1. Repetition & Loops

2. One of the BIG advantages of a computer: ­It can perform tasks over and over again, without getting bored or making mistakes (assuming we gave it the right instructions) Repetition: doing this over and over again Loops: going around in a circle and repeating the action The way a computer performs repetitive tasks is by programming instructions that tell it to repeat a task. The structure that is used for these instructions is called a Loop. The loop structure allows for any number of statement to be performed repeatedly.

3. A Loop in Programming A loop in programming is a block of code that will be executed repeatedly until a certain condition is met. Declare Number As Integer Repeat Write “ Please enter a number” Input Number Write Number Until Number == 0 Write “ List Ended ” The body of the loop is executed repeatedly until the user enters 0. An iteration occurs each time the loop is run. Body

4. Do For a While Programming languages have three statement that allow for the creation of loops known as conditional statements. Conditional statement: keep doing something unless something changes (i.e. a condition is meat) It requires conditional statements ­For…this… do.. until that is true ­While …this do… until that is true ­While…this is true ….do this, otherwise do that…

5. Types of Loops Loops create one of the most important and fundamental algorithm of a program Loops are used perform any function that repeats itself such as loading data, calculating and manipulating data or user interaction They are used in every computer programming language. They can be divided into two basic types depending upon the when the condition is tested. Pre-test: the condition is tested at the beginning Post-test: the condition is tested at the end

6. Pre-test Loop The pre-test loop test the condition before executing the body of the loop The body of the pre-test loop will not be executed if the test condition is initially false While…this is true ….do this, otherwise do that… Pseudocode Example Write “Enter a number: “ Input Number While Number != 0 Write Number Input Number End While Write “Done”

7. Post-test Loop The post-test loop test the condition after executing the body of the loop The body of the post-test loops is always executed at least once For…this… do.. until that is true Pseudocode Example Do Write “Enter a number: “ Input Number Write Number While Number != 0 Write “Done”

8. Pre-test and Post-test Flowcharts

9. Trace a Loop Tracing a loop is It is like tracing a path to see where it leads. Think of it as a manual walk-through – following each step. Psuedocode Example Declare Number As Integer Write “ Enter a number or 0 to quit: ” Input Number While Number > 0 Write Number^2 Input Number End While NumberOutput 3 9 1

10. The while loop ­From Java Text, using the cooking example While there are lumps in the sauce keep stirring. When there are no more lumps stop. while (there are lumps) stir sauce; Initialize Expression While Loop Condition Statement F True

11. Example from Java 4.1 class PrintSquares { public static void main (String[] args) { System.out.println("Give an integer (zero to stop)"); int value = In.getInt(); while (value != 0) { System.out.println(value + " " + value*value); System.out.println("Next integer (zero to stop)"); value = In.getInt(); } Initialize the value Sentinel: 0 – when program stops Condition Note: if condition is never met program will not stop This could create an infinite loop These statement will continue until condition is met

12. Example from Java 4.1 class AddMarks { public static void main (String[] args) { System.out.println("Submit marks (value < 0 to stop"); int totalMarks = 0; int numberOfMarks = 0; int nextMark = In.getInt(); while (nextMark >= 0) { totalMarks += nextMark; numberOfMarks++; System.out.println("Next mark:"); nextMark = In.getInt(); } if (numberOfMarks > 0) { int average = Math.round((float)totalMarks/numberOfMarks); System.out.println("Average for " + numberOfMarks + " students is " + average); }

13. The do loop ­From Java Text, using the cooking example Stir the sauce as long as there are lumps in it. When there are no more lumps stop. do (the stirring) while (there are lumps in it) do Statement While Loop Condition False True

14. Example from Java 4.2 char response; do { System.out.println("Please enter a letter of the alphabet"); response = In.getChar(); } while ((response 'z') && (response 'Z')); } Do the following statements Conditions

15. 1-15 Counting Loops Sometimes you may want to execute a loop a certain number of times without user input or regardless of the condition. This is known as a Counter-Controlled Loop This loop contains a variable which is known as the counter and this variable keeps track of the number of passes through the loop To counter is defined, initialized and increased using the increment operator. int count=0 count=count +1 ­Define a counter as an integer ­Initialize the counter: set the counter to a beginning value. ­Common variable names are counter, Count, i, or j.

16. Output PsuedoCode Example: Use a Counter to Display the Squares of Numbers Declare PositiveInteger As Integer Declare Count As Integer Write “ Please enter a positive integer: “ Input PositiveInteger Set Count = 1 While Count <= PositiveInteger Write Count + “ “ + Count^2 Set Count = Count + 1 End While CountOutput 11 1 22 4 33 9 44 16 55 25

17. The for loop ­From Java Text, using the cooking example Stir the sauce for 100 times (the assumption is that there will be no more lumps if you stir for that many times. for (100 times) stir sauce; The for loop makes use of the count mechanism Initialize Expression 100 times For Loop Condition Statement False True

18. PsuedoCode Example: using the for loop For (Count=0, Count<=15; Count+5) Write Count End For

19. Example from Java 4.3 class IntegerTable { public static void main (String[] args) { final int UPPER_BOUND=10; System.out.println(“Number Square”); for (int i=1; i<=UPPER_BOUND; i++) { Out.print(i); Out.print(i*i); } Set condition Number output Square output This program reads integers and prints them along with their squares Loop expression