[PPT] - Chapter 4: Control Structures I (Selection) C++ Programming 1 Dr. PowerPoint Presentation

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Chapter 4: Control Structures I (Selection)

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Objectives

Learn about control structures Examine relational and logical

perators

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 Learn how to avoid bugs by avoiding partially understood concepts Learn to use the assert function to terminate a program

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Control Structures

A computer can proceed:

In sequence Selectively (branch): making a choice Repetitively (iteratively): looping

Some statements are executed only if certain

conditions are met

A condition is met if it evaluates to true

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Control Structures

false true

Expression

Statement false true

Expression

Statement1 Statement2 Statement1 Statement2 Statement3

Sequence Selection Repetition

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Relational Operators

A condition is represented by a logical (Boolean)

expression that can be true or false

Relational operators:

Allow comparisons between two operands (binary) Evaluate to true or false

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Relational Operators and Simple Data Types

You can use the relational operators with all

three simple data types:

(8 < 15)

evaluates to true

(6 != 6)

evaluates to false

(2.5 > 5.8)

evaluates to false

(5.9 <= 7.5)

evaluates to true

(‘a’ == 97)

evaluates to true

(97.0 == 97)

evaluates to true

(‘A’ > ‘a’)

evaluates to false(65<97)

(a = ‘0’)

evaluates to true

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Relational Operators and Simple Data Types

Logical (Boolean) expressions

Expression with relational operators Returns an integer value of 1 if the logical

expression evaluates to true

Returns an integer value of 0 otherwise

cout << (‘a’ == 97); cout << (‘a’ > ‘z’); cout << (97.0 == 97); 101

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Relational Operators and the string Type

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

f the shorter string

–The shorter string is less than the larger string

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

“Hello” < “Hi” true “Hello” > “Hen” false “Air” < “An” true “Hello” == “hello” false “Air” <= “Bill” true “Hi” > “Bill” true “Hello” > “hello” false “Bill” >= “Billy” false “Big” <= “Bigger” true

Relational Operators and the string Type

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Logical Operators and Logical Expressions

Logical (Boolean) operators enable you to

combine logical expressions

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Logical Operators and Logical Expressions

The negation (!) operator

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Logical Operators and Logical Expressions

The AND (&&) Operator

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Logical Operators and Logical Expressions

The OR ( || ) Operator

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Logical Operators and Logical Expressions

Logical Operators AND (&&) and OR (||) have

2 symbols

If you use only one symbol, you will get the

Bitwise operator: Bitwise AND (&) and Bitwise OR (|) which are done per bit and lead to different results

1 && 2 -> 1 1 & 2 -> 0

1= 0 0 0 0 0 0 0 1 2= 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 =0

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Order of Precedence

Relational and logical operators are evaluated

from left to right

The associativity is left to right Parentheses can override precedence

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Order of Precedence

1. + (positive) , - (negative), ! (not)
2. ++ (increment), -- (decrement)
3. * (multiplication), / (division), % (modulus)
4. + (addition), - (subtraction)
5. <, <=, >, >= (relational comparison)
6. == (equal-to), != (not-equal-to)
7. && (and)
8. || (or)
9. = (assignment), +=, -=, *=, /=, %= (compound assignment)

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

9 && 8 == 7 < 0 / 4 + 1 - 1

Order of Precedence

9 && 8 == 7 < !6 * - 5 / 4 + 3 % 2 - 1 9 && 8 == 7 < 0 * (- 5) / 4 + 3 % 2 - 1 9 && 8 == 7 < 9 && 8 == 9 && 9 && 8 == 7 < 0 + 1 - 1 x = x = x = x = x = x = x = x =

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

int and bool Data Type and Logical Expressions

Logical expressions evaluate to either true (=1) or

false (=0)

The data type bool has logical (Boolean) values true

and false

bool legalAge = (age >= 21);

Earlier versions of C++ did not provide built-in data types

that had Boolean values, so the int data type was used to manipulate logical (Boolean) expressions

int legalAge = (age >= 21);

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Selection: if and if...else

One-Way Selection Two-Way Selection Compound (Block of) Statements Multiple Selections: Nested if Comparing if...else Statements with a

Series of if Statements

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

One-Way Selection

The syntax is:

The Statement is executed if the value of the Expression is true The Statement is bypassed if the value is false and the

program goes to the next statement if (Expression) Statement

false true

Expression Statement

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Examples:

if (score >= 60); grade = ‘P’; if score >= 60 grade = ‘P’; grade = ‘F’; if (score >= 60) grade = ‘P’;

