Transcript CS102 Introduction to Computer Programming

CS102
Introduction to Computer
Programming
Chapter 4 Making Decisions
Chapter 4 Topics
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Relational Operators
Value of a Relationship
What is Truth
The if Statement
Flags
Expanding the if Statement
The if/else Statement
The if/else if Statement
Using the Trailing else
Relational Operators
• Relational Operator
>
<
>=
<=
==
!=
Meaning
greater than
Less than
greater than or equal to
less than or equal to
equivalent to
not equal to
Concept - Relational operators allow you to compare
numeric values
Value of a Relationship
• All expressions have a value
– The value of a relational expression is either 1 if true or
0 if false
– A relational expression is also called a Boolean
expression
• Think of a relational expression as a question the
computer will answer not a statement of fact
Concept - A relational expression is either true or false
Program 4-1
/* This program displays the values
of true and false states.*/
#include <iostream.h>
void main(void)
{
int True, False
int X = 5, Y = 10;
True = (X < Y);
False = (Y = = X);
cout << "True is " << True << endl;
cout << "False is " << False <<
endl;
}
The computer represents the
abstract concept of truth as an
integer value.
1 for true
0 for not true
Program Output
True is 1
False is 0
What is Truth
• C++ deals with the abstract states of true or false
by converting them to numbers
• Because a relational expression is an expression
and has a value, the value can be
– assigned to a variable
– displayed with cout
– used in a mathematical expression
Concept - True and false are values that can be stored in
memory
Check Point
• If x is 5, y is 6
and z is 8
T
T
T
T
T
T
T
x == 5
7 <= (x+2)
z>4
(2+x)!=y
z!=4
x>=0
x<=(y*2)
• Correct or not
F
F
T
T
F
F
(x<=y) == (y>x)
(x!=y) == (y>=x)
(x>=y) == (y<=x)
x>y and x<z then y<z
x>=y and x==z then y==z
x!=y and x!=z then z!=y
The if Statement
• The if statement allows a program to have more
than one path of execution
if (expression)
statement ;
• If the value of the expression is:
– 0, then statement is skipped
– Not 0, then statement is executed
• Note: the ; goes at the end of the if statement not
at the end of the line
Concept - The if statement can cause other statements to
execute only under certain conditions
The if Flow
Chart Symbol
if
expression
0
end
!0
statement
Program 4-2
// This program averages 3 test
scores
#include <iostream.h>
void main(void)
{
int Score1, Score2, Score3;
float Average;
cout << "Enter 3 test scores
and I will average them: ";
cin >> Score1 >> Score2 >>
Score3;
Average = (Score1 + Score2 +
Score3) / 3.0;
cout.precision(1);
cout.setf(ios::showpoint |
ios::fixed);
cout << "Your average is " <<
Average << endl;
if (Average > 95)
cout << "Congratulations!
That's a high score!\n";
}
This program uses a simple if
statement to test the value of average
Program Output
Enter 3 test scores and I will average
them:
80 90 70 [Enter]
Your average is 80.0
Enter 3 test scores and I will average
them:
100 100 100 [Enter]
Your average is 100.0
Congratulations! That's a high score!
The if Statement continued
• Be careful with comparing floating point
numbers
– Rounding errors can cause unwanted results
– stick with < or > relationships
• Any non-zero value is considered true
• = = is not the same as =
x = = y is either 0 or 1
if (x = = y )
x = y is the value of y
if (x = y )Unless y=0 this condition is always true
Flags
• Flag variables are set by the programmer to
trigger a specific operation if a specific
condition has been met
– if the flag = = 0 , then the condition is not met
– if the flag != 0, then the condition has been met
• Note: Variables are not automatically set to
0
– always be sure to initialize flags
Concept - A flag is a variable, usually an integer, that signals
when a condition has been met
Program 4-6
/* This program averages 3 test
scores. It uses the variable
HighScore as a flag.*/
#include <iostream.h>
void main(void)
{
int Score1, Score2, Score3;
int true = 1, false = 0;
float Average;
int HighScore = false;
cout << "Enter your 3 test
scores and I will average
them: ";
cin >> Score1 >> Score2 >>
Score3;
Average = (Score1 + Score2 +
Score3) / 3.0;
if (Average > 95)
HighScore = true;
// Set the flag variable
cout << fixed << showpoint
<<setprecision(1);
cout << "Your average is " <<
Average << endl;
if (HighScore)
cout << "Congratulations!
