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Loop - CIS 1068 Program Design and Abstraction

Loop - CIS 1068 Program Design and Abstraction. Zhen Jiang CIS Dept. Temple University SERC 347, Main Campus Email: zhen.jiang@temple.edu. Table of Contents. Taste of Loop While Loop Do While Loop For Loop Variations Controlling Number of Loop Iterations Loop Development

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Loop - CIS 1068 Program Design and Abstraction

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  1. Loop - CIS 1068 Program Design and Abstraction Zhen Jiang CIS Dept. Temple University SERC 347, Main Campus Email: zhen.jiang@temple.edu

  2. 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

  3. Taste of Loop • Price is right. • Sample execution (click on this link to try) • Before you get the price right, the program will REPEAT…

  4. 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

  5. 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. Not clear?

  6. <initialization>; while (<test>) { <body>; }

  7. 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: 91 First factor: 7

  8. Example, WhileDemo.java, P202

  9. 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 3 Random rand = new Random(); int die; do { die = rand.nextInt(); } while (die == 3);

  10. 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.

  11. Example, DoWhileDemo.java, P206

  12. 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

  13. for (<init>; <test>; <update>) { <body>; }

  14. Example, ForDemo.java, P219

  15. Summary Body first, and then event change/update

  16. For loop?

  17. 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? …

  18. 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)); }

  19. 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!

  20. 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!");

  21. 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.

  22. 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; …

  23. 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!!!"); }

  24. Loops that go on… forever while (true) { <statement(s)>; } If it goes on forever, how do you stop?

  25. 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?

  26. 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);

  27. 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

  28. 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 }

  29. 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(); }

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

  31. Loop Development population  TV purchase 1+2+4+8+... 1+2+3+4+...+99 • http://www.cis.temple.edu/~jiang/LoopDevelopment.htm

  32. 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.

  33. 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 + " "); }

  34. 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 + " "); }

  35. 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

  36. count (x)‏ number to print (y)‏ 1 2 2 7 3 12 4 17 5 22

  37. 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

  38. 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)‏

  39. 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 1 5 1 -20

  40. 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?

  41. 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?

  42. 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 10 2 4 6 8 10 12 14 16 18 20 3 6 9 12 15 18 21 24 27 30 4 8 12 16 20 24 28 32 36 40 5 10 15 20 25 30 35 40 45 50 What is the body? Counter-controlled loop? Initialization and body detail?

  43. 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 …

  44. 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: ****** * * * * * * * * ****** What is the body? Counter controlled loop

  45. 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: * ** *** **** ***** ****** i * each line!

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