Rossella Lau Lecture 3, DCO10105, Semester B,2005-6 DCO10105 Object-Oriented Programming and Design Lecture 3: Control Statements Selection statements

Rossella Lau Lecture 3, DCO10105, Semester B,2005-6

DCO10105 Object-Oriented Programming and Design

Lecture 3: Control Statements

Selection statements• logical expression, if-else, switch-case, ?:

Repetition statements• for-loop, while-loop, do-while-loop

Good programming practice: indentation

-- By Rossella Lau


Limitation of sequential statements

Usually, statements are executed one after each other

But many times we require some statements to be executed only for special situations or some statements to be executed some times in order to avoid long coding

...

One after each other


Control structures

Usually, a high level language, such as Java or C++, supports two other control structures: selection and repetition


Selection statement and program flow

Selection statement allows to change the sequential flow by passing (skipping) some statements

SelectionStatements


Logical expression

A series of logical operations or relational operations

A relational operation compares two values >, <, >=, <=, ==, != The values being compared, usually, are in the same type The result will be a boolean value: true (1) or false (0)

Logical operations binary or bit by bit operations: &, | not in this course boolean computation: &&, || , !

• may short-cut some operations


Examples of logical expressions evaluation

a) (x >= 5) && (x <= 10)

b) (x > 4) && (x < 11)

c) ! (x >=5) i.e., x < 5

d) (x >= 5) && ( x <=10) || (x >= -10) && (x <= -5)

Note: a) and b) are equivalent when x is an integer

Simple is the best E.g., use x < 5 is better than using ! (x >= 5)

using b) is better than a) if x is an integer

Don't be lazy! Use (x >= y) rather than using ( ! (x < y) )


Short-Circuit Evaluation

E.g., (age > = 21) || (x == 5) When age >= 21, the first expression resulted in true and

no matter what the value of x, the result of the whole logical expression will result in true

Once the first expression resulted in true in a Logical Or operation, the execution stops to evaluate the other expression and “short-circuits” the evaluation and returns a value of true

E.g., (grade == ‘A’) && (x >=7) Similarly, once the first expression resulted in false in a

Logical And operation, the evaluation stops with false


Special notes for C++ logical expression

In C++, a numeric expression can also be treated as a logical expression used as a condition

if (age – 12) cout << “when age is ” << age << “, it is true”;

if (!(age-12)) cout << “when age is “ << age << “, it is false”;

When the value of a numerical expression is 0, it will be treated as false, any other value, including a NEGATIVE value will be treated as true


Selection – if statements

Display the amount to charge 100 dollars to people over 12 for a concert ticket.

if ( age > 12 ) cout << 100;

age > 12 display 100true

false

Note that when there is more than one statement to be executed for the if condition, braces should be used to make the statements a block

if ( age > 12 ) { charge = 100 cout << charge;}


Selection – if-else statement

Display the amount to charge 70 for people under 12 and 100 dollars for people 12 or older for a concert ticket.

if ( age < 12 ) cout << 70;else cout << 100;

The if-else statement causes two branches in a program flow statements of if-else are also called branch statements

age < 12

display 100 display 70

truefalse


The only ternary conditional operator ? :

The if-else statement in the previous example can be modified to charge = age < 12 ? 70 : 100; or cout << (age < 12 ? 70 : 100);

This ternary operator, conditional operator, has 3 operands:

First: a logical expression Second is the resulting value if First is true Third is the resulting value otherwise

It makes the program flow more simply – good to use whenever possible


Nested if-else statements

but charge $60 for a senior citizen (65 or over).

if ( age < 12 ) cout << 70;else if ( age >= 65 ) cout << 60; else cout << 100;

age < 12

display 100

display 70

truefalse

age >= 65

display 60

false true


Serial if-else statements

Although the previous example is a nested if-else statement, all of the branches work towards checking on the age It is more common to treat it as a serial of if-else statements and its more acceptable to write the code in the following formats:

if ( age < 12 ) cout << 70;else if ( age >= 65 ) cout << 60;else cout << 100;

if ( age < 12 ) cout << 70;elseif ( age >= 65 ) cout << 60;else cout 100;


switch-case statementIt is another form of selection statement

It is used more often when there are multiple selections on the same variable

// range() results range of ageswitch ( range (age) ) { case 1: cout << 70; break; case 2: cout << 60; break; default: cout << 100; break; //redundant //but good //style}

age < 12 display 70true

age > 65

false

break

display 60 break

default:display 100

false

true


Sample coding on exercises

Malik: 4:4 (with structured programming approach) The cost of an international call from New York to New Delhi is calculated as follows: Connection fee, $1.99; $2.00 for the first three minutes; and $0.45 for each additional minute. Write a program that prompts the user to enter the number of minutes the call lasted and outputs the amount due. Format your output with two decimal places.

