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Repetition-Counter control Loop

Chapter 4: Control Structures. Repetition-Counter control Loop. Why Is Repetition Needed?. How can you solve the following problem: What is the sum of all the numbers from 1 to 100. The answer will be 1 + 2 + 3 + 4 + 5 + 6 + … + 99 + 100. Why Is Repetition Needed?.

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Repetition-Counter control Loop

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  1. Chapter 4: Control Structures Repetition-Counter control Loop

  2. Why Is Repetition Needed? How can you solve the following problem: • What is the sum of all the numbers from 1 to 100. • The answer will be 1 + 2 + 3 + 4 + 5 + 6 + … + 99 + 100.

  3. Why Is Repetition Needed? Here’s some sample Java code: int sum=0; sum = sum+1; sum = sum +2; sum = sum +3; sum = sum +4; … sum = sum +99; sum = sum +100; System.out.println(“The sum from 1 to 100 = “ +sum);

  4. Why Is Repetition Needed? This solution has problems: • It would take a long time to type in. • There is a high risk of making an error while typing it in. • It doesn’t easily scale. This may work for 100 numbers but how would you handle having to add from 1 to a 1000? Or to 1000000?Or to 1000000000?

  5. Why Is Repetition Needed? The Algorithm • Create a variable to hold the sum. • Initialize the sum to zero. • Create a variable to hold a counter from 1 to 100. • Initialize the counter to 1. • While the counter is less-than-or-equal to 100 • add the counter to the sum • add one to the counter • Now repeat • Print the sum

  6. Why Is Repetition Needed? We can use pseudo-code: sum = 0 count = 1 loop while count <= 100 sum = sum + count count++ endloop print sum

  7. Why Is Repetition Needed? loop while condition <body> endloop • This pseudo-code means: before executing the statements in the body, evaluate the condition. If the condition is true then execute the body once. • Once you have executed the body statements once, go back to the loop condition and re-evaluate it. If it is true, execute the body code again. If the condition is not true then the body will not be executed!

  8. The while Looping (Repetition) Structure • Infinite loop: is a loop that continues to execute endlessly. • So, expression is always true in an infinite loop. • Statements must change value of expression to false.

  9. Example start i = 0 loop while (i <= 20) print(i) i = i + 5 end loop i = 0 i <= 20 No Output i Yes 0 5 Print i 0 5 10 15 20 10 i = i + 5 15 end 20 What will happen if you omit (i= i + 5) ? 25

  10. While Loop Types • Counter-Controlled Loop • Sentinel-Controlled Loop • Flag-Controlled Loop

  11. Counter-Controlled Loop • Used when exact number of data or entry pieces is known. • General form: N = … counter = 0 Loop while (counter < N) . . . counter = counter + 1 . . . End loop N = … or Read N (1) Initialization stmt. counter = 0 (2) Loop condition counter < N No Yes do operation counter = counter + 1 (3) Update stmt.

  12. Example • Write a program to allow the user to enter a set of integer numbers, then print the sum of these numbers. Start Program Read setSize counter = 0 sum = 0 Loop while (counter < setSize) Read number sum = sum + number counter = counter + 1 End loop Print sum End Program

  13. start Read setSize counter = 0 Start Program Read setSize counter = 0 sum = 0 Loop while (counter < setSize) Read number sum = sum + number counter = counter + 1 End loop Print Sum End Program sum = 0 counter < setSize No number setSize counter sum Yes 3 0 1 0 Read number Output 1 10 1 sum = sum + number 3 1 10 4 15 11 4 2 counter = counter + 1 3 15 Print sum end

  14. The while Looping (Repetition) Structure • Syntax: while (expression) statement • Statements must change value of expression to false. • A loop that continues to execute endlessly is called an infinite loop (expression is always true).

  15. The while Looping (Repetition) Structure Example 5-1 i = 0; while (i <= 20) { System.out.print(i + " "); i = i + 5; } System.out.println(); Output 0 5 10 15 20

  16. The while Looping (Repetition) Structure Typically, while loops are written in the following form: //initialize the loop control variable(s) while (expression) //expression tests the LCV { . . . //update the loop control variable(s) . . }

  17. Counter-Controlled while Loop • Used when exact number of data or entry pieces is known. • General form: int N = //value input by user or specified //in program int counter = 0; while (counter < N) { . . . counter++; . . }

  18. Counter-Controlled while Loop-Example 5-3 //Counter-controlled while loopimport java.util.*;publicclass CounterControlledWhileLoop{static Scanner console = new Scanner(System.in);publicstaticvoid main(String[] args) {int limit; //store the number of items//in the listint number; //variable to store the numberint sum; //variable to store the sumint counter; //loop control variable System.out.print("Enter the number of " + "integers in the list: "); limit = console.nextInt(); System.out.println(); sum = 0; counter = 0; System.out.println("Enter " + limit+ " integers.");

