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Comprehensive Looping Concepts - Computer Science III

Learn about different types of loops (while, do-while, for) in programming with examples, exercises, and variations for mastering efficient and smart decision-making in programming. Explore temperature/humidity detection and surveillance techniques for seamless monitoring.

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Comprehensive Looping Concepts - Computer Science III

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  1. CSC240 Computer Science III Zhen Jiang Dept. of Computer Science West Chester University West Chester, PA 19383 zjiang@wcupa.edu

  2. Loop • Smart if-decision making, smart program work, talent programmer • Research experience (REU) - click on this link http://www.cs.wcupa.edu/~zjiang/Matlabprogramming.pptx • Temperature/humidity detection every second • A repetition process for the 7x24 hours seamless surveillance • Needs a computer support to REPEAT …

  3. While loop • Format & Logic, page 197, Figure 4-1. • Sample, code 4-3, page 198.

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

  5. Do-while loop • Format, page 208 • Logic, page 209, Figure 4-6. • Sample, code 4-6, page 209.

  6. 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.

  7. For loop • Format, page 212, Figure 4-7. • Logic, page 212, Figure 4-8. • Sample, code 4-7, page 213.

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

  9. Why need to repeat this?

  10. Exercise • population  • http://www.cs.wcupa.edu/~zjiang/6billion.exe • TV purchase • http://www.cs.wcupa.edu/~zjiang/tv563.exe • 1+2+4+8+... • http://www.cs.wcupa.edu/~zjiang/1_2_4.exe • 1+2+3+4+...+99 • http://www.cs.wcupa.edu/~zjiang/1to99.exe

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

  12. 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? …

  13. Exercises • http://www.cs.wcupa.edu/~zjiang/141_ex11.pdf

  14. 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)); }

  15. 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!

  16. 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!");

  17. 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.

  18. 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; …

  19. 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!!!"); }

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

  21. 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?

  22. 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);

  23. 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

  24. 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 }

  25. 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(); }

  26. Development process • http://www.cs.wcupa.edu/~zjiang/LoopDevelopment.htm

  27. 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.

  28. 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 + " "); }

  29. 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 + " "); }

  30. 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

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

  32. 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

  33. 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)‏

  34. 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

  35. 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?

  36. 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?

  37. 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?

  38. 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 …

  39. 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

  40. 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!

  41. 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?

  42. 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?

  43. 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 Controlling Event of Loop Iterations

  44. 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?

  45. 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

  46. 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. 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?

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