Repe$$on CSC 121 Spring 2017 Howard Rosenthal

Transcription

1 Repe$$on CSC 121 Spring 2017 Howard Rosenthal

2 Lesson Goals Learn the following three repetition structures in Java, their syntax, their similarities and differences, and how to avoid common errors when using them: while do-while for Learn nested looping Learn how to use break statements to exit a loop Begin to learn how to deal with situations where faulty or out of range input is supplied Review and expand knowledge of blocks and scope 2

3 Introduc$on to Loops Many of the tasks performed by computers require repetition, often massive repetition Using repetitive syntax can make your code more concise and easier to write and debug Java has three types of repetitive constructs (also known as loops): while, dowhile or for Loops are built out of several fundamental statements because there are three things (in each of the three types of loops) that must be done correctly: The variables used to control the loop must be initialized correctly The ending condition for the loop must be tested correctly The body of the loop or the loop statement must change the condition that is tested Overlooking one of these aspects results in a defective loop and a sometimes difficult to find program bug! Usually each of these three considerations is located in a different spot. No wonder that loops often go wrong! You have to plan out the logic of your program in order to effectively use loops and conditionals 3

4 Types of Loops type counting loop sentinel-controlled loop result-controlled loop description Uses a loop control variable to count upwards or downwards (usually by an integer increment.) Example: Keep going until you drive 100 miles. Loop keeps going until a special value is encountered in the data. Example: Keep going until you hit a red light. Loop keeps going until a test determines that the desired result has been achieved. Example. Keep going until you reach your home. 4

5 The while Statement Here is a program with a loop. It contains a while statement, followed by a block of code. Remember, a block is a group of statements enclosed in braces. // Example of a while loop public class WhileLoopExample1 public static void main (String[] args ) // start count out at one int count = 1; while ( count <= 3 ) System.out.println( "count is:" + count ); count = count + 1; // add one to count System.out.println( "Done with the loop" ); 5

6 The Basic Syntax of the while Statement while (condition) statement 1; statement 2; o 0 o statement n; You execute through the entire block repetitively until the condition is false unless you encounter a break statement The break will typically occur based on a separate condition tested with an if statement 6

7 Basic Terminology of the while Statement The condition is a Boolean expression: something that evaluates to true or false The condition can be complicated, using many relational operators and logical operators. Due to the precision of integers, integers are almost always used as counters for loops A single statement is allowed Without braces only one statement is executed inside the loop The block or single executable statement is sometimes called the loop body If one of the statements in the while block doesn t eventually change the value of the condition then the loop goes on forever The while loop may not be executed at all if the initial condition is false There is no semi-colon after the while statement If you put one there you will end the loop before it executes anything 7

8 Counters Can Be Decremented Rather Than Incremented When The Need Arises Here is a program with a loop It contains a while statement, followed by a block of code The condition is checked when entering the loop, and for each iteration If the condition is false the loop is skipped over Remember, a block is a group of statements enclosed in braces. // Example of a while loop decrementing from 2 to 0 public class WhileLoopExample2 public static void main (String[] args ) int count = 2; // count is initialized while ( count >= 0 ) // count is tested System.out.println( "count is:" + count ); count = count - 1; // count is changed by -1 System.out.println( "Done counting down." ); 8

9 The do-while Statement Here is a program with a loop It contains a do statement, followed by a block of code, followed by a while statement The condition is checked at the end of each execution of the loop which is always executed at least once // Example of a do-while loop that executes only once public class DoWhileLoopExample public static void main (String[] args ) int count = 1000; // initialize do System.out.println( count ); count++ ; while ( count < 10 ); // test System.out.println( "Done with the loop" ); 9

10 The Basic Syntax of the do-while Statement do statement1; statement2; o 0 0 statementn; while (condition); 10

11 Basic Terminology of the do-while Statement The condition is a Boolean expression: something that evaluates to true or false The condition can be complicated, using many relational operators and logical operators A single statement is allowed Without braces to create a block only one statement is executed inside the loop The block or single executable statement is sometimes called the loop body. The do-while is always executed at least once, if the do statement is executed There is a semi-colon after the while statement in a dowhile 11

