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CSC141 Computer Science I Zhen Jiang Dept. of Computer Science
West Chester University West Chester, PA 19383 6/11/2018
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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 … 6/11/2018
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While loop Format & Logic, page 193, Figure 4-1.
Sample, code 4-3, page 194. 6/11/2018
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<initialization>; while (<test>) { <body>; }
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Do-while loop Format, page 204 Logic, page 205, Figure 4-6.
Sample, code 4-6, page 205. 6/11/2018
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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. 6/11/2018
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For loop Format, page 208, Figure 4-7. Logic, page 208, Figure 4-8.
Sample, code 4-7, page 209. 6/11/2018
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for (<init>; <test>; <update>) {
<body>; } 6/11/2018
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Summary Body first, and then event change/update 6/11/2018
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1st iteration? 2nd iteration? 3rd iteration? …
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 squared is 1 2 squared is 4 3 squared is 9 4 squared is 16 1st iteration? 2nd iteration? 3rd iteration? …
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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 1 2 for (int i = * 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-minus 3 2 1 Blastoff!
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What if we wanted the output to be the following?
T-minus 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!");
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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 So is this... or is it? Moral: Always use curly braces and always use proper indentation.
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int sum; for (int i=0, sum; … Extra semicolon in a loop (P218).
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); 385 int sum; for (int i=0, sum; …
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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; } Why is the break statement in an if statement?
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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(); for (int j = 1; j <= 5; i++) {
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Ex10 Ex11 http://www.cs.wcupa.edu/~zjiang/141_ex10.pdf
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Trial population TV purchase 1+2+4+8+... 1+2+3+4+...+99
TV purchase 6/11/2018
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Development process 6/11/2018
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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. 6/11/2018
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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? … Answer: System.out.print(2 * count + " ");
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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? Answer: System.out.print(2 * count " ");
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What statement could we write in the body of the loop that would make the loop print the following output? 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
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17 4 22 12 7 2 number to print (y) 5 3 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 + b 2 = 5 * 1 + b Then b = -3 So the equation is y = 5 * x – 3 y = 5 * count - 3 17 4 22 12 7 2 number to print (y) 5 3 1 count (x)
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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 = 5 * x – 3 y = 5 * count – 3 (which is exactly the equation from the previous slides)
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What statement could we write in the body of the loop that would make the loop print the following output? 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 4 1 9 13 17 number to print 3 2 count 5 -16 -20 -12 -8 -4 count * -4 1 9 13 17 count *
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1st iteration? 2nd iteration? 3rd iteration? …
Code: for (int i = 1; i <= 4; i++) { System.out.println(i + " squared is " + (i * i)); } Output: 1 squared is 1 2 squared is 4 3 squared is 9 4 squared is 16 1st iteration? 2nd iteration? 3rd iteration? … 6/11/2018
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Counter-controlled loop?
Coding (different from execution check): n=keyboard.nextInt(); // try 6! for (int i = 1; i <= n; i++) { System.out.print("*"); } System.out.println(); Output: ****** What is the body? Counter-controlled loop? 6/11/2018
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Counter-controlled loop?
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: ****** What is the body? Counter-controlled loop? 6/11/2018
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Counter-controlled loop? Initialization and body detail?
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: What is the body? Counter-controlled loop? Initialization and body detail? 6/11/2018
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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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Counter controlled loop
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(“*”); } Output: ****** * * What is the body? Counter controlled loop 6/11/2018
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i * each line! Code: Output: 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: * ** *** **** ***** ****** i * each line! 6/11/2018
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How many numbers each line? What are they?
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 How many numbers each line? What are they? 6/11/2018
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Space and numbers? 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 Space and numbers? 6/11/2018
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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. Appropriate status update (or event initializing) for a sequence of iterations 6/11/2018
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Body: exploring the factor. When/what to stop the loop?
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 Body: exploring the factor. When/what to stop the loop? n% f = = 0 divisible! The range of f? Initialization of f? The change of f? 6/11/2018
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Type a non-negative integer: -5
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: -235 Invalid number, try again: -87 Invalid number, try again: 11 11 squared is 121 6/11/2018
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Body: trying different value of n. When/what to stop the loop?
n>= 0 non-negative! The range of n? -> any (since n<0) Initialization of n? The change of n? 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. 6/11/2018
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Enter a nonnegative number: 29107
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: prints out 19 (i.e., ) Hint: Use the % operator to extract the last digit of a number. If we do this repeatedly, when should we stop? 6/11/2018
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When/what to stop? All digits are counted!
Body: adding the last digit and extracting that from the original number (for next round) When/what to stop? All digits are counted! n <= 0 no more need to count! The change of n? -> make the extraction valid Initialization? 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); 6/11/2018
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When/what to stop? K>n (trial from 1 is over)
Body: try all possible numbers k, count k (sum++) only when n is divisible by k. When/what to stop? K>n (trial from 1 is over) The range of k? -> 1 to n Change? k++ Initialization? 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); 6/11/2018
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Scanner keyboard =new Scanner(System. in); int n = keyboard
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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Exercise population TV purchase 1+2+4+8+... 1+2+3+4+...+99
TV purchase 6/11/2018
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Solution 6/11/2018
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Ex 12 Ex 13 http://www.cs.wcupa.edu/~zjiang/141_ex12.pdf
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File writing, page 230-237 Filename PringWriter Println Close
Sample, code 4-17, page 233 6/11/2018
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Appending data to a (existing) file
FileWriter (, true), page 236 6/11/2018
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File Reading, page 237-241 File Scanner nextXXXX( ) close
Sample, code 4-18, page 238. 6/11/2018
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Detecting the end of a file
hasNext Code 4-19, page 241. Detecting the existence of a file exists Code 4-21, page 245. 6/11/2018
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Random number generator
randomNumbers.nextXXX( ) Sample, code 4-23, page 250. 6/11/2018
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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 integer nextInt() Description Method name 6/11/2018
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