Chapter 5: Repetition Control Structures

Transcription

Chapter 5:
Repetition Control Structures
Repetition (looping) control structures.
Count-controlled, sentinel-controlled, flagcontrolled, and EOF-controlled repetition
structures.
Break and continue statements.
nested control structures.
Java Programming: From Problem Analysis to Program Design, Second Edition
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Why Is Repetition Needed?
There are many situations in which the same
statements need to be executed several times.
Example:
Formulas used to find average grades for
students in a class.
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1
The while Looping (Repetition) Structure
Syntax:
while (expression)
statement
Expression is always true in an infinite loop.
Statements must change value of expression to false.
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Example 5-1
i = 0;
while (i <= 20)
{
System.out.print(i + " ");
i = i + 5;
}
System.out.println();
//Line 1
//Line 2
//Line 3
//Line 4
//Line 5
Output
0 5 10 15 20
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2
Typically, while loops are written in the following form:
//initialize the loop control variable(s)
while (expression)
//expression tests the LCV
{
.
.
.
//update the loop control variable(s)
.
.
.
}
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Counter-Controlled while Loop
Used when exact number of data or entry pieces is
known.
General form:
int N = //value input by user or specified
//in program
int counter = 0;
while (counter < N)
{
.
.
.
counter++;
.
.
.
}
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3
Sentinel-Controlled while Loop
Used when exact number of entry pieces is unknown, but
last entry (special/sentinel value) is known.
General form:
Input the first data item into variable;
while (variable != sentinel)
{
.
.
.
input a data item into variable;
.
.
.
}
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Flag-Controlled while Loop
Boolean value used to control loop.
General form:
boolean found = false;
while (!found)
{
.
.
.
if (expression)
found = true;
.
.
.
}
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4
EOF(End of File)-Controlled while Loop
Used when input is from files.
Sentinel value is not always appropriate.
In an EOF-controlled while loop that uses the Scanner
object console to input data, console acts at the
loop control variable.
The method hasNext, of the class Scanner,
returns true if there is an input in the input stream;
otherwise, it returns false.
The expression console.hasNext() acts as the
loop condition.
Expressions such as console.nextInt() update the
value of the loop condition.
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EOF-Controlled while Loop
A general form of the EOF-controlled while loop
that uses the Scanner object console to input
data is:
while (console.hasNext())
{
//Get the next input and store in an
//appropriate variable
//Process data
}
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5
EOF-Controlled while Loop
Suppose that inFile is a Scanner object
initialized to the input file. In this case, the EOFcontrolled while loop takes the following form:
while (inFile.hasNext())
{
//Get the next input and store in an
//appropriate variable
//Process data
}
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Programming Example: Checking
Account Balance
Input file: Customer’s account number, account
balance at beginning of month, transaction type
(withdrawal, deposit, interest), transaction amount.
Output: Account number, beginning balance, ending
balance, total interest paid, total amount deposited,
number of deposits, total amount withdrawn,
number of withdrawals.
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6
Programming Example: Checking
Account Balance
Solution:

Read data.
EOF-controlled loop.
switch structure of transaction types.
Determine action (add to balance or subtract
from balance depending on transaction type).
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Programming Example:
Fibonacci Number

Fibonacci formula for any Fibonacci sequence:
an = an-1 + an-2
Input: First two Fibonacci numbers in sequence,
position in sequence of desired Fibonacci number (n).

int previous1 = Fibonacci number 1
int previous2 = Fibonacci number 2
int nthFibonacci = Position of nth Fibonacci number
Output: nth Fibonacci number.
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7
Programming Example: Fibonacci Number (Solution)
if (nthFibonacci == 1)
current = previous1;
else if (nthFibonacci == 2)
current = previous2;
else
{
counter = 3;
while (counter <= nthFibonacci)
{
current = previous2 + previous1;
previous1 = previous2;
previous2 = current;
counter++;
}
}
Final result found in last value of current.
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The for Looping (Repetition) Structure
Specialized form of while loop.
Simplifies the writing of count-controlled loops.
Syntax:
for (initial statement; loop condition;
update statement)
statement
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8
Execution:
Initial statement executes.
Loop condition is evaluated.
If loop condition evaluates to true, execute for
loop statement and execute update statement.
Repeat until loop condition is false.
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Example 5-8
The following for loop prints the first 10
nonnegative integers:
for (i = 0; i < 10; i++)
System.out.print(i + " ");
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9
Example 5-9
1. The following for loop outputs the word Hello and a star (on
separate lines) five times:
2.
for (i = 1; i <= 5; i++)
{
System.out.println("Hello");
System.out.println("*");
}
The following for loop outputs the word Hello five times and the
star only once:
for (i = 1; i <= 5; i++)
System.out.println("Hello");
System.out.println("*");
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Does not execute if initial condition is false.
Update expression changes value of loop control
variable, eventually making it false.
If loop condition is always true, result is an infinite
loop.
Infinite loop can be specified by omitting all three
control statements.
If loop condition is omitted, it is assumed to be
true.
for statement ending in semicolon is empty.
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10
Programming Example: Classify
Numbers
Input: N integers (positive, negative, and zeros).
int N = 20;
//N easily modified
Output: Number of 0s, number of even integers,
number of odd integers.
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Programming Example: Classify Numbers
(Solution)
for (counter = 1; counter <= N; counter++)
{
number = console.nextInt();
System.out.print(number + " ");
switch (number % 2)
{
case 0: evens++;
if (number == 0)
zeros++;
break;
case 1:
case -1: odds++;
} //end switch
} //end for loop
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11
The do…while Loop (Repetition)
Structure
Syntax:
do
statement
while (expression);
Statements are executed first and then expression is
evaluated.
Statements are executed at least once and then
continued if expression is true.
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do…while Loop (Post-Test Loop)
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12
break Statements
Used to exit early from a loop.
Used to skip remainder of switch structure.
Can be placed within if statement of a loop.
If condition is met, loop is exited immediately.
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continue Statements
Used in while, for, and do...while structures.
When executed in a loop, the remaining statements
in the loop are skipped; proceeds with the next
iteration of the loop.
When executed in a while/do…while structure,
expression is evaluated immediately after continue
statement.
In a for structure, the update statement is executed
after the continue statement; the loop condition
then executes.
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13
Nested Control Structures
Provides new power, subtlety, and complexity.
if, if…else, and switch structures can be
placed within while loops.
for loops can be found within other for loops.
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Nested Control Structures (Example)
for (int i = 1; i <= 5; i++)
{
for (int j = 1; j <= i; j++)
System.out.print(" *");
}
Output:
*
**
***
****
*****
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14
Chapter Summary
Looping mechanisms:

Counter-controlled while loop
Sentinel-controlled while loop
Flag-controlled while loop
EOF-controlled while loop
for loop
do…while loop
break statements
continue statements
Nested control structures
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