12 The for Statement Here is a program with a loop. It contains a for statement, followed by a block of code. A block is a group of statements enclosed in braces. // Example of a for loop public class ForLoopExample public static void main (String[] args ) int sum=0; for ( int count = 0; count <= 5; count++ ) sum = sum + count ; System.out.print( count + " " ); System.out.println( \nsum is: " + sum ); Initialization count = 0 loop condition count <= 5 sum = sum + count Print count count++ Print sum false 12

13 The Basic Syntax of the for Statement for (initialization; loop condition; update statements) statement1; statement2; o 0 0 statementn; 13

14 Basic Terminology of the for Statement initialization Used to initialize basic variables used in the loop condition More than one variable can be set, separated by commas You can declare the variable in the for statement, but then it is only valid within the for loop within which it is created for (int j=3, k=4; ; ) If no variables are set, you must have a ; before the loop condition this typically means that the variable has been set prior to the for statement The initialization portion of the for statement is only executed once, each time you enter the loop. This allows you to nest loops loop condition A boolean expression evaluates as true or false Evaluated at the beginning of each loop Although this statement can be left out, leaving it out could lead to an infinite do loop, unless you execute a break statement inside the loop 14

15 Basic Terminology of the for Statement (2) update statement(s) Increases or decreases a value, typically one or more used in the loop condition Always executed at the end of the loop body Can include more than one statement, separated by commas for( ; ; i++, j= j+5) Usually use postfix notation, but in this case prefix work exactly the same Notes: If you are eliminating multiple parts of a for statement it may be better to use a while or do-while Don t stuff too many extraneous things into the for statement, you ll confuse yourself Most people use for statements when executing a specific number of times 15

16 Nested Loops A nested loop is a (inner) loop that appears in the loop body of another (outer) loop. The inner or outer loop can be any type: while, do while, or for. For example, the inner loop can be a while loop while an outer loop can be a for loop. Of course, they can be the same kind of loops too. Inner loops always stay completely inside their outer loops An inner loop may be skipped over during the execution of an outer loop If the inner loop has a tested condition that starts out false or If the loop is inside of an if block that isn t executed, then that loop wouldn t be executed 16

17 Nested Loop Example // Example of nested loop that prints a rectangle of stars public class NestedLoops public static void main (String[] args ) int height =5; int width = 4; for ( int i = 0 ; i < height ; i++ ) for ( int j = 0 ; j< width ; j++ ) System.out.print( "*" ) ; System.out.println( "" ) ; 17

18 The Mul$plica$on Table Is A Good Example Of Nested Looping

19 How Can We Use Loops To Help Construct This Table 1. Write the Top Row a. Use a loop to write the numbers Draw a horizontal line line 3. For each row a. Write the row number b. Draw a vertical line c. For each column 1) Calculate the product 2) Write the product d. Skip to the next row Now we ll look at the solution in MultTable.java 19

20 The break Statement Used to exit the innermost loop that the break is found within, unless the loop is labeled Loop is labeled with looplabelname: followed by the loop the name begins with an alpha character followed by alphas, number, $, _ characters The next statement executed is the first statement beyond the scope of that loop If you label a loop the break statement can be used to exit through an outer nesting We ll look at SimpleBreakLoop.java and BreakLabeledLoop.java 20

21 The conhnue Statement When executed in a repetition statement, the remainder of the statements in that iteration of the loop body are skipped, and the next iteration test is executed. In the for statement the update statements are executed We ll look at the ContinueExample.java 21

22 The flag or senhnel A flag or sentinel is a value appended to a data collection that signals the end of the data. The idea of a sentinel controlled loop is that there is a special value (the sentinel) that is used to control when the loop is done. In following code fragment, the user enters a zero when the sum is complete. Zero is the sentinel. Here is how the program should work import java.util.scanner; // Add up integers entered by the user. // After the last integer, the user enters a 0 class AddUpNumbers public static void main (String[] args ) Scanner keyboard = new Scanner( System.in ); int value, sum = 0; System.out.print( "Enter first integer (enter 0 to quit): " ); value = keyboard.nextint(); //we initialize value before entering the loop while ( value!= 0 ) sum = sum + value; System.out.print( "Enter next integer (enter 0 to quit): " ); value = keyboard.nextint(); //update value inside the loop System.out.println( "Sum of the integers: " + sum ); 22

