Loops Chapter 4
“while” structures • It repeats a set of statements while a condition is true. • while (condition) • { • execute these statements; • }
“while” structures • It repeats a set of statements while a condition is true. • while ( condition ) • { • execute these statements; • } 1 2 3 • The dynamics of “while” • Evaluate condition: • if TRUE go to 2 • If FALSE go to 3 • Execute statements, and then go to 1 • Continue with next statement. 1. 2. 3.
“while” structures • It repeats a set of statements while a condition is true. • int speedLimit = 55; • int speed = 0; • while ( speed <= speedLimit ) • { • speed = speed + 1; • } • // since we don’t want a ticket… • speed = speed - 1; • What is the value of “speed” after the • last statement? • 55 • 54 • 56 • 0 • none of the above
“while” structures • It repeats a set of statements while a condition is true. • int speedLimit = 55; • int speed = 0; • while ( speed < speedLimit ) • { • speed = speed + 1; • } • // since we don’t want a ticket… • speed = speed - 1; 1 initialize variables in conditional 2 modify variables in conditional
“while” structures: Exercises • Determine the output of the following methods: • public void foo1() { • int i=0; • while (i <= 20) { • System.out.println( i ); • i = i + 4; // or i+= 4; • } • } • public void foo2() { • int i = 20; • while (i > 0) { • i = i / 2; • System.out.println( i ); • } • } • 0 4 8 12 16 20 • 4 8 12 16 • 0 4 8 12 16 20 24 • 0 4 8 12 16 • none of the above • (on separate lines) • 20 10 5 2.5 1.25 • 10 5 2 1 0 • 20 5 2 1 • 20 5 2 1 0 • none of the above • (on separate lines)
Computer Work • Write a program that reads an integer, and displays (using System.out.println) all numbers from the number down to 0. • For example, if 8 is entered, the program should display numbers 7, 6, 5, 4, 3, 2, 1, 0, in this order and with each number in one line. • Write a program that reads an integer, and displays (using System.out.println) all even numbers between 0 and the number received, and finally the number of numbers displayed. • For example, if the parameter was 8, the method should display numbers 2, 4 and 6, in this order and with each number in one line, and return a value of 3 (which is how many even numbers were between 0 and 8).
“for” structures • It (also) repeatsstatements while a condition is true. Works best when loop will be repeated a known number of times 1 2 4 loop condition initial statement modify statement for ( ; ; ) { statements; } 3 5 • The dynamics of “for” • Initialize condition variables. • Evaluate loop condition: • if TRUE go to 3 • If FALSE go to 5 • Execute statements; then go to 4 • Modify condition variables; then go to 2 • Continue with next statements.
prime using for Write a program that prints “yes” or “no” to indicate whether an integer parameter named number is a prime number. Use a for loop. How to write Java: Answer the questions: Exactly how do you do this? What is a step by step description? WRITE IN ENGLISH. NOT Java or Draw pictures. or both Debug the English: Trace through with numbers. especially with going through the loop 0 times, 1 time, >= 1 times. Fix the ENGLISH.
Brainstorm. How do you figure out if a number is prime? prime(11) prime(12). Talk to each other. • Write an algorithm for that • Translate the algorithm to Java
Prime Number Program import java.util.Scanner; public class Prime { public static void main(String[ ] args) { Scanner kbd = new Scanner(System.in); System.out.print("Enter a number: "); int nbr = kbd.nextInt( ); if (nbr < 2) // only look at positive numbers { System.out.println("no"); System.exit(0); // means leave the program right now } for (int i=2;i<nbr;i++) { if (nbr % i == 0) { System.out.println("no"); System.exit(0); } } System.out.println("yes"); } } for loop Leaves loop and program outside the loop
Embedded loops • Write a program to draw a square box after reading in its size • Write the first line (for loop printing n stars) • println • for each internal line (n-2 of them) • print a * • print n-2 spaces • println a * • print the last line
import java.util.Scanner; public class DrawStars { public static void main(String[ ] args) { Scanner kbd = new Scanner(System.in); System.out.print("Enter a number: "); int nbr = kbd.nextInt( ); if (nbr < 2) // only look at positive numbers System.exit(0); // means leave the program right now // do first line for (int i=0;i<nbr;i++) System.out.print("*"); // add enter System.out.println( ); // nothing in parentheses // continued on next page
// print n-2 internal lines for (int i=0;i<nbr-2;i++) // i is never used. it only counts { // nbr of times through loop // print first character on line System.out.print("*"); // print number of spaces minus one * on each end for (int j=0;j<nbr-2;j++) // embedded loop for one internal line System.out.print(" "); // single space in between quotes // print last * on line -- also put enter in there System.out.println("*"); } // and outer for loop that does all internal lines // print last line. same as first line for (int i=0;i<nbr;i++) System.out.print("*"); System.out.println( ); // nothing in parentheses } // end main } // end class
Embedded Loop Practice • Write a loop that prints a times table 0-2: 0 0 1 2 0 0 0 0 1 0 1 2 2 0 2 4 • Write the first line • for each number on the left (0, 1, 2) • print the left number (the LHS of the table) • for each number on the right (0, 1, 2) • print the product • println. 2a
