1 February 2013Birkbeck College, U. London1 Introduction to Programming Lecturer: Steve Maybank Department of Computer Science and Information Systems

1 February 2013 Birkbeck College, U. London 1

Introduction to Programming

Lecturer: Steve Maybank

Department of Computer Science and Information Systems

[email protected] 2013

Week 4: Numbers and Strings

Overview Java Lab 2, Exercise 5 Arithmetical Expressions Parentheses Formatted output Lexicographic ordering of strings Example: a vending machine See Java for Everyone, Ch. 2 and 3


Java Lab 2, Exercise 5 We are given a string str and two positions i and j (i

comes before j). Set first to the substring from the start of the string

to the last position before i. Set middle to the substring from position i+1 to j-1. Set last to the substring from position j+1 to the end

of the string. Concatenate the following five strings: first, the

string containing just the character at position j, middle, the string containing just the character at position i, and last.


Example Input: str = “Boston”, i=2, j=4. Output: “Bootsn”. first = “Bo”. si = “s”; middle = “t”. sj = “o”. last = “n”.


Strategy First version of the program:

choose simple input and output. Divide the string str into five pieces Reassemble the pieces to obtain

the result. Later versions: improve input and

output, add error checking, examine special cases carefully.


String Operations

Check the properties of the operators and methods you might use.

Concatenation: + Substring: str.substring(0,1)


Properties of substring str.substring(i,j): return the string

consisting of the characters in str from position i to position j-1, counting from 0.

Note i, j are integers such that i<j. Example: “Sally”.substring(1,4) is “all”. If j is omitted the string of characters from

position i to the end of str is returned. Example: “Sally”.substring(1) is “ally”.


Central Part of the CodeString str = “Boston”;int i = 2, j = 4;String first = str.substring(0, i);String si = str.substring(i, i+1);String middle = str.substring(i+1, j);String sj = str.substring(j, j+1);String last = str.substring(j+1);String result = first+sj+middle+si+last;


Arithmetic Expressions


math. exp. Java expression

(x+y)/2

x*y/2

Math.pow(1+r/100,n)

Math.sqrt(a*a+b*b)

Math.PI

2

yx

2

yx

nr

1001

22 ba

Parentheses Each left parenthesis in an expression

must be paired with a right parenthesis. Incorrect expression: (3+9)+1)*(4+(2*3) Correct expression: ((3+9)+1)*(4+(2*3)) Check: working from left to right, count +1

for a left parenthesis and –1 for a right parenthesis. If the count drops below 0 or is not 0 at the end then there is an error.

How about working right to left?


Evaluation of an Expression


Work from the “inside outwards”, e.g. evaluate(3+4*(9-7))*((2*7)-5)

First step: evaluate 9-7 and 2*7 to obtain (3+4*2)*(14-5)

Second step: evaluate 4*2 and 14-5 to obtain (3+8)*9

Third step: evaluate 3+8 to obtain11*9

Fourth step: evaluate 11*9 to obtain 99.

Formatted Outputdouble price = 1.21997;System.out.println(price);/* 1.21997 */System.out.printf(“%.2f”, price);/* 1.22with no newline */System.out.printf(“%10.2f”, price);/* 1.22with no newline*/


Format Specifier %10.2f 10: width of field in characters. 2: number of digits after the decimal

point. f: fixed floating point. If 47.21997 is printed using this format

specifier, then how many spaces are there to the left of the printed number?


Example

price=1.21997;System.out.printf(“Price per litre: %10.2f“,

price);

/* The printed output isPrice per litre: 1.22with no newline*/


Decimal Integer Format

int volume = 10;System.out.printf(“The volume is %5d”, volume);/* %5d: decimal integer format specifier. The

field width is 5.Printed output isThe volume is 10with no newline*/


Lexicographic Ordering of Strings

Dictionary order, eg. “anteater” is before “zebra” even though “anteater” is longer.

Java lexicographic ordering:uppercase letters precede lower case

lettersnumbers precede lettersthe space character precedes all

printable characters.


Algorithm for Lexicographic Ordering

Given two distinct strings, compare them character by character starting from position 0.

Stop at the first pair of characters which fail to match.The strings have the same order as the corresponding characters.

If one string ends, then it precedes the other string.


Examples


preceding string

following string

car cat

car cart

cart cat

cart wheel cartwheel

car car7

7car car

Zebra zebra

Program to Simulate a Vending Machine

A person pays for an item with a dollar note and receives change. All prices are multiples of 25 cents (quarters) and change is given in dollar notes and quarters. Compute the numbers of dollars and quarters to be returned. (See JFE, Section 2.3.)


Inputs and Outputs Inputs:

Denomination of the note, in dollarsPrice of purchased item

Outputs:Number of dollars to be returnedNumber of quarters to be returned


Example Note: $5 Price: $2.25 Change: $2.75 which is $2 and 3x25 cents Strategy: work in cents. Number of cents

in the change is 500-225=275. Number of dollars in 275 cents is 275/100. Number of quarters is (275%100)/25


Pseudocode

amount due = (100xnote value)-(price in cents)dollars = amount due/100 (discard remainder)amount due = amount due%100quarters = amount due/25


Variables and Constants noteValue itemPrice amountDue dollars quarters CENTS_PER_DOLLAR CENTS_PER_QUARTER


Input and OutputSystem.out.print(“Enter note value (1=$1 note,

5=$5 note, etc.): ”);int noteValue=in.nextInt();System.out.print(“Enter price in cents: ”);int itemPrice=in.nextInt();…System.out.printf(“Dollars: %6d”, dollars);System.out.printf(“Quarters: %6d”, quarters);


Class and main methodimport java.util.Scanner;/** This program simulates a vending machine that gives change. */public class VendingMachine{

public static void main(String[] args){

Scanner in = new Scanner(System.in);final int CENTS_PER_DOLLAR = 100;final int CENTS_PER_QUARTER = 25;….. //input hereint amountDue = CENTS_PER_DOLLAR*noteValue-itemPrice;int dollars = amountDue/CENTS_PER_DOLLAR;amountDue=amountDue%CENTS_PER_DOLLAR;int quarters = amountDue/CENTS_PER_QUARTER;…//output here

}}


Find Five Compile Time Errors

public class HasErrors{

public static void main(String[] args);{System.out.print(Pleese enter two numbers: )x=in.readDouble();y=in.readDouble();System.out.println(“The sum is ”+x+y);}

}

1 February 2013 JFE Review Exercises, R2.8 26

Find Two Run Time Errorsimport java.util.Scanner;public class HasErrors{

public static void main(String[] args){

int x=0, y=0;Scanner in = new Scanner(“System.in”);System.out.print(“Please enter an integer: ”);x=in.nextInt();System.out.print(“Please enter another integer:

”);x=in.nextInt();System.out.println(“The sum is ”+x+y);

}}1 February 2013 JFE Review Exercises R2.6 27

Documents

1 February 2013Birkbeck College, U. London1 Introduction to Programming Lecturer: Steve Maybank Department of Computer Science and Information Systems