23 The hasnext() method of the Scanner class The hasnext() method keeps reading data unto there is no more data to read When there is no more data to read it returns false Once we start reading from files in Chapter 9, that means that it will test to see if we have reached the end of the file Working interactively we can use a while or do while and enter <Ctrl> z on a Windows System or <Ctrl> d on a Mac to indicate end of data See HasNextTest.java 23

24 Bad Input data There are cases when erroneous data is entered There are many options Request revised data Exit the program In order to do either you must check the data first Note: Some inputs will cause the program to crash Can you name 0ne? Look at ErroneousData.java We will look at GeneralAverage.java in a few minutes 24

25 Scope of a Variable (1) A variable s scope is determined by the highest level of braces within which it is defined. It works at that level and all nested level If you use the same variable name in an outer and nested blocks, an error will occur. If you exit a block where a variable is defined you can reuse the variable name It s values are separate from the originally named variable Don t use the same variable name twice in a program it only confuses things. You are allowed to continuously redeclare a variable in the same statement within a loop again this allows for nesting Scope is extremely important to understand in nested loops. Once you leave the loop a variable declared in the loop no longer exists. 25

26 Scope of a Variable (2) / Demonstrate block scope. public class Scope public static void main(string args[]) int n1; // Visible in main n1 = 10; if (n1 == 10) // start new scope int n2 = 20; // visible only to this block // n1 and n2 both visible here. System.out.println( n1 and n2: + n1 + + n2); // n2 = 100; // Error! n2 not known here // n1 is still visible here. System.out.println("n1 is " + n1); 26

27 Scope of a Variable (3) public class LifeTime public static void main(string args[]) int i; for(i = 0; i < 3; i++) int y = -1; System.out.println("y is : " + y); Variable y is declared inside for loop block Each time when control goes inside for loop block the y variable is declared and used in loop When control goes out of the for loop then the variable becomes inaccessible 27

28 Scope of a Variable (4) public class ScopeInvalid public static void main(string args[]) int num = 1; // creates a new scope int num = 2; // Compile-time error // num already defined public class ScopeValid public static void main(string args[]) // creates a new scope int num = 1; // creates a new scope int num = 2; 28

29 Programming Exercises Class (1) Exercise 2 Pictures Write a program that reads an integer n and prints the following right triangle with base and height n 1 x 2 xx 3 xxx 4.. n xxxxxx.x 29

30 Programming Exercises Class(2) Exercise 9 GeneralAverage Write a program that calculates the average of n test scores, such that each score is an integer in the range 0 to 100. There should be a minimum of two scores. Your program should first prompt for an integer n and then request n scores. There should be a minimum of two scores. Your program should also test for invalid data. If a user enters a number outside the correct range, the program should prompt for another value. Round the average to the closest integer. 30

31 Programming Exercises Lab (1) Exercise 3 MorePictures Write a program that accepts an integer n and prints out the following picture of a diamond with 2n-1 rows 1 X 2 X X X 3 X X X X X n XXXXXXXXXXX (2n-1 times). 2n-1 X 31

32 Pseudo Code for More Pictures Read in n to determine total number of rows as 2n-1 Calculate the width = 2*n-1 For each row in top half up from 1 to n Calculate the number of x s (2*rows-1) Calculate the blanks = (width x s)/2 Print the blanks (a loop) Print the x s (a loop) Skip to next line For each row in the bottom half from n-1 to 1 Calculate the number of x s (2*rows-1) Calculate the blanks = (width x s)/2 Print the blanks (a loop) Print the x s (a loop) Skip to next line 32

33 Programming Exercises Lab (2) Exercise 10 Modified Average Write a program that accepts a list of n test scores, and then finds the average of the n-1 highest scores on the list that is, the lowest score on the list is not included in the average. For example, if the test scores are 90, 80, 70, and 60, the average is computed as ( )/3. The low score of 60 is excluded Your program should first prompt for an integer n and then request n scores. There should be a minimum of two scores. Your program should also test for invalid data. If a user enters a number outside the correct range, the program should prompt for another value. Round the average to the closest integer. 33