for, while, etc - WCU Computer Science
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Transcript for, while, etc - WCU Computer Science
CSC141 Computer Science I
Zhen Jiang
Dept. of Computer Science
West Chester University
West Chester, PA 19383
[email protected]
4/12/2016
1
Loop
Smart if-decision making, smart program work, talent
programmer
Research experience (REU) - click on this link
Temperature/humidity detection every second
A repetition process for the 7x24 hours seamless surveillance
Needs a computer support to REPEAT …
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2
While loop
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Format & Logic, page 193, Figure 4-1.
Sample, code 4-3, page 194.
3
<initialization>;
while (<test>) {
<body>;
}
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4
Do-while loop
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Format, page 204
Logic, page 205, Figure 4-6.
Sample, code 4-6, page 205.
5
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How does this differ from
the while loop?
The controlled
<statement(s)> will
always execute the first
time, regardless of
whether the <test> is
true or false.
6
For loop
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Format, page 208, Figure 4-7.
Logic, page 208, Figure 4-8.
Sample, code 4-7, page 209.
7
for (<init>; <test>; <update>) {
<body>;
}
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8
Summary
Body first, and then
event change/update
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9
Initialization, test, and body, and
execution results of loop
Code:
for (int i = 1; i <= 4; i++) {
System.out.println(i + " squared is " + (i * i));
}
Output:
1
2
3
4
squared
squared
squared
squared
is
is
is
is
1
4
9
16
10
Variations
The initial and final values for the loop counter/event variable can be
arbitrary expressions:
Example:
for (int i = -3; i <= 2; i++) {
System.out.println(i);
}
Output:
-3
-2
-1
0
1
2
Example:
for (int i = 1 + 3 * 4; i <= 5248 % 100; i++) {
System.out.println(i + " squared is " + (i * i));
}
11
The update can be a -- (or any other operator).
Caution: This requires changing the test from <= to >= .
System.out.println("T-minus");
for (int i = 3; i >= 1; i--) {
System.out.println(i);
}
System.out.println("Blastoff!");
Output:
T-minus
3
2
1
Blastoff!
12
What if we wanted the output to be the following?
T-minus 3 2 1 Blastoff!
System.out.print prints the given output without
moving to the next line.
System.out.print("T-minus ");
for (int i = 3; i >= 1; i--) {
System.out.print(i + " ");
}
System.out.println("Blastoff!");
13
When controlling a single statement, the {} braces are optional.
for (int i = 1; i <= 6; i++)
System.out.println(i + " squared is " + (i * i));
This can lead to errors if a line is not properly indented.
for (int i = 1; i <= 3; i++)
System.out.println("This is printed 3 times");
System.out.println("So is this... or is it?");
Output:
This is printed 3 times
This is printed 3 times
This is printed 3 times
So is this... or is it?
Moral: Always use curly braces and always use proper indentation.
14
Extra semicolon in a loop (P218).
int i;
for (i = 1; i <= 6; i++);
System.out.println(i + " squared is " + (i * i));
Output:
7 squared is 49
Comman in a loop (P220).
int sum;
for (int i=0, sum; …
int i, sum;
for (i = 1, sum = 0; i <= 10; i++)
sum = sum + i * i;
System.out.println("Result is " + sum);
Output:
385
15
Invalidation: Loops that never execute.
for (int i = 10; i < 5; i++) {
System.out.println("How many times do I print?");
}
ERROR: Loop tests that never fail.
A loop that never terminates is called an infinite loop.
for (int i = 10; i >= 1; i++) {
System.out.println("Runaway Java program!!!");
}
16
Loops that go on… forever
while (true) {
<statement(s)>;
}
If it goes on forever, how do you stop?
17
break statement: Immediately exits a loop (for, while,
do/while).
Example:
while (true) {
<statement(s)>;
if (<test>) {
break;
}
<statement(s)>;
}
Why is the break statement in an if statement?
