Topic 3: Selection Control Structure · 2017-10-01 · C++ Programming: From Problem Analysis to...

Selection Control Structure

1) Boolean values2) Relational Operators

3) Logical Operators

4) Type of selection control structures

Boolean values

• Boolean values only have two possible values: true (1) and false (0).

• To declare a boolean variable, we use the keyword bool(data type).

• when converting integers to booleans, the integer zero resolves to boolean false, whereas non-zero integers all resolve to true.

bool bValue;

bValue = true;

Relational operators

Relational operator

Relational operators are used to compare two values to form a condition.

Math C++ Plain English

= == equals [example: if(a==b) ]

[ (a=b) means put the value of b into a ]

< < less than

<= less than or equal to

> > greater than

>= greater than or equal to

!= not equal to

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.

Simple Boolean Expressions

• Example:a) 8 < 15 = true

b) 2.5 >= 5.8 = false

• Exercise:a) 6 != 6

b) 5.9 <= 7.5

Logical operators

Logical operator

• The AND operator will give the value TRUE when all expression are TRUE

• The OR operator will give the value TRUE if any expression is TRUE

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

OPERATOR DESCRIPTION

! Not

&& AND

|| OR

logical operator

• Truth table

1

1

1 1

Precedence of operator

• Precedence of operators (from highest to lowest)– Parentheses ( … )

– Unary operators !

– Multiplicative operators * / %

– Additive operators + -

– Relational ordering < <= >= >

– Relational equality == !=

– Logical and &&

– Logical or ||

– Assignment =

Logical expression

Logical expression

• Expression that can be thought of as being true or false.– True if correct or false if not correct.

• The expression uses relational operators such as == and < and logical operators such as &&, ||, and !

Logical expression

• Logical expressions can be unpredictable• The following expression appears to represent a

comparison of 0, num = 12, and 10: 0 <= num <= 10

• It always evaluates true because 0<=12 evaluates to 1, and 1 <= 10 is true.

• A correct way to write this expression is:

0 <= num && num <= 10

Logical expression

• Example:

a) (true || false)

b) (7>3 && 5<2)

c) (5*4<20 || false)

Exercises • Assume x =5, y = 6 and z =7

Boolean expression Value

x<y && z>y

x<=z && x>0

x>=y && z==3

y<15||!(y>0)

x>y||z>y && !(z<=0)

Exercises Evaluate the following expression:

(17 < 4 * 3 + 5) || (8 * 2 == 4 * 4) &&

!(3 + 3 == 6)

C++ Programming: From Problem

Analysis to Program Design, Fourth

Edition

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Comparing Characters



Edition

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Relational Operators and thestring 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 of the shorter string

– The shorter string is less than the larger string



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Relational Operators and thestring Type (continued)

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

string str2 = "Hi";

string str3 = "Air";

string str4 = "Bill";

string str4 = "Big";



Edition

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Edition

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Edition

29




Edition

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Logical (Boolean) Operators and Logical Expressions

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

unary

binary

binary



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Logical (Boolean) Operators and Logical Expressions (continued)

Decision control structure

Decision control instruction

Example 1 Example 2

If (it is raining)wear a rain coatbring an umbrella

If (you have a test tomorrow)study tonight

Elsewatch a movie

You use the decision control structure or selection control sturcture when

you want program to make a decision or comparison and then select one of two paths,

depending on the result of that decision or comparison.

Decision control structure

1

One way selection

2

Two way selection

3

Multiple selection

One way selection

• The syntax of one-way selection is:

if (expression)

statement;

• Statement is executed if the value of the expression is true (1)

• Statement is by passed if the value is false(0); program goes to the next statement

Boolean expression in if condition

• if (quantity <50)

• if (age>=25)

• if (onhand == target)

• if (quantity != 7500)

One way selection

• Pseudocode:

• Flowchart:

if (expression) then

statements

end if

Expression

True

False

Statements

Example

Problem statement:

If the speed of the car is greater than 80km/h, then output a message “you are too fast!”