  19. Counter-Controlled while Loop-Example 5-3 (continued) while (counter < limit) { number = console.nextInt(); sum = sum + number; counter++; } System.out.printf("The sum of the %d " +"numbers = %d%n", limit, sum); if (counter != 0) System.out.printf("The average = %d%n",(sum / counter)); else System.out.println("No input."); } } Sample Run: Enter the number of integers I the list: 4 Enter 4 Integers 2 1 5 8 The sum of the 4 numbers = 16 The average = 4

  20. Counter-Controlled Loop: Another way for expressing it While loop For loop N = … counter = 1 Loop while (counter<=N) . . counter = counter + 1 End loop N = … For(counter = 1, counter <= N, counter = counter + 1) . . End For Initialization Condition Increment \ Decrement counter = 1 to N Step 1

  21. Counter-Controlled Loop –For Loop The for loop does not have a standard flowcharting method and you will find it done in different ways. N = … or Read N N = … or Read N Can be simplified to: counter = 1 For counter = 1 to N, step 1 counter <= N No do operation Yes do operation Next counter counter = counter + 1

  22. Example • Write down an algorithm and draw a flowchart to find and print the largest of N (N can be any number) positive numbers. Read numbers one by one. • Verify your result by tracing the developed algorithm. (Assume N to be 4 and the following set to be the numbers {5 2 6 1})

  23. start Solution Read N max = 0 Start Program Read N max = 0 For(counter = 1 to N, step 1) Read number if (number > max) max = number End For Print max End Program (1) For counter = 1 to N, step 1 Yes Read number number > max No Yes max = number Next counter end

  24. start start (2) • Trace the developed algorithm. • Assume N to be 4 and the following set to be the numbers {5 2 6 1} Read N max = 0 5 2 2 For counter = 1 to N, step 1 Yes 6 3 6 Read number number counter max N 5 1 4 4 1 0 number > max No Yes 5 max = number Next counter Print max  6 end

  25. The for Looping (Repetition) Structure • Specialized form of while loop. • Its primary purpose is to simplify the writing of counter-controlled loops. For this reason, the forloop is typically called a counted or indexed for loop. . • Syntax: for (initial statement; loop condition; update statement) statement

  26. The for Looping (Repetition) Structure • Execution: • Initial statement executes. • Loop condition is evaluated. • If loop condition evaluates to true, • execute for loop statement and • execute update statement. • Repeat step 2 until loop condition is false.

  27. The for Looping (Repetition) Structure Example 5-9 The following for loop prints the first 10 nonnegative integers: for (i = 0; i < 10; i++) System.out.print(i + " ");

  28. The for Looping (Repetition) Structure Example 5-10 • The following for loop outputs the word Hello and a star (on separate lines) five times: for (i = 1; i <= 5; i++) { System.out.println("Hello"); System.out.println("*"); } 2. The following for loop outputs the word Hello five times and the star only once: for (i = 1; i <= 5; i++) System.out.println("Hello"); System.out.println("*");

  29. The for Looping (Repetition) Structure • Does not execute if loop condition is initially false. • Update expression changes value of loop control variable, eventually making it false. • If loop condition is always true, result is an infinite loop. • Infinite loop can be specified by omitting all three control statements. • If loop condition is omitted, it is assumed to be true. • Action of for loop ending in semicolon is empty.

  30. For Loop Programming Example: Classify Numbers • Input: Nintegers (positive, negative, and zeros). int N = 20; //N easily modified • Output: Number of 0s, number of even integers, number of odd integers.

  31. For Loop Programming Example: Classify Numbers (solution) for (counter = 1; counter <= N; counter++) { number = console.nextInt(); System.out.print(number + " "); switch (number % 2) { case 0: evens++; if (number == 0) zeros++; break; case 1: case -1: odds++; } //end switch } //end for loop

  32. While Loop Programming Example: Fibonacci Number • Fibonacci formula for any Fibonacci sequence: an = an-1 + an-2 • Input: First two Fibonacci numbers in sequence, position in sequence of desired Fibonacci number (n). • intprevious1 = Fibonacci number 1 • intprevious2 = Fibonacci number 2 • intnthFibonacci = Position of nth Fibonacci number • Output: nth Fibonacci number.

  33. While Loop Programming Example: Fibonacci Number if (nthFibonacci == 1) current = previous1; else if (nthFibonacci == 2) current = previous2; else { counter = 3; while (counter <= nthFibonacci) { current = previous2 + previous1; previous1 = previous2; previous2 = current; counter++; } } • Final result found in last value of current.

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