selection structures: if and switch statements. 2 selection statements –in this chapter we study...

Selection Structures: if and switch Statements

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Selection Statements

– In this chapter we study statements that allow alternatives to straight sequential processing. In particular:• if statements (do this only if a condition is

true)• if-else statements (do either this or that)• Logical expressions (evaluate to true or

false)• Boolean operators (not: ! and: && or: ||)

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4.1 Control Structures

– Programs must often anticipate a variety of situations.

– Consider an Automated Teller Machine:• ATMs must serve valid bank customers. They must

also reject invalid PINs.

• The code that controls an ATM must permit these different requests.

• Software developers must implement code that anticipates all possible transactions.

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4.2 Logical Expressions

Declarationint month = 9;Expression is(month > 9) False

(month < 9) False

(month >= 9) True

(month <= 9) True

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Boolean Variables

bool variable Included with C++ compiler

bool leapYear;

leapYear = true; // Non zero return value

leapYear = false; // Zero return value

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Boolean Expressions

Examples (Write T for True, or F for False):

int n1 = 55;

int n2 = 75;

n1 < n2 // _____

n1 > n2 // _____

(n1 + 35) > n2 // _____

(n1-n2) < 0.1 // _____

n1 == n2 // _____

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Logical Expressions

– Logical expressions often use these relational operators:

> Greater than< Less than>= Greater than or equal<= Less than or equal== Equal!= Not equal

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Logical Operators

Logical operator (&& means AND) used in an if...else statement:

( (tryIt >= 0) && (tryIt <= 100) )

Logical operator (| | means OR) used in an if...else statement:

( (tryIt >= 0) | | (tryIt <= 100) )

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Using &&

Assume tryIt = 90, Is tryIt within the range of 0 and 100 ?

( (tryIt >= 0) && (tryIt <= 100) )

( ( 90 >= 0) && ( 90 <= 100) )

( 1 && 1 )

1

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Using &&

Assume tryIt = 99 Is tryIt outside the range of 0 and 100 ?

( (tryIt < 0) ¦¦ (tryIt > 100) )

( ( 99 < 0) ¦¦ ( 99 > 100) )

( 0 ¦¦ 0 )

0

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Truth Tables for Boolean Operators

Truth tables Logical operators !, ¦¦, &&– 1 is a symbol for true– 0 is a symbol for false

Operation Result Operation Result Operation Result! 0! 1

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1 ¦¦ 11 ¦¦ 00 ¦¦ 10 ¦¦ 0

1110

1 && 11 && 00 && 10 && 0

1000

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Precedence of Operators

Precedence: most operators are evaluated (grouped) in a left-to-right order:– a / b / c / d is equivalent to

(((a/b)/c)/d) Assignment operators group in a right-to-

left order so the expression – x = y = z = 0.0 is equivalent to

(x=(y=(z=0.0)))

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Operator Description GroupingHighest ::

()Scope resolutionFunction call

Left to right

Unary !, +, - Not, unary plus/minus Right to left

Multiplicative * / % Multipy/divide/remainder Left to right

Additive + - Binary plus, minus Left to right

Input/Output >> << Extraction / insertion Left to right

Relational < ><= >=

Less/Greater thanLess/Greater or equal

Left to right

Equality == != Equal, Not equal Left to right

and && Logical and Left to right

or ¦¦ Logical or Left to right

Assignment = Assign expression Right to left

Precedence of Operators

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Boolean Assignment

bool same;

same = true;

Form:

variable = expression;

Example:

same = (x = = y);

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4.3 Introduction to the if Dependent Control Statement

– The if is the first statement that alters strict sequential control.

– General form

if ( logical-expression )

true-part ; • logical-expression: any expression that evaluates to

nonzero (true) or zero (false).

• In C++, almost everything is true or false.

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if Control Statementswith Two Alternatives

– The logical expression is evaluated. When true, the true-part is executed and the false-part is disregarded. When the logical expression is false, the false-part executes.

– General Formif ( logical-expression ) true-part ;else false-part ;

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What happens when an if statement executes?

• After the logical expression of the if statement evaluates, the true-part executes only if the logical expression was true.

gross >100.0

False

net=gross-tax net=gross True

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Programming Tip

Using = for == is a common mistake. For example the following statements are legal:

int x = 25;

Because assignment statements evaluate to the

expression on the right of =, x = 1 is always

1, which is nonzero, which is true:

if (x = 1) // should be (x == 1)

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4.4 if Statements with Compound Alternatives

– General form (also known as a block): {

statement-1 ; statement-2 ;

...statement-N ;

} – The compound statement groups together many

statements that are treated as one.

