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Introduction to Programming

Lecturer: Steve Maybank

Department of Computer Science and Information Systems

sjmaybank@dcs.bbk.ac.ukSpring 2013

Week 7: loops

Overview

Java Lab 6, Exercises 2 and 3 while statement for statement See Java for Everyone, Ch. 4

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JavaLab 6, Qu. 2

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Score Grade

90-100

A

80-89 B

70-79 C

60-69 D

<60 E

Obtain a score of type int fromthe keyboard and print out thecorresponding letter grade.

Solution to Qu. 2: first partimport java.util.Scanner;public class QuizGrading{

public static void main(String[] args){

Scanner in = new Scanner(System.in);System.out.print("Please type in the score: ");int score = in.nextInt();… // more code here

System.out.print("The grade is: "+grade);

}}

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Solution to Qu. 2: second part

char grade = ' ';if (score >= 90)

{grade = 'A';}else if (score >= 80)

{grade = 'B';}else if (score >= 70)

{grade = 'C';}else if (score >= 60)

{grade = 'D';}else

{grade = 'E';}

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Diagram for Question 2

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50 807060 10090

>=A

>= D

>= C

>= B

E

JavaLab 6, Qu. 3 Obtain a year of type int from the

keyboard. Print true if it is a leap year, otherwise print false.

Usually years that are divisible by 4 are leap years. However, years that are divisible by 100 are not leap years, unless the year is also divisible by 400.

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Decision Tree

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divisibleby 4?

yesno

not a leapyear

divisibleby 100

?leapyear

no yes

divisibleby 400

?not a leapyear

no yes

leapyear

Two Paths through the Decision Tree

a: (year%4 == 0); b: (year%100 !=0); c: (year%100 == 0) && (year%400 ==

0); First path: a && b; Second path: a && c;

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Boolean Test for a Leap Year

Solution: (a && b)||(a && c) Equivalent solution: a && (b||c) Proof of equivalence:

check a = false (both are false)check a = true (both reduce to b||c)

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Solution to Qu. 3: first partimport java.util.Scanner;public class LeapYear{

public static void main(String[] args){

Scanner in = new Scanner(System.in);System.out.print("Please type in the year: ");int year = in.nextInt();… // more code here

}}

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Solution to Qu. 3: second part

boolean a = year%4 == 0;boolean b = year%100 != 0;boolean c = (year%100==0) && (year%400==0);if (a && (b||c)){

System.out.println("Leap year");}else{

System.out.println("Not a leap year");}

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Syntax for the while Loop

while (boolean condition){

statements}/* The statements are carried out

while the condition tests true. */

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Flowchart for the while Loop

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testcondition

true

false

executestatements

Example: compound interest

double balance = 10000;int year = 0;while (balance < TARGET){

year++;double interest = balance*RATE/100;balance = balance+interest;

}

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Access to Variablesdouble balance = 10000;…while (balance < TARGET){

double interest = balance*RATE/100;…

}/* The variable balance is declared outside the loop and is

available inside and outside the loop. The variable interest is declared inside the loop and is only available inside the loop. A new version of the variable interest is created on each iteration. */

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Compute Time to Double an

Investment public class DoubleInvestment{

public static void main(String[] args){

final double RATE = 5;final double INITIAL_BALANCE = 10000;final double TARGET = 2*INITIAL_BALANCE;double balance = INITIAL_BALANCE;int year = 0;/* while loop here */System.out.println("Investment doubled after "+year+" years");

}}

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Test Cases Use very simple test data to check that the

while loop is correct. Eg. if

RATE = 100.1%,TARGET = 2*INITIAL_BALANCEthen the balance is slightly more than doubled at the end of the first year.

Check the value of year on exiting the loop.

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while Loop Example 1

int i = 0, sum = 0;while(sum < 10){

i++; sum = sum+i;System.out.print(i+" "+sum+", ");

}/* output: 1 1, 2 3, 3 6, 4 10, */

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while Loop Example 2

int i = 0, sum = 0;while(sum < 10){

i++; sum = sum-i;System.out.print(i+" "+sum+", ");

}/* output: 1 -1, 2 -3, 3 -6, 4 -10, … */

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The for Loop

for(int i = 1; i <=10; i++){

System.out.println(i);}/* Use a for loop when the number of

iterations is known in advance.The for loop is count controlled.The while loop is event controlled. */

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Parts of the for Statement

for (int i = 0; i <= 10; i++){

…}/* i = 0: initialisation, executed once on entering the loop.

i <= 10: condition to be checked before each iteration.i++: update executed after each iteration. */

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Examples of for Loops

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Loop values of I comment

for(int i = 0; i <= 5; i++) 0 1 2 3 4 5 The loop is executed 6 times

for(int i = 5; i >= 0; i--) 5 4 3 2 1 0 The value of i can decrease

for(int i = 0; i < 9; i = i+2) 0 2 4 6 8 Step size not equal to 1

for(int i = 0; i != 9; i = i+2) 0 2 4 6 8 … Infinite loop

for(int i = 1; i <= 20; i = i*2) 1 2 4 8 6 Any rule for modifying i can be used

for(int i = 0; i<str.length(); i++)

0 1 … str.length()-1

Step through the characters in a string

Avoid Confusing for Loops

double x, sum = 0;

for(System.out.print(“Input: ”); in.hasNextDouble(); sum += x)

{

x = in.nextDouble();

}

/* The compiler does not check whether the initialisation, condition and update are related.

in.hasNextDouble() tests to see if the next input is a number. */

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Sum and Average Valuedouble total = 0;int count = 0;while (in.hasNextDouble()){

double input = in.nextDouble();total = total+input;count++;

}double average = 0;if (count > 0){average = total/count;}

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Counting Matches

int upperCaseLetters = 0; // initialise a counterfor (int i = 0; i < str.length(); i++){

char ch = str.charAt(i); //obtain the ith character in strif (Character.isUpperCase(ch)) //test for upper case{

upperCaseLetters++;}

}/* Character.isUpperCase is a method in the class Character

which is in the package java.lang. */

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