Use of if-else statement – ch4PEx4a.cpp Use of switch-case statement – ch4PEx4b.cpp Use of conditional operator ?: – ch4PEx4.cpp


Use if statements when checking on file status

When using cin to get user input, cin may be re-directed to an input file

the input file may not exist there may not be data or sufficient data in the file A simple check on the above two cases can be:if (cin) // cin does not have the above two problems

Similarly, when using an input file stream, one can check, e.g., if (inFile) // input ok or if (!inFile) //input not ok

Example program: Malik: 4 Average test score


assert() if (!inFile) {

cout << “Cannot open the input file. ” <<“The program terminates.” << endl; return EXIT_FAILURE; }

can use an alternative way to terminate the program:assert (inFile);

When inFile cannot be opened, the program will terminate with messages:

Assertion failed: inFile, file D:/acadWeb/teaching/04-05/dco10105/lab/lab4/Ch4_AverageTestScore.cpp, line 20

This application has requested the Runtime to terminate it in an unusual way. Please contact the application's support team for more information.

When using assert(), one should have #include <cassert> in the header


Efficiency concerns – avoid redundancy

Observe that the first and the last statements of each branch are the same they can be moved up or down from the branchesif ( minutes > 3) { sum = CONNECTION_FEE + FIRST_3_MINUTES + (minutes - 3) * ADDITIONAL; return sum;}else { sum = CONNECTION_FEE + FIRST_3_MINUTES; return sum;}

sum = CONNECTION_FEE + FIRST_3_MINUTES;if (minutes > 3) sum += (minutes - 3) * ADDITIONAL;return sum;


Efficiency concerns – return logical expression

If we add a function to determine if a quadratic equation is solvable, we may have:

However, it is more efficient to return the expression directly

bool isSolvable(double a, double b, double c) { if ( (b * b – 4 * a * c) >= 0 ) return true; else return false;}

bool isSolvable(double a, double b, double c) { return ( (b * b – 4 * a * c) >= 0 );}


Indentation for selection statement

Proper consistent indentation

Each branch opens a deeper level of its associated selection statements and should have a deeper indentation

Same levels should align on the same column

An indentation usually is 4 spaces

An indentation may also simply be a “Tab” but note that “Tab” may not be “portable” among different editors


Position of braces for selection statement

Consistent braces usage

It is generally accepted that no braces are used for a single statement of a branch

Positions Open at the same line after an if or else statement or an

independent new line under the first character of “if” or “else”

Close at the same column of the function or the if-else statement on a new line


A new problem

Wendy plans to deposit 300 dollars at the beginning of each month into her fixed deposit account. The interest is added to the account and rounded to the nearest cent at the end of each month. Assume that the annual interest rate is 6%.

Write a program to display the total amount she can get back by the end of the year


The calculation

// the following data may be input from user double deposit = 300.0; double rate = 0.06; double balance = 0.0;

// The first month balance += deposit; balance += round ( balance * rate / 12.0, 2); // The second month balance += deposit; balance += round ( balance * rate / 12.0, 2 );

…… // repeat the last 2 statements 10 more times

cout << “Wendy can get ” << balance << endl;


The solution with loop

// the following data may be input from user double deposit = 300.0; double rate = 0.06; double balance = 0.0;

loop for 12 times balance += deposit; balance += round ( balance * rate / 12.0, 2 ); loopend

cout << “Wendy can get ” << balance << endl;


Loops in a program

Loop, iteration, or repetition statements allow a program to repeatedly execute a portion of the codes.

When viewing the program flow, an iteration diverts the sequential flow to a previous statement

Back to previous

…


Initialization Before the loop starts, there should be

preparation Condition

To specify if a loop goes on and the variables to specify this condition are called control variables

Update It updates the control variables of the

condition in order to allow the condition to become false. Otherwise, the loop becomes infinitive

Body The statements being repeatedly

executed are the body of the loop

General structure of a loop

…


Loop statements

A higher level language usually supports three kinds of loops and they can be easily transformed to each other: for-loop, while-loop, and do-while-loop.

for (initialization; condition; update) { body}

initializationwhile (condition) { body update}

initializationdo { body update} while (condition)


The sample loops for Wendy's problem

for ( int i = 0; i < 12; i++ ) { balance += deposit; balance += round ( balance * rate / 12.0 ,0); }

int i = 0;while ( i < 12 ) { balance += deposit; balance += …… i++;}

int i = 0; do { balance += deposit; balance += …… i++;} while ( i < 12 )


