Loop - CIS 1068 Program Design and Abstraction Zhen Jiang CIS Dept. Temple University SERC 347, Main Campus Email: [email protected] 1

Loop - CIS 1068 Program Design and Abstraction

Zhen Jiang

CIS Dept.

Temple University

SERC 347, Main Campus

Email: [email protected]

1

Table of Contents Taste of Loop While Loop Do While Loop For Loop Variations Controlling Number of Loop Iterations Loop Development Mapping Iterations to Counter Values Controlling Event of Loop Iterations Random number generator Fencepost problem (an interesting scenario) Summary of Learning Materials 2

Price is right. Sample execution (click on this link to try)

Before you get the price right, the program will REPEAT…

Taste of Loop

3

while loop while loop: A control structure that repeatedly

performs a test and executes a group of statements if the test evaluates to true.

while loop, general syntax: <initialization>;while (<test>) { <body, consisting of statement(s)>;}

Example:int number = 1;while (number <= 200) { System.out.print(number + " "); number *= 2;}

Output:1 2 4 8 16 32 64 128

4

The <initialization> prepares the variable declarations and their values that are used in the test, update, and body of the loop.

The <test> checks whether the repetition of the loop body can stop.

The statement or group of statements to be repeated is called the <body> of the loop.

Each repetition of the loop body is called an <iteration> of the loop.

5

<initialization>;while (<test>) {

<body>;}

6

Finds and prints a number's first factor other than 1:

Scanner console = new Scanner(System.in);System.out.print("Type a number: ");int number = console.nextInt();int factor = 2;while (number % factor != 0) { factor++;}System.out.println("First factor: " + factor);

Sample run:Type a number: 91First factor: 7 7

Example, WhileDemo.java, P202

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Variant 1: do/while

do/while loop: A control structure that executes statements repeatedly while a condition is true, testing the condition at the end of each repetition.

do/while loop, general syntax: <initialization>; do { <statement(s)>;} while (<test>);

Example:// roll until we get a number other than 3Random rand = new Random();int die;do { die = rand.nextInt();} while (die == 3);

9

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.

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Example, DoWhileDemo.java, P206

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for loop: A block of Java code that executes a group of statements repeatedly until a given test fails.

General syntax:for (<initialization>; <test>; <update>) { <statement>; <statement>; ... <statement>;}

Example:for (int i = 1; i <= 30; i++) { System.out.println("I will not throw...");}

Variant 2: for

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for (<init>; <test>; <update>) { <body>;

}

13

Example, ForDemo.java, P219

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Summary

Body first, and thenevent change/update

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16

17

18

Initialization, test, and body, and execution results of loop

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Code:for (int i = 1; i <= 4; i++) { System.out.println(i + " squared is " + (i * i));}

Output:1 squared is 12 squared is 43 squared is 94 squared is 16

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:-3-2-1012

Example:for (int i = 1 + 3 * 4; i <= 5248 % 100; i++) { System.out.println(i + " squared is " + (i * i));}

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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-minus321Blastoff!

21

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!");22

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 timesThis is printed 3 timesThis is printed 3 timesSo is this... or is it?

Moral: Always use curly braces and always use proper indentation.

23

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 i, sum;for (i = 1, sum = 0; i <= 10; i++)

sum = sum + i * i;System.out.println("Result is " + sum);

Output:

385

int sum;for (int i=0, sum; …

24

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!!!");

}

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Loops that go on… forever

while (true) {

<statement(s)>;}

If it goes on forever, how do you stop?

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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? 27

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);

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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 no longer exists here} // x ceases to exist here

x's scope i’s scope

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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}

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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();}

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3204/21/23

Exercises http://www.cis.temple.edu/~jiang/1068LoopExecution.pdf

http://www.cis.temple.edu/~jiang/LoopDevelopment.htm

Loop Development

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population TV purchase 1+2+4+8+...1+2+3+4+...+99

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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. Counter =0, counter++, counter <number

Counter = 1, counter++, counter <=number

Use for loop for an easy development.

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36

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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 + " ");} 38

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 + " ");}

39


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.

1720174

22

12

7

2

number to print

25

15

10

5

count * 5

225

123

72

21

count * 5 - 3count

40

-10

-5

0

5

10

15

20

25

-2 0 2 4 6 174

22

12

7

2

number to print (y)

5

3

2

1

count (x)

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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 + b2 = 5 * 1 + bThen b = -3

So the equation isy = m * x + by = 5 * x – 3y = 5 * count - 3

174

22

12

7

2

number to print (y)

5

3

2

1

count (x)

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Algebraically, if we always take the value of y atx = 1, then we can solve for b as follows:

y = m * x + by1 = m * 1 + by1 = m + bb = 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 equationy = m * x + by = 5 * x – 3y = 5 * count – 3

(which is exactly the equation from the previous slides)

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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 ...

