1. Introduction to Programming
Lecturer: Steve Maybank
Department of Computer Science and Information Systems
Spring 2013
Week 4: Numbers and Strings
1 February 2013
Birkbeck College, U. London

2. Birkbeck College, U. London
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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3. Birkbeck College, U. London
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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4. Birkbeck College, U. London
Example
Input: str = “Boston”, i=2, j=4.
Output: “Bootsn”.
first = “Bo”.
si = “s”;
middle = “t”.
sj = “o”.
last = “n”.
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5. Birkbeck College, U. London
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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7. 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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8. Central Part of the Code
String 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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9. 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
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10. Birkbeck College, U. London
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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11. 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 obtain
11*9
Fourth step: evaluate 11*9 to obtain 99.
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12. Birkbeck College, U. London
Formatted Output
double price = ; System.out.println(price); /* */ System.out.printf(“%.2f”, price); /* 1.22 with no newline */ System.out.printf(“%10.2f”, price); /* 1.22 with 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 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= ; System.out.printf(“Price per litre: %10.2f“, price); /* The printed output is Price per litre: 1.22 with no newline */
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15. 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 is The volume is 10 with no newline */
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16. Lexicographic Ordering of Strings
Dictionary order, eg. “anteater” is before “zebra” even though “anteater” is longer.
Java lexicographic ordering:
uppercase letters precede lower case letters
numbers precede letters
the space character precedes all printable characters.
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17. 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
preceding string
following string
car
cat
cart
cart wheel
cartwheel
car7
7car
Zebra
zebra
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19. 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 dollars
Price of purchased item
Outputs:
Number of dollars to be returned
Number of quarters to be returned
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21. Birkbeck College, U. London
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 =275.
Number of dollars in 275 cents is 275/100.
Number of quarters is (275%100)/25
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22. Birkbeck College, U. London
Pseudocode
amount due = (100xnote value)-(price in cents) dollars = amount due/100 (discard remainder) amount due = amount due%100 quarters = amount due/25
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23. Variables and Constants
noteValue
itemPrice
amountDue
dollars
quarters
CENTS_PER_DOLLAR
CENTS_PER_QUARTER
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Input and Output
System.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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25. Birkbeck College, U. London
Class and main method
import 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 here int 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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26. 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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JFE Review Exercises, R2.8 26

27. Find Two Run Time Errors
import 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: ”); System.out.println(“The sum is ”+x+y); }
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JFE Review Exercises R2.6 27