1 / 32

Intro to CS – Honors I Control Flow: Loops

Intro to CS – Honors I Control Flow: Loops. Georgios Portokalidis gportoka@stevens.edu. Repeating an Action. Calculating student grades without loops read student name read number of correct answers of student c alculate letter grade read student name

dalton
Télécharger la présentation

Intro to CS – Honors I Control Flow: Loops

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Intro to CS – Honors IControl Flow: Loops Georgios Portokalidis gportoka@stevens.edu

  2. Repeating an Action • Calculating student grades without loops • read student name • read number of correct answers of student • calculate letter grade • read student name • read number of correct answers of student • calculate letter grade • read student name • read number of correct answers of student • calculate letter grade

  3. Repeating an Action • Calculating student grades with a loop • read student name • read number of correct answers of student • calculate letter grade • repeat until all students have been processed • What do you need to determine when designing a loop? • The actions the body of the loop will perform • How/when will the loop stop

  4. The while Statement • SYNTAX • while (Boolean_Expression) • Body • Body can consist of multiple statements enclosed in braces { } Start Evaluate expression False True Execute body End loop

  5. Example: Calculating the Sum inthowManyNumbers= 0; int sum = 0; Scanner keyboard = new Scanner(System.in); howManyNumbers= keyboard.nextInt(); while (howManyNumbers-- > 0) sum += keyboard.nextInt(); System.out.println(“Sum: “ + sum); A good programmer would not assume that the input is well formed.

  6. Using A Negative Number Terminate the Loop • Add numbers till the user enters 0 or a smaller number intnextNumber = 0; int sum = 0; Scanner keyboard = new Scanner(System.in); while (nextNumber >= 0) { sum += nextNumber; nextNumber = keyboard.nextInt(); } System.out.println(“Sum: “ + sum); A while loop can perform zero iterations What if I also wanted to allow zero to be a valid input?

  7. Adding 0 to Termination Inputs • Add numbers till the user enters a negative number • Can you use a while loop to code an if statement? int sum = 0; intnextNumber= keyboard.nextInt(); while (nextNumber> 0) { sum += nextNumber; nextNumber= keyboard.nextInt(); } System.out.println(“Sum: “ + sum); intnextNumber = 0; int sum = 0; Scanner keyboard = new Scanner(System.in); while (nextNumber >= 0) { sum += nextNumber; nextNumber = keyboard.nextInt(); } System.out.println(“Sum: “ + sum);

  8. Terminating the Loop inthowManyNumbers= 0; int sum = 0; Scanner keyboard = new Scanner(System.in); howManyNumbers= keyboard.nextInt(); while (howManyNumbers-- > 0) sum += keyboard.nextInt(); System.out.println(“Sum: “ + sum); The condition of the while statement needs to at some point return false. Otherwise the loop will never terminate  Infinite loop

  9. One Loop is Not Enough int sum = 0; intnextNumber= keyboard.nextInt(); while (nextNumber> 0) { sum += nextNumber; nextNumber= keyboard.nextInt(); } System.out.println(“Sum: “ + sum); The while loop is not robust enough for this example

  10. The do-while Statement • SYNTAX • do • Body • while (Boolean_Expression); • Body can consist of multiple statements enclosed in braces { } • The body is always executed at least once Start Notice the semicolon Execute body Evaluate expression False True End loop

  11. Revisiting the Previous Example • Add numbers till the user enters a negative number int sum = 0; intnextNumber = 0; do { sum += nextNumber; nextNumber= keyboard.nextInt(); } while (nextNumber >= 0); System.out.println(“Sum: “ + sum);

  12. Reasoning About Loops How many times will this loop execute? int n = 1;double x = 0, s = 0; do {s = 10 / n * n;x = x + s;n++;} while (s > 0.1); int n = 1;double x = 0, s = 1.0; while (s > 0.1) {s = 10 / n * n;x = x + s;n++;} Transform it to a while loop

  13. Nested Loops • int sum = 0, nextNumber = 0, noNumbers = 0; • String answer; • do { • nextNumber = keyboard.nextInt(); • while (nextNumber >= 0) • { • sum += nextNumber; • noNumbers++; • nextNumber= keyboard.nextInt(); • } • System.out.println(“Average: “ + (sum / noNumbers); • System.out.println(“Do you want to go again (yes/no)?”); • answer = keyboard.next(); • } while (answer.equalsIgnoreCase(“yes”)); These numbers not initialized with the proper values after the first iteration of the outer loop

  14. Nested Loops • int sum, nextNumber, noNumbers; • String answer; • do { • sum = NoNumbers = 0; • nextNumber = keyboard.nextInt(); • while (nextNumber >= 0) • { • sum += nextNumber; • noNumbers++; • nextNumber= keyboard.nextInt(); • } • System.out.println(“Average: “ + (sum / noNumbers); • System.out.println(“Do you want to go again (yes/no)?”); • answer = keyboard.next(); • } while (answer.equalsIgnoreCase(“yes”)); Initialization should be in the loop