Begin

input speed

if speed > 80

output “you are too fast!”

end if

End speed>80

Input speed

TYou are too

fast

F

Start

End

Example #include <iostream>

using namespace std;

int main()

{

int speed;

cout<< “Enter your speed “;

cin >> speed;

if (speed > 80)

cout << “you are too fast!”;

}

Exercises 1#include <iostream>


int main()

{

int mark;

cin >> mark;

if (mark >= 50)

cout << “PASSED!”;

return 0;

}

Exercises 2#include <iostream>


int main()

{

int mark;

cin >> mark;

if (mark >= 50)


cout<< “thank you for using the program”<<endl;

return 0;

}

Compound statements

– Use the symbols { and } to group several statements as a single unit.

– Simple statements within the compound statements end with semicolons.

– Compound Statements are sometimes called a statement block.

– Use compound statements in an if statement if you want several actions executed when the Boolean expression is true.

Compound statements (cont.)

int main()

{

int mark;

cin >> mark;

if (mark >= 50)


cout<< “thank you”<<endl;

return 0;

}

int main()

{

int mark;

cin >> mark;

if (mark >= 50)

{


cout<< “thank you”<<endl;

return 0;

}

}

Single statement

compound statements

Two way selection• Two-way selection takes the form:

if (expression)

statement1;

else

statement2;

• If expression is true, statement1 is executed otherwise statement2 is executed

• statement1 and statement2 are any C++ statements

• if and else is a reserved word

Two way selection

• Pseudocode:

if (expression) then

statement1

else

statement2

end if

Two way selection

• Flowchart:

Expression

True

False

Statement1

Statement2

Example

Program segment Flowchart

if (cgpa < 2.0)

status = ‘F’;

else

status = ‘P’;

if (choice == 1)

gender = ‘F’;

else

gender = ‘M’;

cgpa < 2.0

P F

choice == 1

M F

true

true

false

false

Example

Problem statement:

If student marks greater than or equal to 50, then output a message “pass”, else “fail”.

Pseudocode:

Begin

input marks

if marks >= 50 then

output “pass”

else

output “fail”

end if

End

Example

• Flowchart

marks>=40

Input marks

TPass

F

Start

End

Fail

Example #include <iostream>

Using namespace std;

int main()

{

int marks;

cin >>marks;

if (marks >= 50)

cout << “pass”;

else

cout << “fail”;

return 0;

}

Compound statements

Compound statements (cont.)

if (marks >= 50)

{

cout << “pass”<<endl;

cout << “Good job!!”;

}

else

{

cout << “fail”;

cout << “Try again!!”;

}

Compound statements (cont.)• Syntax and code:#include <iostream>


int main()

{

int marks;

cin >> marks;

if (marks >= 50)

cout << “pass”;

else

{

cout << “fail”;

cout << “you have to have to repeat”;

}

return 0;

}

Exercise

1. Assume that a program need to determine whether a student’s gender is male or female. If student’s gender equal to ‘f’ then display female; otherwise display male. Write pseudocode for the selection structure.

2. Write a C++ if statement that corresponds to the pseudocode in question 1.

Exercise (cont.)

3. Problem statement:

A shopkeeper sells a magazine for RM2.00 each. However, if the customer buys more than 10 magazines, the price of the magazine is only RM1.50 each. Calculate the total the customer has to pay based on the quantity that he buys.

Begin

input quantity

if quantity > 10 then

price = 1.50

else

price = 2.00

total = price * quantity

output total

End

Input quantity

T

Display total

F

quantity>10

price = 2.00 price = 1.50

Total = quantity * price

Start

End

• Syntax and code#include <iostream>


int main()

{

int quantity = 0;

float total = 0;

cin >> quantity;

if (quantity > 10)

price = 1.50;

else

price = 2.00

total = quantity * price;

cout << total;

return 0;

}

Exercise (one way VS two way)

• A group of one way selection structure:

• Two way selection structure:

int x = -3;

if(x<10)

x = * x;

else

x = x + 10;

cout<<x;

int x = -3;

if(x<10)

x = * x;

if(x<100)

x = x + 10;

cout<<x;

Determine the output of both program segment:

Topic 3: Selection Control Structure · 2017-10-01 · C++ Programming: From Problem Analysis to...

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