One-Way Selection

CORRECT – but we do need the grade = ‘F’; assignment before the if to make sure the grade will have a value if the score is lower than 60 INCORRECT – missing parentheses SYNTACTICALLY CORRECT – it has a valid if with an empty statement – that does not do (if (score >=

60);) , followed by an assignment grade = ‘P’ but

LOGICALLY INCORRECT – it will make the grade =

‘P’ for any score so you need to delete the ; at the

end of the first line

if (score >= 60) grade = ‘P’;

SYNTACTICALLY CORRECT but LOGICALLY INCORRECT – it will make the grade = ‘P’ for scores larger than 60 but will not assign any value for scores smaller than 60

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

One-Way Selection

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Two-Way Selection

If expression is true,

Statement1 is executed;

therwise, Statement2 is

executed if (Expression) Statement1 else Statement2

false true

Expression Statement1 Statement2

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Two-Way Selection

Example:

if (hours > 40.0) wages = 40.0 * rate + 1.5 * rate * (hours – 40.0); else wages = hours * rate;

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Compound (Block of) Statements

Compound statement (block of statements): A compound statement is a single statement

{ Statement1; Statement2; … StatementN; }

Statement1 Statement2 StatementN

…

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Compound (Block of) Statements

Example:

if (age > 18) { cout << "Eligible to vote." << endl; cout << "No longer a minor." << endl; } else { cout << "Not eligible to vote." << endl; cout << "Still a minor." << endl; }

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Multiple Selections: Nested if

Nesting: one control statement

in another

An else is associated with the

most recent if that has not been paired with an else

You can nest control statement

in compound statements to remove some of the ambiguity

r organize your code

if (expression1) statement1; else if (expression2) statement2; else statement3; if (expression1) statement1; else { if (expression2) statement2; else statement3; }

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Multiple Selections: Nested if

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Multiple Selections: Nested if

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Comparing if…else Statements with a Series of if Statements

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Short-Circuit Evaluation

Short-circuit evaluation: evaluation of a logical

expression stops as soon as the value of the expression is known

Example:

(age >= 21) || ( x == 5) (grade == 'A') && (x >= 7)

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Comparing Floating-Point Numbers for Equality: A Precaution

Comparison of floating-point numbers for equality may

not behave as you would expect

Math:

=
=
=1

C++:

int: 3/7 + 2/7 + 2/7 = 0 !=1 float: 3.0/7.0 + 2.0/7.0 + 2.0/7.0 = 0.42857142857+0.28571428571+0.28571428571 = 0.9999994 !=1

Use a tolerance value fabs(x – y) < 0.000001

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Associativity of Relational Operators: A Precaution

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Associativity of Relational Operators: A Precaution

0<=num<=10 0<=num && num<=10

5

5 15 0<=-5 <=10 0<=5 <=10 0<=15<=10 0 <=10 1 <=10 1 <=10 1 1 1 0 10

Always

true

5

5 15 0<=-5 && -5<=10 0<=5 && 5<=10 0<=15 && 15<=10 0 && 1 1 && 1 1 && 0 1 0 10 true

nly if

between 0 and 10

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Avoiding Bugs by Avoiding Partially Understood Concepts and Techniques

Must use concepts and techniques correctly;

Otherwise solution will be either incorrect or

deficient

If you do not understand a concept or technique

completely

Don’t use it Save yourself an enormous amount of

debugging time

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Input Failure and the if Statement

If input stream enters a fail state

– All subsequent input statements associated with that

stream are ignored

– Program continues to execute – May produce erroneous results

Can use if statements to check status of input

stream

If stream enters the fail state, include

instructions that stop program execution

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Confusion Between the Equality (==) and Assignment (=) Operators

C++ allows you to use any expression that can be

evaluated to either true or false as an expression in the if statement: (x=5) or (x==5)

The appearance of ‘=‘ in place of ‘==‘ resembles a

silent killer

It is not a syntax error, it is a logical error

if (x = 5) cout << "The value is five." << endl; else cout << "The value is five." << endl;

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Conditional Operator (?:)

Conditional operator (?:) takes three arguments

(ternary operator)

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

max = (a >= b) ? a : b;

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Conditional Operator (?:)

With conditional operator:

variable = expression1 ? expression2 : expression3

With if…else statement:

if (expression1) variable = expression2; else variable = expression3;

Example:

max = (a >= b) ? a : b;

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Conditional Operator (?:)

With conditional operator:

cout << expression1 ? expression2 : expression3

With if…else statement:

if (expression1) cout << expression2; else cout << expression3;