  15. The for Statement • All loops • …have an initialization/setup phase • …modify the condition that control when the looping ceases • The for loop enables you to easily write a loop that clearly defines these steps • Such as loops controlled by a counter • SYNTAX • for (Initializing_Action; Boolean_Expression; Update_Action) • Body • Body can consist of multiple statements enclosed in braces { }

  16. Start Execute Initializing_Actioin Evaluate expression False True End loop Execute body Execute Update_Action

  17. Easy Counting With for Loops int count; for (count = 0; count < 3; count++) { System.out.println(“Counter: “ + count); } intcountDown; for (countDown = 10; count > 0; count--) { System.out.println(count); System.out.println(“and counting”); } System.out.println("Blast off!");

  18. for and while Loops for (Initializing_Action; Boolean_Expression; Update_Action;) Statements . . . } Initializing_Action; while (Boolean_Expression) { Statements . . . Update_Action; } For loops are essentially a specialization of while loops

  19. Multiple Statements in for Loops Initialization can include multiple statements separated by commas Same stands for the update statements inti, j; for (i = 0, j = 0; i < 10; i++, j += 2) { System.out.println(i* j); } What can you do for multiple conditions

  20. Declaring Variables in for Loops for (inti= 0, j = 0; i < 10; i++, j += 2) { System.out.println(i* j); }

  21. Common Errors A semicolon at the end means that the body of the for loop is empty int product = 1, number; for (number = 1; number <= 10; number++); { product = product * number; } System.out.println("Product of the numbers 1 through " + "10 is " + product);

  22. A Semicolon and a while Loop int product = 1, number = 1; while (number <= 10); { product = product * number; number++; } System.out.println("Product of the numbers 1 through " + "10 is " + product); What happens here?

  23. Picking the Right Loop • Use a do-while loop only when you are certain that the loop needs to execute exactly once • If your loop has well-defined initialization and update statements use a for loop • Otherwise use while • These are general guidelines, the final decision depends on the problem

  24. for-each Statement • Can be applied on variables that can only have certain values • Example: enumerations • We will later see other data that can be used with for-each loops enum Suit {CLUBS, DIAMONDS, HEARTS, SPADES}; for (Suit nextSuit : Suit.values()) System.out.print(nextSuit + " "); System.out.println();

  25. Programming with Loops 1. Display instructions to the user. 2. Initialize variables. 3. Repeat the following for the appropriate number of times. { Read a number into the variable next. sum = sum + next Display the number and the sum so far. } • 1. Display instructions to the user • 2. Initialize variables • 3. Read a number into the variable next. • 4. sum = sum + next • 5. Display the number and the sum so far. • 6. Read another number into the variable next. • 7. sum = sum + next • 8. Display the number and the sum so far. • 9. Read another number into the variable next. • 10. sum = sum + next • 11. Display the number and the sum so far. • 12. Read another number into the variable next. Write out the sequence of actions that you need to accomplish Identify patterns Write the sequence using a loop

  26. Using Boolean Variables to End a Loop int next, sum = 0; booleanthereAreMoreNumbersToRead = true; while (thereAreMoreNumbersToRead) { next = keyboard.nextInt(); if (next < 0) thereAreMoreNumbersToRead = false; else sum = sum + next; }

  27. The break Statement • A break statement within a loop causes an immediate exit from the loop body • Use them wisely • To handle errors • If they actually make your code more readable • To handle rare conditions occurring • Using break frequently makes loops harder to reason with • In nested loops break only stops the innermost loop

  28. Example using break inthowManyNumbers= 0; int sum = 0; Scanner keyboard = new Scanner(System.in); howManyNumbers= keyboard.nextInt(); while (howManyNumbers-- > 0) sum += keyboard.nextInt(); System.out.println(“Sum: “ + sum); inthowManyNumbers= 0; int sum = 0, nextNumber; Scanner keyboard = new Scanner(System.in); howManyNumbers= keyboard.nextInt(); while (howManyNumbers-- > 0) { nextNumber= keyboard.nextInt(); if (nextNumber < 0) break; sum += nextNumber; } System.out.println(“Sum: “ + sum);

  29. The continue Statement • A continue statement within a loop ends the current iteration and begins the next one • Use them wisely • If they actually make your code more readable • To handle rare conditions occurring • Using continue frequently makes loops harder to reason with • In nested loops continue corresponds to the innermost loop • In for loops it also triggers the update actions

  30. Example using continue inthowManyNumbers= 0; int sum = 0; Scanner keyboard = new Scanner(System.in); howManyNumbers= keyboard.nextInt(); while (howManyNumbers-- > 0) sum += keyboard.nextInt(); System.out.println(“Sum: “ + sum); inthowManyNumbers= 0; int sum = 0, nextNumber; Scanner keyboard = new Scanner(System.in); howManyNumbers= keyboard.nextInt(); while (howManyNumbers-- > 0) { nextNumber= keyboard.nextInt(); if (nextNumber < 0) continue; sum += nextNumber; } System.out.println(“Sum: “ + sum);

  31. Loop Bugs • Infinite loops • Off-by-one errors • Executing one more or less iteration of a loop • Due to erroneous boolean expression • Example: Using < instead of a <=

More Related