Control Structures II. Repetition (Loops)

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Lucy Bishop
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1 Control Structures II Repetition (Loops)

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

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; Systemoutprintln( 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 ?Or to ?

5 Why Is Repetition Needed? The Algorithm 1 Create a variable to hold the sum 2 Initialize the sum to zero 3 Create a variable to hold a counter from 1 to Initialize the counter to 1 5 While the counter is less-than-or-equal to add the counter to the sum 7 add one to the counter 8 Now repeat 9 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 8

Example i = 0 loop while (i <= 20) print(i) i = i + 5 end loop i 0 5 10 15 20 25 Output 0 5 10

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

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 = (1) Initialization stmt (2) Loop condition (3) Update stmt or counter = 0 counter < N Yes do operation counter = counter + 1 Read N No

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 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 setsize Output counter sum number start Read setsize counter = 0 sum = 0 counter < setsize Yes Read number sum = sum + number counter = counter + 1 Print sum end No

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

15 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 counter = 1 Can be simplified to: counter <= N Yes do operation counter = counter + 1 No For counter = 1 to N, step 1 do operation Next counter

16 Example 1 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 2 Verify your result by tracing the developed algorithm (Assume N to be 4 and the following set to be the numbers { })

17 start Read N max = 0 Solution (1) 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 For counter = 1 to N, step 1 Yes Read number number > max Yes max = number Next counter No end

(2) Trace the developed algorithm Assume N to be 4 and the following set to be the numbers {5 2 6 1} N 4 max 0 counter 1 number 5 start

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

Chapter 4: Control structures. Repetition

Chapter 4: Control structures. Repetition Chapter 4: Control structures Repetition Loop Statements After reading and studying this Section, student should be able to Implement repetition control in a program using while statements. Implement repetition