Lecture An introduction to computer science using java (2nd Edition): Chapter 6 - S.N. Kamin, D. Mickunas, E. Reingold

Chapter 6

Iteration
Lecture Slides to Accompany

An Introduction to Computer
Science Using Java (2nd Edition)
by
S.N. Kamin, D. Mickunas, E. Reingold
 

 


Chapter Preview
In this chapter we will:
• discuss the use of repetition (iteration) in
programming algorithms
• describe the Java while, for, and do-while
statements
• demonstrate the use of loop invariants to
verify the correctness of loops
• show the use of loops in data entry and
computer animation
 

 


while Loop
• Repeated executes body of the loop which

can be a single or compound statement
• A while loop will execute as long as its
condition is true
• An infinite loop will occur if the condition
never becomes false
• Example:

 

count = 1;
while (count <= 10)
count = count + 1;
 


while Loop From Temperature
Conversion Program
OutputBox out = new OutputBox( );
out.println(“\tDEGREES C\tDEGREES F”);
// initialize the loop variable
cent = LOW_TEMP;
// test the loop variable
while (cent <= HIGH_TEMP) {
// convert C to F
fahr = (9.0 / 5.0) * cent + 32.0;
out.println(“\t” + cent “\t\t” fahr);
// increment loop variable
cent = cent + 1.0;
}

 

 


 

 


for Statement
• Used to implement pre-test loops that are
controlled by incrementing a variable each
time the loop is executed
• Loop variable computations for initialization,
testing, and incrementing appear in a single
statement
• Example:
for(count=1; count <= 10; count = count + 1);

 

 


Comparing for and while
• Done using for

• Equivalent while loop

for ( statement1;
condition;
statement2 )
statement 3;

 

 

statement1;
while (condition) {
statement3;
statement2;
}


Temperature Conversion Program
Loop Implemented Using for
OutputBox out = new OutputBox( );
out.println(“\tDEGREES C\tDEGREES F”);
for (cent = LOW_TEMP;
cent <= HIGH_TEMP;
cent = cent + 1.0;) {
// convert C to F
fahr = (9.0 / 5.0) * cent + 32.0;
out.println(“\t” + cent “\t\t” fahr);
}
 

 

do-while Statement
• Used to implement post-test loops that always
execute the loop body at least one time
• A do-while loop will continue execute as long as its
condition is true
• An infinite loop will occur if the condition never
becomes false
• Example:
count = 1;
do {
count = count + 1;
} while (count <= 10);

 

 


Comparing do-while and while
• Done using do-while
do
statement;
while (condition);

 

 

• Equivalent while loop
statement;
while (condition)
statement;


Comparing do-while and while
• Done using do-while

• Equivalent while loop

numberOfDigits = 0;
rest = number;
do {
rest = rest / 10;
numberOfDigits++;
} while (rest != 0);

numberOfDigits = 0;
rest = number;
if (number = 0)
numberOfDigits = 1;
else
while (rest > 0) {
rest = rest / 10;
numberOfDigits++;
};

 

 


Reading Input Using a Loop
• Done using do-while

• Done using while

read score
if ( not end of input)
do {
process score
read score
} while ( not end of input );

read score
while ( not end of input ) {
process score
read score
};

 

 


Sentinel Controlled Loop
int i = 0;
InputBox in = new InputBox();
In.setPrompt(“Enter data or –1 to terminate”);

// read first score
score = in.readInt();
while ( score != -1 ) {
i++;
// insert code to process score
// read next score
score = in.readInt();

}
 

 


Reading to End of Input
int i = 0;
InputBox in = new InputBox();
In.setPrompt(“Enter score (empty input ends data”);
// read first score
score = in.readInt();
while ( !in.eoi() ) {
i++;
// insert code to process score
// read next score
score = in.readInt();

}
 

 

Loop-and-a-Half
• Done using while

• Done using do-while

read score
while ( !in.eoi( ) ) {
process score
read score
};

do {
read score
if ( !in.eoi( ) )
process score
} while ( !in.eoi( ) );

 

 


 

 


Infinite Loop with Escape

• Done using do-while

• Done using while

do {
read
if ( in.eoi( ) ) return
process
} while ( true );

while (true) {
read
if ( in.eoi( ) ) return
process score
};

 

 


 

 


Drawing in Java
// code to read and display GIF image from file
import java.awt.*;
Import CSLib.*;

…
DrawingBox g = new DrawingBox();
…
Toolkit tools = Toolkit.getDefaultToolkit():
Image mouse =
tools.getImage(“C:/KMR/images/mouse.gif”);
g.drawImage(mouse, x, y);

 

 


Nested Loops
int advance = 0;
// Draw clock and read input repeatedly
do {
for (int i = 1; i <= advance; i++) {
// Advance time
c.setMinute(c.getMinute()+1);
// Update clock display
d.clear();
c.display(d, SIZE/2, SIZE/2, SIZE/2);
}
in.setPrompt(“Advance how many minutes?”);
advance = in.readInt();
} while (!in.eoi());