18
Sentinel loop using break:
Scanner console = new Scanner(System.in);
int sum = 0;
while (true) {
System.out.print("Enter a number (-1 to quit): ");
int inputNumber = console.nextInt();
if (inputNumber == -1) { // don't add -1 to sum
break;
}
sum += inputNumber; // inputNumber != -1 here
}
System.out.println("The total was " + sum);
19
Special case: If a variable is declared in the
<initialization> part of a for loop, its scope is
the for loop.
public static void main(String [] args) {
int x = 3;
int i;
for (i = 1; i <= 10; i++) {
System.out.println(x); i’s scope x's scope
}
// i no longer exists here
} // x ceases to exist here
20
ERROR: Using a variable outside of its scope.
public static void main(String[] args) {
for (int i = 1; i <= 10; i++) {
int y = 5;
System.out.println(y);
}
System.out.println(i); // illegal
System.out.println(y); // illegal
}
21
COMMON ERROR: Using the wrong loop counter variable.
But barely possible when you develop code with our process.
What is the output of the following piece of code?
for (int i = 1; i <= 10; i++) {
for (int j = 1; i <= 5; j++) {
System.out.print(j);
}
System.out.println();
}
What is the output of the following piece of code?
for (int i = 1; i <= 10; i++) {
for (int j = 1; j <= 5; i++) {
System.out.print(j);
}
System.out.println();
}
22
Ex10
http://www.cs.wcupa.edu/~zjiang/141_ex10.pdf
Ex11
http://www.cs.wcupa.edu/~zjiang/141_ex11.pdf
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23
Trial
population
TV purchase
http://www.cs.wcupa.edu/~zjiang/tv563.exe
1+2+4+8+...
http://www.cs.wcupa.edu/~zjiang/6billion.exe
http://www.cs.wcupa.edu/~zjiang/1_2_4.exe
1+2+3+4+...+99
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http://www.cs.wcupa.edu/~zjiang/1to99.exe
24
Development process
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http://www.cis.temple.edu/~jiang/LoopDevelopment.htm
25
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Controlling Number of
Loop Iterations
If the number of iterations is known before
the loop starts, the loop is called a count-
controlled loop.
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Counter =0, counter++, counter
<number
Counter = 1, counter++, counter
<=number
Use for loop for an easy development.
27
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29
Mapping iterations to counter values
Suppose that we have the following loop:
for (int count = 0; count < 49; count++) {
...
}
What statement could we write in the body of the loop that
would make the loop print the following output?
0 2 4 6 8 …
Answer:
for (int count = 0; count < 49; count++) {
System.out.print(2 * count + " ");
}
30
Now consider another loop of the same style:
for (int count = 0; count < 49; count++) {
...
}
What statement could we write in the body of the loop that
would make the loop print the following output?
3 5 7 9 11
Answer:
for (int count = 0; count < 49; count++) {
System.out.print(2 * count + 3 + " ");
}
31
What statement could we write in the body of the loop that would
make the loop print the following output?
2 7 12 17 22
To find the pattern, it can help to make a table.
Each time count goes up by 1, the number should go up by 5.
But count * 5 is too big by 3, so we must subtract 3.
count
number to print
count * 5
count * 5 - 3
1
2
5
2
2
7
10
7
3
12
15
12
4
17
20
17
5
22
25
22
32
25
20
15
10
5
count (x)
number to print (y)
1
2
2
7
3
12
4
17
5
22
0
-2
0
-5
-10
2
4
6
33
Caution: This is algebra, not assignment!
Recall: slope-intercept form (y = mx + b)
Slope is defined as “rise over run” (i.e. rise / run). Since the “run” is
always 1 (we increment along x by 1), we just need to look at the
“rise”. The rise is the difference between the y values. Thus, the slope
(m) is the difference between y values; in this case, it is +5.
