Transcript CS 140 Introduction to Computing I

Chapter 3
More Flow of Control
Goals:
• To analyze the use of Boolean expressions
• To introduce the notion of enumerated types
• To explore the switch statement as an alternative to
multiway if-else statements
• To view the conditional statement as a alternative to a
simple if-else statement
• To examine the for-statement as a looping option
• To demonstrate the design of good nested loops
Precedence Rules for Boolean Expressions
With the addition of Boolean operators, the precedence rules
that C++ uses to evaluate expressions become more complex.
• Parentheses still take the highest precedence
• Multiplication, division, and modulus come second.
• Addition and subtraction take the next precedence.
• Order-related inequality operators (<, >, <=, >=) are next.
• The pure equality/inequality (==, !=) operators are next.
• The Boolean AND operator (&&) takes the next precedence.
• The Boolean OR operator (||) takes the lowest precedence.
• Precedence ties are still handled in left-to-right fashion.
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Precedence Rules Examples
int x = 7,
if (x == y
cout <<
else
cout <<
y = 7, z = 7;
== z)
“YES”;
“NO”;
Output: NO
The left equality is checked and
evaluates to true (numerical 1),
which is not equal to the z-value!
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int a =
if (a <
cout
else
cout
5, b = 4, c = 3;
b < c)
<< “YES”;
<< “NO”;
Output: YES
The left inequality is checked and
evaluates to false (numerical 0),
which is less than the c-value!
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Boolean Expressions Short-Circuit Evaluation of &&
When a Boolean expression using the && operator is
evaluated, the first subexpression is evaluated first. if it
evaluates to false, then the second subexpression is ignored.
if ((x != 0) && (y/x > 1))
z = 100;
else
z = -1;
When this code segment is
encountered and x’s value is
zero, the program crashes due to
the attempt to divide by zero!
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When this code segment is
encountered and x’s value is
zero, z will be assigned either
the value 100 or the value -1.
if ((y/x > 1) && (x != 0))
z = 100;
else
z = -1;
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Boolean Expressions Short-Circuit Evaluation of ||
When a Boolean expression using the || operator is
evaluated, the first subexpression is evaluated first. if it
evaluates to true, then the second subexpression is ignored.
if ((ct == 0) || (total/ct > 70))
cout << “NO PROBLEMO”;
When this code segment
is encountered and ct’s
value is zero, the program
crashes due to the attempt
to divide by zero!
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When this code segment is
encountered and ct’s
value is zero, the message
is output.
if ((total/ct > 70) || (ct == 0))
cout << “NO PROBLEMO”;
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Enumerated
Types
To enhance the readability of one’s code, a
programmer can define an enumerated type,
which consists of a set of integer constants.
#include <iostream>
using namespace std;
enum MonthNumber { JANUARY = 1, FEBRUARY, MARCH,
APRIL,
MAY,
JUNE,
JULY,
AUGUST,
SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER };
void main()
{
MonthNumber monthCount = JANUARY;
double monthlyRate = 12.34, totalCost = 0.0;
cout.setf(ios::fixed);
cout.setf(ios::showpoint);
cout.precision(2);
cout << "Computing Annual Costs:\n\n";
while (monthCount <= DECEMBER)
{
totalCost += monthlyRate;
monthCount = MonthNumber(monthCount + 1);
}
cout << endl << endl << "Total: $" << totalCost << endl << endl;
return;
}
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switch Statements
C++ switch statements provide a simple alternative to the
use of convoluted nested if-else statements.
if (month == 2)
{
if (year % 4 == 0)
daysInMonth = 29;
else
daysInMonth = 28;
}
else if ((month == 4) ||
(month == 6) ||
(month == 9) ||
(month == 11))
daysInMonth = 30;
else
daysInMonth = 31;
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switch (month)
{
case 2: {
if (year % 4 == 0)
daysInMonth = 29;
else
daysInMonth = 28;
break;
}
case 4:
case 6:
case 9:
case 11: { daysInMonth = 30; break; }
default: { daysInMonth = 31; break; }
}
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#include <iostream>
using namespace std;
The break statements
void main()
ensure that the switch
{
statement is exited once the
char letter;
appropriate case is handled.
bool gotAVowel = false;
do
Without the break
{
cout << "Please enter a letter: ";
statements, all cases might
cin >> letter;
execute.
switch(letter)
{
case 'a': case 'A':
cout << "Apple\n"; gotAVowel = true; break;
case 'e': case 'E':
cout << "Egg\n";
gotAVowel = true; break;
case 'i': case 'I':
cout << "Iodine\n"; gotAVowel = true; break;
case 'o': case 'O':
cout << "Oval\n";
gotAVowel = true; break;
case 'u': case 'U':
cout << "Upper\n"; gotAVowel = true; break;
default:
cout << "That is not a vowel. Please try again.\n";
}
The default case is executed if no
}while(!gotAVowel);
}
other case value matches.
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Conditional Statements
C++ conditional statements provide a concise alternative to the
use of relatively simple if-else statements.
if (val > 0)
cout << “GOOD”;
else
cout << “BAD”;
if (year % 4 == 0)
daysInMonth = 29;
else
daysInMonth = 28;
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(val > 0) ? (cout << “GOOD”) : (cout << “BAD”);
daysInMonth = (year % 4 == 0) ? 29 : 28;
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The for Statement
The C++ for statement is designed to facilitate looping when
the control of the loop contains three standard features:
• An initialization action that is performed just before the loop is
started the first time.
• A Boolean expression that is checked just before entering the
loop at each iteration.
• An update action that is performed just after each iteration of
the loop is completed.
for (initAction; booleanExp; updateAction)
bodyOfLoop
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A for-loop Example
#include <iostream>
using namespace std;
const double PAY_RATE = 10.15;
void main()
dayOfWeek is initialized to
{
1when the for-loop is first
double payForWeek = 0.0;
encountered.
int dayOfWeek;
double hoursWorked;
This inequality is checked just
before the body of the loop is
entered (or re-entered).
This increment statement
is executed right after each
execution of the body of
the loop.
for(dayOfWeek = 1; dayOfWeek <= 7; dayOfWeek++)
{
cout << "How many hours did you work on day " << dayOfWeek << "? ";
cin >> hoursWorked;
(dayOfWeek == 1) ?
(payForWeek += 1.5 * hoursWorked * PAY_RATE) :
(payForWeek += hoursWorked * PAY_RATE);
}
cout.setf(ios::fixed);
cout.setf(ios::showpoint);
cout.precision(2);
cout << endl << endl << "Your Pay For \n"
<< "The Week Is $" << payForWeek
<< endl << endl;
}
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Nested for-loops
#include <iostream>
using namespace std;
void main()
{
int iterationNbr, repetitionNbr, indentSpaceNbr;
for (iterationNbr = 1; iterationNbr <= 25; iterationNbr++)
for (repetitionNbr = 1; repetitionNbr <= 1000; repetitionNbr++)
{
cout << '\r';
for (indentSpaceNbr = 1; indentSpaceNbr <= iterationNbr; indentSpaceNbr++)
cout << ' ';
cout << "HAPPY NEW YEAR!";
}
cout << endl << endl;
return;
}
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