5-16

-20

-12

-8

-4

count * -4

1

9

13

17

count * -4 + 21

54

1

9

13

17

number to print

5

3

2

1

count

44

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Coding (different from execution check):n=keyboard.nextInt(); // try 6!for (int i = 1; i <= n; i++) {

System.out.print("*");}System.out.println();

Output:******

04/21/23

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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:************************************

04/21/23

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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 6 7 8 9 102 4 6 8 10 12 14 16 18 203 6 9 12 15 18 21 24 27 304 8 12 16 20 24 28 32 36 405 10 15 20 25 30 35 40 45 50

04/21/23

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 …

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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:******* ** ** ** *******

04/21/23

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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:*********************

04/21/23

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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:122333444455555666666

04/21/23

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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 44445555504/21/23

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.

Appropriate status update (or event initializing) for a sequence of iterations

04/21/23 53

Controlling Event of Loop Iterations

04/21/23 54

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Finds and prints a number's first factor other than 1:

int n = keyboard.nextInt(); // try 91int f = 2;while (n % f != 0) { f++;}System.out.println("First factor:" + f);

Sample run:First factor:7

04/21/23

56

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 again: -1



Invalid number, try again: 11

11 squared is 12104/21/23

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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.04/21/23

58

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: 29107prints 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?04/21/23

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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);

}}

04/21/23

60

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);

04/21/23

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);

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Exercises (Wednesday lab)

population TV purchase 1+2+4+8+...1+2+3+4+...+99

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63

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Complete a Loop program http://www.cis.temple.edu/~jiang/1068LoopComplete.pdf

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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

returns a random real number in the range [0.0, 1.0)nextDouble()

returns a random integer in the range [0, max)

in other words, from 0 to one less than max

nextInt(max)

returns a random integernextInt()

DescriptionMethod name

Random Number Generator

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Random rand = new Random();

int randomNum = rand.nextInt(10);

// randomNum has a random value between 0 and 9

What if we wanted a number from 1 to 10?int randomNum = rand.nextInt(10) + 1;

What if we wanted a number from min to max (i.e. an arbitrary range)?int randomNum = rand.nextInt(<size of the range>) + <min>

where <size of the range> equals (<max> - <min> + 1)

67

Given the following declaration, how would you get:

A random number between 0 and 100 inclusive?



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Given the following declaration, how would you get:

A random number between 0 and 100 inclusive?int random1 = rand.nextInt(101);

A random number between 1 and 100 inclusive?int random1 = rand.nextInt(100) + 1;

A random number between 4 and 17 inclusive?int random1 = rand.nextInt(14) + 4;

69

Write a program that simulates the rolling of two six-sided dice until their combined result comes up as 7.

Sample run:Roll: 2 + 4 = 6Roll: 3 + 5 = 8Roll: 5 + 6 = 11Roll: 1 + 1 = 2Roll: 4 + 3 = 7You won after 5 tries!

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import java.util.*;

public class Roll { public static void main(String[] args) { Random rand = new Random(); int sum = 0; int tries = 0; while (sum != 7) { int roll1 = rand.nextInt(6) + 1; int roll2 = rand.nextInt(6) + 1; sum = roll1 + roll2; System.out.println("Roll: " + roll1 + " + " + roll2 + " = " + sum); tries++; } System.out.println("You won after " + tries + " tries!"); }}

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Fencepost Problem: Write a class named PrintNumbers that reads in an integer called max and prints each number from 1 to max, separated by commas.

Example:java PrintNumbers

Please enter a maximum integer: 5

should print:1, 2, 3, 4, 5

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Fencepost Problem

We want to print n numbers but need only n - 1 commas.

Similar to the task of building a fence If we repeatedly place a post and wire, the last post has an extra dangling wire.

A flawed algorithm:for (length of fence) {

plant a post.

attach some wire.

}

73

import java.util.Scanner;

public class PrintNumbers {

public static void main(String [] args) {

Scanner keyboard = new Scanner(System.in);

int max = keyboard.nextInt();

for (int i = 1; i <= max; i++) {

System.out.print(i + ", ");

}

System.out.println(); // to end the line

}

} Output when user enters 5:

1, 2, 3, 4, 5, // notice extra comma at end!

unnecessary

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public class PrintNumbers




for (int i = 1; i <= max; i++) {

System.out.print(", " + i);

}


}


, 1, 2, 3, 4, 5 // comma at beginning

unnecessary

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The solution is to add an extra statement outside the loop that places the initial "post."

This is called a fencepost loop.

The revised algorithm:plant a post.

for (length of fence - 1) {

attach some wire.

plant a post.

}

76


public class PrintNumbers




System.out.print(1);

for (int i = 2; i <= max; i++) {

System.out.print(", " + i);

}


}


1, 2, 3, 4, 5 // no extra comma!

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WhileDemo.java, p202 DoWhileDemo.java, p206 ForDemo.java, p219 Exercises, slides 32, 45-52, 55-65, 70-71, 77

Summary

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Test (controlling boolean expression, P201), body (P200), iteration (P200), and initialization (P228)

While (P201-204), do-while (P204-9), and for loop (P217-221) Counter-controlled (P229) and event controlled loop Mapping iterations to counter values Trace and development template Random number generator The omission of {} and its side effect (P218) Extra semicolon (P222) and comma (P224) Invalidation and infinite loop (P213) The use of break and its function (P236-7) Scope of loop variable (P223) Fencepost problem and its solution

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Documents

Loop - CIS 1068 Program Design and Abstraction Zhen Jiang CIS Dept. Temple University SERC 347, Main Campus Email: [email protected] 1