Loops. Eng. Mohammed Alokshiya. Islamic University of Gaza. Faculty of Engineering. Computer Engineering Department

Transcription

1 Islamic University of Gaza Faculty of Engineering Computer Engineering Department Computer Programming Lab (ECOM 2114) Lab 6 Loops Eng. Mohammed Alokshiya November 9, 2014

2 Loop is a powerful construct that controls how many times an operation or a sequence of operations is performed in succession. It simply tells a program to execute statements repeatedly. In java, there are three types of loop: while loop, do-while loop, and for loop. The while Loop A while loop executes statements repeatedly while the condition is true. The syntax for the while loop is: while (loop-continuation-condition) { // Loop body Statement(s); And the flowchart is: 2

3 Example: (The 3n + 1 Problem) Consider the following algorithm to generate a sequence of numbers. Start with an integer n. If n is even, divide by 2. If n is odd, multiply by 3 and add 1. Repeat this process with the new value of n, terminating when n = 1. For example, the following sequence of numbers will be generated for n = 22: Write a program to read a positive integer n and generate a sequence of numbers until 1. Code: Main.java import java.util.scanner; public class Main { public static void main(string[] args) { Scanner input = new Scanner(System.in); int n = input.nextint(); while (n > 1) { System.out.print(n + " "); if (n % 2 == 0) n /= 2; else n = n * 3 + 1; System.out.println(n); Output: 3

4 Example: Write a program to read a file contains unknown number of lines, each one contains exactly two integers A and B. For each line, print the sum of both numbers, in the following format: Line #N: S Where N is the line number, and S is the sum of two numbers in that line. Sample input: Code: Main.java import java.io.file; import java.io.filenotfoundexception; import java.util.scanner; public class Main { public static void main(string[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("input.txt")); int a, b, line = 1; while (input.hasnext()) { a = input.nextint(); b = input.nextint(); System.out.println("Line #" + line + ": " + (a + b)); // System.out.printf("Line #%d: %d\n", line, a + b); line++; Output: 4

5 break Statement You can use break in a loop to immediately terminate the loop. Example: Write a program that asks the user to type a positive integer. When the user types a negative value the program writes ERROR and asks for another value. When the user types 0 that means that the last value has been typed and the program must write the average of the positive integers. If the number of typed values is zero the program writes 'NO AVERAGE'. Code: import java.util.scanner; Main.java public class Main { public static void main(string[] args) { Scanner input = new Scanner(System.in); int n, sum = 0, count = 0; while (input.hasnext()) { n = input.nextint(); if (n == 0) { break; else if (n < 0) { System.out.println("Error! try again"); else { sum += n; count++; if (count > 0) System.out.println("Average is: " + (sum * 1.0 / count)); else System.out.println("No Average"); 5

6 The for Loop The syntax of a for loop is: for (initial-action; loop-continuationcondition; action-after-each-iteration) { // Loop body Statement(s); And the flowchart is: 6

7 Example: (Count Uppercase Letters) write a program that counts how many uppercase letters exist in a given string. Sample Input Sample Output P@ssWoRd 3 Hello World 2 welcome 0 Code: Main.java import java.util.scanner; public class Main { public static void main(string[] args) { Scanner input = new Scanner(System.in); String line = input.nextline(); int count = 0; for (int i = 0; i < line.length(); i++) { if (Character.isUpperCase(line.charAt(i))) { count++; System.out.println(count); continue Statement You can use continue in a loop to terminate the current iteration of a loop. 7

8 Example: Write a program that prints all numbers between 0 and 100, except that which divide by 7. Code: import java.util.scanner; Main.java public class Main { public static void main(string[] args) { for (int i = 0; i <= 100; i++) { if (i % 7 == 0) continue; System.out.println(i); 8

9 Netbeans Tips & Tricks for + TAB Use this shortcut for writing for loop header quickly. Write: for, then press TAB button and see the result: 9

10 Exercises: Part 1: An integer N greater than 1 is prime if its only positive divisor is 1 or itself. In other words, (N % X!= 0, for all integers X where 1 < X < N). Write a program that reads an integer and check if it is a prime number or not. Sample Input Sample Output 2 Prime 5 Prime 8 Not a prime Part 2: (Process string) Write a program that prompts the user to enter a string and displays the characters at odd positions. Sample Input Sample Output Hello World! HloWrd Welcome to Java Wloet aa Part 3: Write a program that asks the user to type 10 integers and writes the smallest value. 10

11 Extra Question (Not Assessed): Find the smallest number X, such 2 that X > 1, and X, X 0). Hint: 3 5 and X are all integers (fractional part is 3 X = X 1 3 To calculate X 1 3 in java, you can use Math.pow method: int X = input.nextint(); double result = Math.pow(X, 1.0 / 3); 11