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Writing Compound Statements

if (transactionType == 'c'){ // process check cout << "Check for $" << transactionAmount << endl;

balance = balance - transactionAmount;}else{ // process deposit cout << "Deposit of $" << transactionAmount << endl;

balance = balance + transactionAmount;}

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4.5 Decision Steps in Algorithms

Algorithm steps that select from a choice of actions are called decision steps. The algorithm in the following case contains decisions steps to compute an employee’s gross and net pay after deductions. The decision steps are coded as if statements.

Payroll Case Study

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Decision Steps in Algorithms

Statement:  Your company pays its hourly workers once a week. An employee’s pay is based upon the number of hours worked (to the nearest half hour) and the employee’s hourly pay rate. Weekly hours exceeding 40 are paid at a rate of time and a half. Employees who earn over $100 a week must pay union dues of $15 per week. Write a payroll program that will determine the gross pay and net pay for an employee.

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Decision Steps in Algorithms

Analysis:  The problem data include the input data for hours worked and hourly pay and two required outputs, gross pay and net pay. There are also several constants: the union dues ($15), the minimum weekly earnings before dues must be paid ($100), the maximum hours before overtime must be paid (40), and the overtime rate (1.5 times the usual hourly rate). With this information, we can begin to write the data requirements for this problem. We can model all data using the money (see Section 3.7) and float data types.

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Decision Steps in Algorithms

Program Design:  The problem solution requires that the program read the hours worked and the hourly rate before performing any computations. After reading these data, we need to compute and then display the gross pay and net pay. The structure chart for this problem (Figure 4.6) shows the decomposition of the original problem into five subproblems. We will write three of the subproblems as functions. For these three subproblems, the corresponding function name appears under its box in the structure chart.

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Decision Steps in Algorithms

– Display user instructions

(function instructUser).– Enter hours worked and hourly rate.– Compute gross pay (function computeGross).– Compute net pay (function computeNet).– Display gross pay and net pay.

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PayrollFunctions.cpp

// File: payrollFunctions.cpp

// Computes and displays gross pay and net pay

// given an hourly rate and number of hours

// worked. Deducts union dues of $15 if gross

// salary exceeds $100; otherwise, deducts no

// dues.

#include <iostream>

#include "myMoney.h"

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PayrollFunctions.cpp

using namespace std;

// Functions used ...

void instructUser();

money computeGross(float, money);

money computeNet(money);

const money MAX_NO_DUES = 100.00;

const money dues = 15.00;

const float MAX_NO_OVERTIME = 40.0;

const float OVERTIME_RATE = 1.5;

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PayrollFunctions.cpp

int main ()

{

float hours;

float rate;

money gross;

money net;

// Display user instructions.

instructUser();

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PayrollFunctions.cpp

// Enter hours and rate.

cout << "Hours worked: ";

cin >> hours;

cout << "Hourly rate: ";

cin >> rate;

// Compute gross salary.

gross = computeGross(hours, rate);

// Compute net salary.

net = computeNet(gross);

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PayrollFunctions.cpp

// Print gross and net.

cout << "Gross salary is " << gross << endl;

cout << "Net salary is " << net << endl;

return 0;

}

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PayrollFunctions.cpp

// Displays user instructions

void instructUser()

{

cout <<

"This program computes gross and net salary." <<

endl;

cout << "A dues amount of " << dues <<

" is deducted for" << endl;

cout << "an employee who earns more than " <<

MAX_NO_DUES << endl << endl;

cout << "Overtime is paid at the rate of " <<

OVERTIME_RATE << endl;

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PayrollFunctions.cpp

cout <<

"times the regular rate for hours worked over "

<< MAX_NO_OVERTIME << endl << endl;

cout <<

"Enter hours worked and hourly rate" << endl;

cout <<

"on separate lines after the prompts. " << endl;

cout <<

"Press <return> after typing each number." <<

endl << endl;

} // end instructUser

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PayrollFunctions.cpp

// FIND THE GROSS PAY

money computeGross (float hours, money rate)

{

// Local data ...

money gross;

money regularPay;

money overtimePay;

// Compute gross pay.

if (hours > MAX_NO_OVERTIME)

{

regularPay = MAX_NO_OVERTIME * rate;

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PayrollFunctions.cpp

overtimePay = (hours - MAX_NO_OVERTIME) *

OVERTIME_RATE * rate;

gross = regularPay + overtimePay;

}

else

gross = hours * rate;

return gross;

} // end computeGross

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PayrollFunctions.cpp

// Find the net pay

money computeNet (money gross)

{

money net;

// Compute net pay.

if (gross > MAX_NO_DUES)

net = gross - dues;

else

net = gross;

return net;

} // end computeNet

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Payroll.cpp

Program outputThis program computes gross and net salary.