To compute the y-intercept (b), plug in the value of y at x = 1 and
solve for b. In this case, y = 2.
y = m * x + b
2 = 5 * 1 + b
Then b = -3
So the equation is
y = m * x + b
y = 5 * x – 3
y = 5 * count - 3
count (x)
number to print (y)
1
2
2
7
3
12
4
17
5
22
34
Algebraically, if we always take the value of y at
x = 1, then we can solve for b as follows:
y = m * x + b
y1 = m * 1 + b
y1 = m + b
b = y1 – m
In other words, to get the y-intercept, just subtract the
slope from the first y value (b = 2 – 5 = -3)
This gets us the equation
y = m * x + b
y = 5 * x – 3
y = 5 * count – 3
(which is exactly the equation from the previous slides)
35
What statement could we write in the body of the loop that would
make the loop print the following output?
17 13 9 5 1
Let's create the loop table together.
Each time count goes up 1, the number should ...
But this multiple is off by a margin of ...
count
number to print
count * -4
count * -4 + 21
1
17
-4
17
2
13
-8
13
3
9
-12
9
4
5
-16
5
5
1
-20
1
36
Code:
for (int i = 1; i <= 4; i++) {
System.out.println(i + " squared is " + (i * i));
}
Output:
1
2
3
4
squared
squared
squared
squared
is
is
is
is
1
4
9
16
http://www.cs.wcupa.edu/~zjiang/141_ex11.pdf
4/12/2016
37
Coding (different from execution check):
n=keyboard.nextInt(); // try 6!
for (int i = 1; i <= n; i++)
{
System.out.print("*");
}
System.out.println();
Output:
******
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38
More complicate case:
n=keyboard.nextInt(); // try 6!
for (int i = 1; i <= n; i++)
{
for (int j = 1; j <= n; j++)
{
System.out.print("*");
}
System.out.println();
}
Output:
******
******
******
******
******
******
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39
Code:
n=keyboard.nextInt(); // try 5!
for (int i = 1; i <= n; i++)
{
for (int j = 1; j <= 10; j++)
{
System.out.print(
(i * j) + " ");
}
System.out.println();
}
Output:
1
2
3
4
5
2 3 4 5 6 7 8 9 10
4 6 8 10 12 14 16 18 20
6 9 12 15 18 21 24 27 30
8 12 16 20 24 28 32 36 40
10 15 20 25 30 35 40 45 50
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40
How to confirm the initialization correct?
On preparing the 1st iteration …
How to ensure the detail of the body?
A consistent view of 1st, 2nd, 3rd iterations …
Map of the counter value to the iteration
expression …
41
Code:
n=keyboard.nextInt(); // try 6!
for (i = 1; i<=n; i++) System.out.print(“*”);
System.out.println(“”);
for (i = 1; i <= n-2; i++) {
System.out.print(“*”);
for (int j = 1; j <= n-2; j++)
System.out.print(“ ”);
System.out.println(“*”);
}
for (i = 1; i<=n; i++) System.out.print(“*”);
System.out.println(“”);
Output:
******
*
*
*
*
*
*
*
*
******
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42
Code:
n=keyboard.nextInt(); // try 6!
for (int i = 1; i <= n; i++) {
for (int j = 1; j <= i; j++) {
System.out.print("*");
}
System.out.println();
}
Output:
*
**
***
****
*****
******
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43
Code:
n=keyboard.nextInt(); // try 6!
for (int i = 1; i <= n; i++) {
for (int j = 1; j <= i; j++) {
System.out.print(i);
}
System.out.println();
}
Output:
1
22
333
4444
55555
666666
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44
Code:
n=keyboard.nextInt(); // try 5!
for (int i = 1; i <= n; i++) {
for (int j = 1; j <= (n - i); j++) {
System.out.print(" ");
}
for (int k = 1; k <= i; k++) {
System.out.print(i);
}
System.out.println();
}
Output:
1
22
333
4444
55555
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45
Controlling Event of Loop
Iterations
Otherwise (unknown or unclear), the loop is
called a event-controlled loop.