That's a high score!\n";
}
Program Output with Example Input
Enter your 3 test scores and I will average
them: 100 100 100 [Enter]
Your average is 100.0
Congratulations! That's a high score!
The conditional expression in an if
statement does not have to be a
relational expression.
Expanding the if Statement
• If you need to execute more than one statement
when the condition is true:
– start with an open bracket
{
– enter all the statements to be executed ending each one
with a ;
– End with a closing bracket
}
Concept - The if statement can conditionally execute a
block of statements enclosed in brackets
Program 4-7
/* This program averages 3 test
scores. It uses the variable
HighScore as a flag.*/
#include <iostream.h>
void main(void)
{
int Score1, Score2, Score3;
float Average;
bool HighScore = false;
cout << "Enter 3 test scores
and I will average them: ";
cin >> Score1 >> Score2 >>
Score3;
Average = (Score1 + Score2 +
Score3) / 3.0;
if (Average > 95)
HighScore = true;
// Set the flag variable
If the condition is true everything
inside the braces is executed
cout << fixed << showpoint
<< setprecision(1);
cout << "Your average is " <<
Average << endl;
if (HighScore)
{
cout <<
"Congratulations!\n";
cout << "That's a high
score.\n";
cout << "You deserve a pat
on the back!\n";
}
}
Program Output
Enter your 3 test scores and I will
average them: 100 100 100
[Enter]
Your average is 100.0
Congratulations!
That's a high score!
Your deserve a pat on the back
Programming Style and the if
Statement
• The conditionally executed statement
should appear on the line after the if
statement.
• The conditionally executed statement
should be indented one “level” from the if
statement.
• Note: Each time you press the tab key, you
are indenting one level.
The if/else Statement
• The if/else statement selects one of two
statements or blocks of code to execute
if (expression)
statement or block;
else
statement or block;
Concept - The if/else statement will execute one group of
statements if the expression is true and another if it is false
The if/else Flow Chart Symbol
Statement
or block
False
if
expression
end
True
Statement
or block
Program 4-9
/* This program uses the modulus
operator to determine if a number is
odd or even. If evenly divided the
number is by 2, it is an even
number. A remainder indicates it is
odd.*/
#include <iostream.h>
void main(void)
{
int Number;
cout << "Enter an integer and I will
tell you if it\n";
cout << "is odd or even. ";
cin >> Number;
if (Number % 2 == 0)
cout << Number << " is even.\n";
else
cout << Number << " is odd.\n";
}
The if/else allows the control
of two paths with one
statement
Program Output
Enter an integer and I will tell
you if it is odd or even.
17 [Enter]
17 is odd.
Program 4-10
/* This program asks the user
for two numbers, Num1 and
Num2. Num1 is divided by Num2
and the result is displayed.
Before the division
operation, however, Num2 is
tested for the value 0. If it
contains 0, the division does
not take place.*/
#include <iostream.h>
void main(void)
{
float Num1, Num2, Quotient;
cout << "Enter a number: ";
cin >> Num1;
cout << "Enter another
number: ";
cin >> Num2;
}
if (Num2 == 0)
{
cout << "Division by zero
is not possible.\n";
cout << "Please run the
program again and
enter\n";
cout << "a number besides
zero.\n";
}
else
{
Quotient = Num1 / Num2;
cout << "The quotient of
" << Num1 << " divided by
";
cout
<< Num2 << " is "
<< Quotient << ".\n";
} This program uses the if
statement to avoid an undesirable
operation
Program Output
(When the user enters 0 for Num2)
Enter a number: 10 [Enter]
Enter another number: 0 [Enter]
Division by zero is not possible.