Types of loopsThe loop for the previous application is called counter-

controlled repetition It uses i as the loop control variable to count the number

of times the loop should be executed This can be applied when the number of repeated times is

known before the loop is executed

It is quite common for the number of repeated times not to be known before the loop is executed

sentinel-controlled repetition flag-controlled repetition EOF-controlled repetition


Sentinel-Controlled repetition

A loop is continuously executed until the loop control variable stores a special value, the sentinel

E.g., a user is allowed to enter radius to cirArea.cpp until -1 is entered (the program is terminated if -1 is entered)

radius = inputRadius(); while (radius != -1) { area = calculateArea(radius); displayResult(radius, area); radius = inputRadius(); } // in cirArea.cpp (v3.0)


Flag-controlled repetition

The loop control variable of a loop is a boolean and the loop is continuously executed the loop control variable becomes false

E.g., a user can continuously enter the radius until he/she replies ‘N’, for the additional prompt

bool goOn = true;while (goOn) { radius = inputRadius(); area = calculateArea(radius); displayResult(radius, area); goOn = inputContinue(); }//in cirArea.cpp (v3.1)

bool inputContinue() { char prompt; cerr << "Continue?(Y/N):"; cin >> prompt; return prompt == 'Y';

}


EOF-controlled repetition

Use EOF as the loop control variable

radius = inputRadius(); while (cin) { area = calculateArea(radius); displayResult(radius, area); radius = inputRadius(); }//in cirArea.cpp (v3.2)

The input can be ended by pressing ctrl-D

Another example: copyFile.cpp


Which loop statement is to be used?

A suggestion:

If the repeating times of the body is known before it is executed, use for loop.

Otherwise, use

while loop for user input process

do-while loop if the loop body is at least executed once

while loop otherwise


Indentation and braces for repetition statement

Proper consistent indentation

Each loop body opens a deeper level and should have a deeper indentation

The level of indentation is the same for selection statements

Similar to selection statements, the position of braces It is generally accepted that no braces are used for a single statement

of a branch Open at the same line after for(), do, while() or an independent new

line under the first character of “for”, “while”, or “do” Close at the same column of loop statement on a new line


Early termination of a loop

A loop may need to be terminated before the condition is failed

Such termination is an (or another) ‘exit’ of a loop

Same as Java, the key word for such branching is ‘break’

The execution after an exit is the statement following the loop

In other words, an exit is an early termination of a loop


An example of early termination of a loop

Modify the above problem for printing which month Wendy can get an interest of 50 dollars or more.

double deposit = 300.0;double rate = 0.06;double balance = 0.0;double interest = 0.0;for ( int i = 0; i < 12; i++ ) { balance += deposit; interest += roundAmount ( balance * rate / 12.0 ); if ( interest >= 50.0 ) { stdOut.println("Wendy gets $50 at month"+(i+1)); break; } balance += interest;}


Early re-start of a loop

A loop body may start the next repeated execution before all the statements in the body are executed.

Such repeating causes a re-start point of a loop.

Same as Java, the key word for such branching is ‘continue’


An example of early re-start of a loop

Modify the first problem for finding how much Wendy can get if there is no interest for the first three months.

double deposit = 300.0;double rate = 0.06;double balance = 0.0;

for ( int i = 0; i < 12; i++ ) { balance += deposit; if ( i <= 3 ) { continue; //no interest } balance += roundAmount ( balance * rate / 12.0 );}cout << “Wendy can get " << balance << endl;


Avoid “continue” and “break”

Although they are supported, it is not recommended to use these two statements as much as possible

It causes more than one exit of a loop and increases difficulty in understanding the program and tracing program logic difficult to debug!


Summary I – for selection statement

We have studied three kinds of selection statements: if-else, switch-case, and ? :

Selection statements allow a program to divert the flow of handling different states of data

assert() is a convenient way to terminate a program with unexpected intermediate result(s)

Indentation for branch statements significantly affect the readability of a program

For efficiency, redundant statements on different branches should be avoided and logical expression should be returned directly


Summary II – for repetition statement

Make the four parts (initialization, condition, update, body) of a loop explicit for better maintainability

Make sure to properly initialize loop control variables. Ask question: e.g., start from 0 or 1

Make sure to set up a fail situation for terminating a loop

Make sure the interval of the count of a loop is correct E.g., from 0 to 20 or from 1 to 20?

Minimize the use of ‘continue’ and ‘break’ in a loop body

Indent the loop body


Reference

Malik: 4-5

Programming styles: http://personal.cityu.edu.hk/~dcrosela/teaching/04-05/dco1

0105/help/style_white.htm

-- END --

Documents

Rossella Lau Lecture 3, DCO10105, Semester B,2005-6 DCO10105 Object-Oriented Programming and Design Lecture 3: Control Statements Selection statements