Use a while loop or a do-while loop for an
easy checkpoint development.
Asking the user before each iteration if it is time to
end the loop is called the ask-before-iterating
technique.
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Appropriate status update (or event initializing) for
a sequence of iterations
46
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Finds and prints a number's first factor other than 1:
int n = keyboard.nextInt(); // try 91
int f = 2;
while (n % f != 0) {
f++;
}
System.out.println("First factor:" + f);
Sample run:
First factor:7
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48
Write a program that will repeatedly prompt the user to
type a number until the user types a non-negative number,
then square it.
Example log:
Type a non-negative integer: -5
Invalid number, try
Invalid number, try
Invalid number, try
Invalid number, try
11 squared is 121
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again:
again:
again:
again:
-1
-235
-87
11
49
System.out.print("Type a non-negative integer: ");
int n = keyboard.nextInt();
while (n < 0) {
System.out.print("Invalid number, try again: ");
n = keyboard.nextInt();
}
int square = n * n;
System.out.println(n + " squared is " + square);
Notice that the number variable had to be declared outside
the while loop in order to remain in scope.
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50
Write a class named DigitSum that reads an integer from
the user and prints the sum of the digits of that number.
You may assume that the number is non-negative.
Example:
Enter a nonnegative number:
29107
prints out 19 (i.e.,2+9+1+0+7 )
Hint: Use the % operator to extract the last digit of a
number. If we do this repeatedly, when should we stop?
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import java.util.Scanner;
public class DigitSum {
public static void main(String [] args) {
Scanner keyboard = new Scanner(System.in);
int n = keyboard.nextInt();
int sum = 0;
while (n > 0) {
sum += n % 10;
// add last digit to sum
n = n / 10;
// remove last digit
}
System.out.println(“sum = “ + sum);
}
}
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52
Write a program named CountFactors that reads in an
integer and displays its number of factors.
For example, if the user enters 60, CountFactors displays 12
because 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, and 60 are all factors
of 60.
Scanner keyboard = new Scanner(System.in);
int n = keyboard.nextInt();
int sum = 0, k = ?;
while (
) {
}
System.out.println(“sum = “ + sum);
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53
Scanner keyboard =new Scanner(System.in);
int n = keyboard.nextInt();
int k = 1;
int sum = 0;
while (k<=n)
{
if(n%k==0)
sum ++;
k++;
}
System.out.print("sum = " + sum);
54
Exercise
population
TV purchase
http://www.cis.temple.edu/~jiang/tv563.exe
1+2+4+8+...
http://www.cis.temple.edu/~jiang/6billion.exe
http://www.cis.temple.edu/~jiang/1_2_4.exe
1+2+3+4+...+99
http://www.cis.temple.edu/~jiang/1to99.exe
4/12/2016
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Solution
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57
Ex 12
http://www.cs.wcupa.edu/~zjiang/141_ex12.pdf
Ex 13
http://www.cs.wcupa.edu/~zjiang/141_ex13.pdf
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58
File writing, page 230-237
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Filename
PringWriter
Println
Close
Sample, code 4-17, page 233
59
Appending data to a (existing) file
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FileWriter (, true), page 236
60
File Reading, page 237-241
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File
Scanner
nextXXXX( )
close
Sample, code 4-18, page 238.
61
Detecting the end of a file
hasNext
Code 4-19, page 241.
Detecting the existence of a file
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exists
Code 4-21, page 245.
62
Random number generator
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randomNumbers.nextXXX( )
Sample, code 4-23, page 250.
63
Objects of the Random class generate pseudo-random
numbers.
Class Random is found in the java.util package.
import java.util.*;
The methods of a Random object
Method name
Description
nextInt()
returns a random integer
nextInt(max)
returns a random integer in the range [0, max)
in other words, from 0 to one less than max
nextDouble() returns a random real number in the range [0.0, 1.0)
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