Please run the program again and enter
a number besides zero.
The if/else if Statement
• The tests are performed one after another until one
is found to be true.
• The if/else statement selects one of two
statements or blocks of code to execute
if (expression)
statement or block;
else if (expression)
statement or block;
Concept - The if/else if statement is a chain of if
statements
Flow Chart for the if/else if
False
True
Else
if
expression
False
end
if
expression
Statement
or block
True
Statement
or block
Program 4-11
/* This program uses an if/else
if statement to assign a
letter grade (A, B, C, D, or
F) to a numeric test score.*/
if (TestScore < 60)
Grade = 'F';
else if (TestScore < 70)
Grade = 'D';
else if (TestScore < 80)
Grade = 'C';
else if (TestScore < 90)
Grade = 'B';
else if (TestScore <= 100)
Grade = 'A';
cout << "Your grade is "
<< Grade << ".\n";
#include <iostream.h>
void main(void)
{
int TestScore;
char Grade;
cout << "Enter your numeric
test score and I will\n";
cout << "tell you the letter
grade you earned: ";
cin >> TestScore;
As soon as a true condition is met the
if/else if statement terminates
}
Program Output
Enter your test score and I will tell you
the letter grade you earned:
88 [Enter]
Your grade is B.
Program 4-12
// This program uses
independent if statements to
assign a letter grade (A, B,
C, D, or F) to a numeric test
score.
#include <iostream.h>
if (TestScore < 60)
Grade = 'F';
if (TestScore < 70)
Grade = 'D';
if (TestScore < 80)
Grade = 'C';
if (TestScore < 90)
Grade = 'B';
if (TestScore <= 100)
Grade = 'A';
cout << "Your grade is "
<< Grade << ".\n";
void main(void)
{
int TestScore;
char Grade;
cout << "Enter your test
score and I will tell you\n";
cout << "the letter grade you
earned: ";
cin >> TestScore;
Do you think it will work?
}
Program Output
Enter your test score and I will tell you
the letter grade you earned: 40 [Enter]
Your grade is A.
Using the Trailing else
Can be used to to take care of any case not
considered by the if expressions
if (expression)
statement or block;
else if (expression)
statement or block;
else
statement or block;
Concept - The trailing else placed at the end of an if/else if
statement provides default action when none of the ifs are true
Program 4-13
/* This program uses an if/else
if statement to assign a
letter grade (A, B, C, D, or
F) to a numeric test score. A
trailing else has been added
to catch test scores > 100.*/
#include <iostream.h>
void main(void)
{
int TestScore;
cout << "Enter your test
score and I will tell you\n";
cout << "the letter grade you
earned: ";
cin >> TestScore;
if (TestScore < 60)
{
cout << "Your grade is
F.\n";
cout << "This is a
failing grade. Better see
your ";
cout << "instructor.\n";
}
else if (TestScore < 70)
{
cout << "Your grade is
D.\n";
cout << "This is below
average. You should get ";
cout << "tutoring.\n";
}
Program continues
else if (TestScore < 80)
{
cout << "Your grade is C.\n";
cout << "This is average.\n";
}
else if (TestScore < 90)
{
cout << "Your grade is B.\n";
cout << "This is an above
average grade.\n";
}
else if (TestScore <= 100)
{
cout << "Your grade is A.\n";
cout << "This is a superior
grade. Good work!\n";
}
else
else
{
cout << TestScore << " is an
invalid score.\n";
cout << "Please enter
scores no greatr than
100.\n";
}
}
Program Output
Enter your test score and I will tell you
the letter grade you earned: 104 [Enter]
104 is an invalid score.
Please enter scores no greater than 100.