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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
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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.
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Example Input: str = “Boston”, i=2, j=4. Output: “Bootsn”. first = “Bo”. si = “s”; middle = “t”. sj = “o”. last = “n”.
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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.
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String Operations
Check the properties of the operators and methods you might use.
Concatenation: + Substring: str.substring(0,1)
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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”.
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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;
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Arithmetic Expressions
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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?
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Evaluation of an Expression
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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*/
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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?
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Example
price=1.21997;System.out.printf(“Price per litre: %10.2f“,
price);
/* The printed output isPrice per litre: 1.22with no newline*/
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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*/
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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.
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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.
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Examples
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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.)
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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
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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
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Pseudocode
amount due = (100xnote value)-(price in cents)dollars = amount due/100 (discard remainder)amount due = amount due%100quarters = amount due/25
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Variables and Constants noteValue itemPrice amountDue dollars quarters CENTS_PER_DOLLAR CENTS_PER_QUARTER
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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);
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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
}}
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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);}
}
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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