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

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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

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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

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if STATEMENT : ONE-WAY SELECTION

• The syntax of one-way selection is:

if (expression)

statement

• statement is executed only if the value of expression 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)

statement1else

statement2• If expression is true, statement1 is executed.

Otherwise, statement2 is executed• else is a reserved word• statement1 and statement2 must both be

single C++ statements.

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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 PRICE_FIRST_CLASS = 30.00;

const float PRICE_ECONOMY = 5.00;

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

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• Consider the following structure:

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num = 0? print warning

input num

result ← sum / num

num ← 10

true

false

COMPOUND STATEMENTS

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 statementnum = 10;

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• 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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COMPOUND STATEMENTS

• Example: cin >> num;if (num == 0) {

cout << "Invalid value!";num = 10;

}result = sum / num;

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compound statement

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):

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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 with the closest unpaired if.

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switch STRUCTURE

• switch structure: alternate to if-else.

• Example:switch (code) {

case 1:cout << "one";

case 2:cout << "two";

default:cout << "Error";

}

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First, evaluate the switchexpression

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

If there is no match, start from the default case

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

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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 case is optional in a switch

statement.

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THE break STATEMENT

• The break statement may be used to jump out of a switch 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 statement is

found or the end of switch 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 label, and if no match, the entire switch statement is skipped

• A break statement causes an immediate exit from the switch structure

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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?

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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

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string COMPARISON EXAMPLE

• Suppose we have the following declarations:string str1 = "Hello";

string str2 = "Hi";

string str3 = "Air";

string str4 = "Bill";

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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?

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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 && and || are binary operators

• Putting ! in front of a logical expression reverses its value

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CLASS ACTIVITY 3.9

• Problem:

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Substance A

Substance B

) (][put incontainer E1

) (][put incontainer E1

Substance C

put incontainer E2

put incontainer E2

put incontainer E3

put incontainer E4

put in container E1

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

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LOGICAL (BOOLEAN) EXPRESSIONS

• The bool Data Type and Logical (Boolean) Expressions

– The data type bool has logical (Boolean) values

true and false

–bool, true, and false are reserved words

– The identifier true has the value 1

– The identifier false has the value 0

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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 0 or 1, and 0 <= 10 is true and 1 <= 10 is true

• A correct way to write this expression is:

0 <= num && num <= 10

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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<num2) ? num2 : num1;

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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

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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

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Requirements

• Ask user for account number and customer code

• Assume R or r stands for residential customer and B or b stands for business customer

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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

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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

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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

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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

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Formulas

Billing for residential customers:

amountDue = RES_BILL_PROC_FEES +

RES_BASIC_SERV_COST

+ numOfPremChannels *

RES_COST_PREM_CHANNEL;

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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;

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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

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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

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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

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SUGGESTED READING (ROYO)

• Chapter 4: Control structures I (Selection)

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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

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YOU SHOULD NOW KNOW…

• relational operators• using relational operators to compare string

values• logical operators• bool variables• the conditional operator

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