A dues amount of $15.00 is deducted for an

employee who earns more than $100.00

Overtime is paid at the rate of 1.5 times the

regular rate on hours worked over 40

Enter hours worked and hourly rate on separate

lines after the prompts. Press <return> after

typing each number.

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Payroll.cpp

Program output

Hours worked: 50

Hourly rate: 6

Gross salary is $330.00

Net salary is $315.00

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4.6 Checking the Correctness of an Algorithm

Verifying the correctness of an algorithm is a critical step in algorithm design and often saves hours of coding and testing time.

We will now trace the execution of the refined algorithm for the payroll problem solved in the last section.

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Checking the Correctness of an Algorithm

1. Display user instructions.

2. Enter hours worked and hourly rate.

3. Compute gross pay.

3.1. If the hours worked exceed 40.0 (max hours before overtime)

3.1.1. Compute regularPay.

3.1.2. Compute overtimePay.

3.1.3. Add regularPay to overtimePay to get gross.

else

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Checking the Correctness of an Algorithm

3.1.4. Compute gross as hours * rate.

4. Compute net pay.

4.1. If gross is larger than $100.00

4.1.1. Deduct the dues of $15.00 from gross pay.

else

4.1.2. Deduct no dues.

5. Display gross and net pay.

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4.7 Nested if Statements and Multiple Alternative Decisions

– Nested logic is one control structure containing another similar control structure.

– An if...else inside another if...else. e.g. (the 2nd if is placed on the same line as the 1st):

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Example of nested logic

if(x > 0)

numPos = numPos + 1;

else

if(x < 0)

numNeg = NumNeg + 1;

else

numZero = numZero + 1;

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Example of nested logic

X numPos numNeg numZero

3.0 _______ _______ _______

-3.6 _______ _______ _______

4.0 _______ _______ _______

Assume all variables initialized to 0

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Writing a Nested if as a Multiple-Alternative Decision

Nested if statements can become quite complex. If there are more than three alternatives and indentation is not consistent, it may be difficult to determine the logical structure of the if statement.

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Function displayGrade

void displayGrade ( int score)

{

if (score >= 90)

cout << "Grade is A " << endl;

else if (score >= 80)

cout << "Grade is B " << endl;

else if (score >= 70)

cout << "Grade is C " << endl;

else if (score >= 60)

cout << "Grade is D " << endl;

else

cout << "Grade is F " << endl;

}

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Order of Conditions

if (score >= 60)

cout << "Grade is D " << endl;

else if (score >= 70)

cout << "Grade is C " << endl;

else if (score >= 80)

cout << "Grade is B " << endl;

else if (score >= 90)

cout << "Grade is A " << endl;

else

cout << "Grade is F " << endl;

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Short Circuit Evaluation

(single == ‘y’ && gender == ‘m’ && age >= 18) – If single is false, gender and age are not

evaluated

(single == ‘y’ || gender == ‘m’ || age >= 18)– If single is true, gender and age are not evaluated

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4.8 The switch Control Statement

switch ( switch-expression ) { case value-1 :

statement(s)-1

break ; ... // many cases are allowed

case value-n :

statement(s)-n

break ;

default : default-statement(s) }

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Switch Control

– When a switch statement is encountered, the switch-expression is evaluated. This value is compared to each case value until switch-expression == case value. All statements after the colon : are executed

– It is important to include the break statement

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Example switch Statement:

switch(watts) // Assume char option = '?’

{ case 25:

cout << " Life expectancy is 2500 hours. " << endl;

break;

case 40:

case 60:

cout << " Life expectancy is 1000 hours. " << endl;

break;

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Example switch Statement

case 75:

case 100:

cout << " Life expectancy is 750 hours. " << endl;

break;

default: cout << "Invalid Bulb !!" << endl;

} // end switch

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Trace the previous switch

– Show output when

• watts = '?' ____________?

• watts = ’40’ ____________?

• watts = ’10'____________?

• watts = ’200' ____________?

• watts = ’100' ____________?

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4.9 Common Programming Errors

Failing to use { and } if(Grade >= 3.5)

// The true-part is the first cout only

cout <<"You receive an A !!!";

cout <<"You made it by " << (Grade-3.5) << " points";

else <<<< Error >>>>

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Common Programming Errors

There are no compile time errors next, but there is an intent error.

else

cout << "Sorry, you missed an A. ";

cout << "You missed it by " << 3.5-Grade << " points"; With the above false part, you could get this

confusing output (when Grade = 3.9):

You received an A !!!.

You made it by 0.4 points.You missed it by -0.4 points

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Corrected Version:

if(Grade >= 3.5)

{

cout << "You received an A !!! " << endl;

cout << "You made it by " << (Grade-3.5) << " points";

// Do other things if desired

}

else

{

cout << " You did NOT receive an A !!! ";

cout << "You missed it by " << (3.5-Grade) <<" points";

}