Example:

cout << ( (a >= b) ? a : b );

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Program Style and Form (Revisited): Indentation

If your program is properly indented

– Spot and fix errors quickly – Show the natural grouping of statements

Insert a blank line between statements that are

naturally separate

Two commonly used styles for placing braces

– On a line by themselves – Or left brace is placed after the expression, and

the right brace is on a line by itself

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Using Pseudocode to Develop, Test, and Debug a Program

Pseudocode, or just pseudo

Informal mixture of C++ and ordinary language Helps you quickly develop the correct structure

f the program and avoid making common

errors

Use a wide range of values in a walk-through to

evaluate the program

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

switch Structures

switch structure:

alternate to if-else

switch (integral)

Expression is evaluated first

Value of the expression

determines which corresponding action is taken

Expression is sometimes

called the selector

switch (Expression) { case Value1: Statement1 break; case Value2: Statement2 break; … case ValueN: StatementN break; default: Statements }

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Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming false true

Expression ==Value1 Expression ==Value1 Statement1

false true

Expression ==Value2 Expression ==Value2 Statement2

false true

Expression ==ValueN Expression ==ValueN StatementN

…

Statements

switch Structures

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

switch Structures

One or more statements may follow a case label Braces are not needed to turn multiple

statements into a single compound statement

The break statement may or may not appear

after each statement

switch, case, break, and default are reserved

words

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

switch Structures

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Avoiding Bugs by Avoiding Partially Understood Concepts and Techniques: Revisited

A missing break statement will cause the next

statement to be called (even if the case value is met)

To output results correctly

The switch structure must include a break

statement after each cout statement

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Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Terminating a Program with the assert Function

Certain types of errors that are very difficult to

catch can occur in a program

Example: division by zero can be difficult to

catch using any of the programming techniques examined so far

quotient=numerator / denominator;

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

The assert Function

The predefined function, assert, is useful in

stopping program execution when certain elusive errors occur

Syntax:
Expression is any logical expression
If Expression evaluates to true, the next statement

executes

If Expression evaluates to false, the program

terminates and indicates where in the program the error

ccurred
To use assert, include cassert header file

assert (Expression);

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

The assert Function

assert is useful for enforcing programming

constraints during program development

After developing and testing a program, remove or

disable assert statements

Place the preprocessor directive #define NDEBUG

before the directive #include <cassert>

#define NDEBUG #include <cassert> assert(denominator); //stops if denominator is 0 quotient = numerator / denominator;

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Programming Example: Cable Company Billing

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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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Programming Example: Cable Company Billing

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/$5.00 for

each additional one

– Premium channel cost: $50.00 per channel for any number of

connections

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Programming Example: Cable Company Billing

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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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Programming Example: Cable Company Billing

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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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Programming Example: Cable Company Billing

Program Analysis

Purpose: calculate and print billing amount
Calculating billing amount requires:

– Customer for whom the billing amount is calculated (residential or

business)

– Number of premium channels to which the customer subscribes – Bill processing fees and the cost of a premium channel

For a business customer, you need:

– Number of basic service connections – Number of premium channels

– The program should print the billing amount to two decimal

places

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Programming Example: Cable Company Billing

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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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Variables and Named Constants

Programming Example: Cable Company Billing

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Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Programming Example: Cable Company Billing

Formula for billing for residential

customers:

amountDue = RES_BILL_PROC_FEES + RES_BASIC_SERV_COST + numOfPremChannels * RES_COST_PREM_CHANNEL;

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Programming Example: Cable Company Billing

Formula for 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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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Programming Example: Cable Company Billing

Algorithm
1. Output floating-point numbers in fixed decimal

with decimal point and trailing zeros

Output floating-point numbers with two decimal places

and 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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Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Programming Example: Cable Company Billing

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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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Programming Example: Cable Company Billing

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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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Summary

Control structures alter normal control flow Most common control structures are selection and repetition Relational

perators: ==, <,

<=, >, >=, != Logical expressions evaluate to 1 (true) or 0 (false) Logical operators: ! (not), && (and), || (or)

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Dr. Adriana Badulescu Kallas

Introduction to Computer Science & Programming Programming Fundamentals I C++ Programming

Summary

Two selection structures: one-way selection and two- way selection The expression in an if or if...else structure is usually a logical expression A sequence of statements enclosed between braces, { and }, is called a compound statement or block of statements Using assignment in place of the equality

perator creates a

semantic error switch structure handles multiway selection break statement ends switch statement Use assert to terminate a program if certain conditions are not met