

, and

false

are reserved words – The identifier

true

has the value 1 – The identifier

false

has the value 0 FTSM :: TK1914, 20112012 59

Logical (Boolean) Expressions (continued)

• Logical expressions can be unpredictable • The following expression appears to represent a comparison of 0 , num , and 10 : 0 <= num <= 10 • It always evaluates true because 0 <= num evaluates to either and 1 <= 10 is 0 true 1 , and 0 <= 10 is true • A correct way to write this expression is: 0 <= num && num <= 10 FTSM :: TK1914, 20112012 60

CONDITIONAL OPERATOR

• Conditional operator ( ?: ) takes three arguments (ternary) • Syntax for using the conditional operator: expression1 ? expression2 : expression3 • If expression1 is

true

, the result of the conditional expression is expression2 . Otherwise, the result is expression3.

• Example: larger = (num1

Programming Example

• This programming example calculates a customer’s bill for a local cable company • There are two types of customers: – Residential – Business • Two rates for calculating a cable bill: – One for residential customers – One for business customers FTSM :: TK1914, 20112012 62

Rates

• For residential customer: – Bill processing fee: $4.50

– Basic service fee: $20.50

– Premium channel: $7.50 per channel • For business customer: – Bill processing fee: $15.00

– Basic service fee: $75.00 for first 10 connections and $5.00 for each additional connection – Premium channel cost: $50.00 per channel for any number of connections FTSM :: TK1914, 20112012 63

Requirements

• Ask user for account number and customer code • Assume R and B or b or r stands for residential customer stands for business customer FTSM :: TK1914, 20112012 64

Input and Output

• Input: – Customer account number – Customer code – Number of premium channels – For business customers, number of basic service connections • Output: – Customer’s account number – Billing amount FTSM :: TK1914, 20112012 65

Program Analysis

• The purpose of the program is to calculate and print billing amount • Calculating the billing amount requires: – Customer for whom the billing amount is calculated (residential or business) – Number of premium channels to which the customer subscribes • For a business customer, you need: – Number of basic service connections – Number of premium channels FTSM :: TK1914, 20112012 66

Program Analysis (continued)

• Data needed to calculate the bill, such as bill processing fees and the cost of a premium channel, are known quantities • The program should print the billing amount to two decimal places FTSM :: TK1914, 20112012 67

Algorithm Design

• Set precision to two decimal places • Prompt user for account number and customer type • If customer type is R or r – Prompt user for number of premium channels – Compute and print the bill • If customer type is B or b – Prompt user for number of basic service connections and number of premium channels – Compute and print the bill FTSM :: TK1914, 20112012 68

Formulas

Billing for residential customers: amountDue = RES_BILL_PROC_FEES + RES_BASIC_SERV_COST + numOfPremChannels * RES_COST_PREM_CHANNEL; FTSM :: TK1914, 20112012 69

Formulas (continued)

Billing for business customers : if (numOfBasicServConn <= 10) amountDue = BUS_BILL_PROC_FEES + BUS_BASIC_SERV_COST + numOfPremChannels * BUS_COST_PREM_CHANNEL; else amountDue = BUS_BILL_PROC_FEES + BUS_BASIC_SERV_COST + (numOfBasicServConn - 10) * BUS_BASIC_CONN_COST + numOfPremChannels * BUS_COST_PREM_CHANNEL; FTSM :: TK1914, 20112012 70

Main Algorithm

1. Output floating-point numbers in fixed decimal with decimal point and trailing zeros • Output floating-point numbers with two decimal places, set the precision to two decimal places 2. Prompt user to enter account number 3. Get customer account number 4. Prompt user to enter customer code 5. Get customer code FTSM :: TK1914, 20112012 71

Main Algorithm (continued)

6. If the customer code is r or R , – Prompt user to enter number of premium channels – Get the number of premium channels – Calculate the billing amount – Print account number and billing amount FTSM :: TK1914, 20112012 72

Main Algorithm (continued)

7. If customer code is b or B , – Prompt user to enter number of basic service connections – – – – – Get number of basic service connections Prompt user to enter number of premium channels Get number of premium channels Calculate billing amount Print account number and billing amount 8. If customer code is other than r , R , b , or B , output an error message FTSM :: TK1914, 20112012 73

SUGGESTED READING (ROYO)

• Chapter 4: Control structures I (Selection) FTSM :: TK1914, 20112012

YOU SHOULD NOW KNOW…

• the syntax of the

if

statement • how to translate a selection structure in a flowchart into a C++

if

statement • • one-way selection • two-way selection • compound statement • • nested

if

statements

switch

statement

break

statement FTSM :: TK1914, 20112012 75

YOU SHOULD NOW KNOW…

• relational operators • using relational operators to compare string values • logical operators •

bool

variables • the conditional operator FTSM :: TK1914, 20112012 76

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