Chapter 8: Loops, Arrays, Strings

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Chapter 8 Loops, Arrays, Strings

• Loop statements
• do
• while
• for
• Arrays
• declaration, allocation, initialization, access
• multi-dimensional
• heterogeneous
• String class

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Loop Statements

• Sometimes we want to execute a particular piece
  of code multiple times
• Easy to with a loop
• Java loops
• do loop
• while loop
• for loop

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Do Loop

• Format
• do
• Stmts
• while (Expr)
• results in
• Stmts
• if Expr true done
• Stmts
• if Expr true done
• Stmts
• ...

• Do loop repeats its statement block until the
  control expression is false
• Always executes at least once
• Expr must be of type boolean

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While Loop

• Format
• while (Expr) Stmt
• results in
• if Expr false done
• Stmt
• if Expr false done
• Stmt
• if Expr false done
• ...

• While loop checks expression, executes statement
  if expr true
• May not execute at all
• Best to make Stmt a block as in
• while (Expr)
• Stmts

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For Loop

• Format
• for (InitE TestE IncE)
• Stmt
• results in
• InitE
• if TestE false done
• Stmt
• IncE
• if TestE false done
• Stmt
• IncE

• For loop
• executes InitE
• then
• checks TestE, if true
• executes Stmt, IncE
• repeats
• Always executes InitE, may not execute Stmt or
  IncE
• Use block for Stmt

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Calculate sum of 1 to N
int total 0 int j 1 if (N gt 0) do
total j j while (j lt N)
int total 0 int j 1 while (j lt N)
total j j
int total 0 int j for (j 1 j lt N j)
total j
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Tricky Aspects of Loops

• QWhat if condition to stop loop never met?
• A loop runs FOREVER
• QWhats wrong with the following
• for (j 1 j lt N j)
• total j
• A loop is running the empty statement () for j
  from 1 to N

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Arrays

• Not primitive types - no preset values
• Not classes - cant change access methods
• Array is a group of elements (primitive or class)
• same type
• index (numbered) from 0 to size-1

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Array Declaration

• Form
• Type Aname
• Type Aname
• examples
• int numbers
• Button myButtons
• Note declaration creates a pointer to an array,
  no space for array allocated

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Array Allocation

• Uses the new operator
• Form
• ArrayVarName new Typesize
• size should be a positive integer
• Allocates group numbered 0 to size-1
• Can be combined with declaration
• int numbers new int10

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Array Initialization

• Can be done at declaration (replaces allocation)
• Type Name Value0, Value1,
• Size of array determined by number of values
• Values must be of same type
• Elements of array set to Value0, Value1, etc. in
  order
• double taxRates 0.0, 0.08, 0.11, 0.13, 0.28
  

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Accessing Array Elements

• Form ArrayNameExpression
• can treat as variable of base type of array
• e.g. in array of ints numbers, numbers0 can be
  used like any other int (written, assigned, etc.)
• Expression should be of type int
• Java may throw an exception if the Expression is
  not between 0 and SizeofArray-1

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Array Processing

• Generally done with loops
• Process all (for loop)
• for (j 0 j lt ArraySize j)
• process Aj
• total sum of elements
• total 0
• for (j 0 j lt N j)
• total numbersj

• find smallest
• smallest 0
• for (j 1 j lt N j)
• if (numbersj lt numberssmallest)
• smallest j

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Array Processing

• Search for element (while loop)
• j 0
• while ((j lt ArraySize) (Aj not what we
  want))
• j
• Find first non-zero element
• j 0
• while ((j lt N) (Aj 0))
• j

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Multi-Dimensional Arrays

• Forms (2-dimensional)
• Type Name new TypeSize1Size2
• Type Name new TypeSize1Size2
• Example int twodnums new int43

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Non-Rectangular Arrays

• Need not allocate subarrays of same size
• int nonrect new int4
• nonrect0 new int1
• nonrect1 new int2
• nonrect2 new int3
• nonrect3 new int4

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Heterogeneous Arrays

• Arrays may be of class parent type, can hold
  different types of elements (as long as parent is
  a superclass of type)
• Example
• array of Employees
• Employees may be Salaried or Hourly

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public abstract class Employee public float
get2WeekSal() public class SalariedEmployee
extends Employee private float
YearlySalary public float get2WeekSal()
return YearlySalary / 26.0 public class
HourlyEmployee extends Employee private
float HoursPerWeek private float PayPerHour
public float get2WeekSal() return
HoursPerWeek PayPerHour 2
Employee bees new Employee20 totalSal
0.0 for (j 0 j lt 20 j) totalSal
beesj.get2WeekSal()
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Sorting

• Take an array of elements and order them by some
  index
• array of integers - order from smallest to
  largest
• array of employees - order alphabetically
• Some methods
• selection sort
• insertion sort
• shell sort
• quick sort

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Selection Sort
Find smallest element, put at position 0 Find
next smallest element, put at position 1 (neednt
look at 0) Find next smallest element, put at
position 2 (ignore 0 and 1) etc. for (j 0 j lt
ArraySize-1 j) smallest j for (k
j1 k lt ArraySize k) if (Arrayk lt
Arraysmallest) smallest k
swap(Arrayj,Arraysmallest)
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Insertion Sort
Like sorting a deck of cards Start with empty
sorted hand Add 1 card in the right place
(sorted hand of 1 card) Add 1 card in the right
place (sorted hand of 2 cards) etc. for (j 1
j lt ArraySize j) k j while ((k gt 0)
(Arrayk lt Arrayk-1))
swap(Arrayk,Arrayk-1) k--
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String Class

• String is a group of characters viewed as a
  single entity
• A String, lvljsjlfl, (empty string)
• Constructors
• YourString - converts things between to
  string
• String(char c) - array of chars to string
• String(String s) - makes copy of String s
• Special Method (append)
• abc 123 produces abc123

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String Methods

• boolean equals(Object obj) - true if strings same
• boolean equalsIgnoreCase(Object obj) - same as
  equals but upper and lower case chars match (a
  equals A)
• int compareTo(String s) - compares
  alphabetically, -1 if this string less, 0 if
  same, 1 if greater
• boolean endsWith(String s) - true if string ends
  with s
• boolean startsWith(String s) - true if string
  starts with s
• int length() - number of chars in string

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String Methods (cont)

• char charAt(int index) - returns char at position
  index
• int indexOf(char c) - location of first
  occurrence of c
• int indexOf(char c, int s) - first occurrence
  after loc s
• int indexOf(String s) - first occurrence of
  string s
• int indexOf(String s, int st) - first occurrence
  of s after st
• these routines return -1 if not found
• there are four similar lastIndexOf routines that
  find the last occurrence of the given char or
  string
• char toCharArray() - returns chars of string as
  array
• void getChars(int s, int e, char c, int cst) -
  returns chars of string from s to e in array c
  starting at cst

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String Methods (cont)

• static String valueOf(boolean b) - returns string
  corresponding to boolean value
• similar versions for char, int, long, float,
  double
• String substring(int s) - new string using chars
  from s on
• String substring(int s, int e) - new string,
  chars from s to e
• String concat(String s) - equivalent to
• String toLowerCase() - new string, all lower case
• String toUpperCase() - new string, all upper case
• String trim() - new string, all leading white
  space gone
• String replace(char old, char newc) - string with
  old replaced by newc

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Related Methods

• static Boolean valueOf(String s) - examines s,
  returns the boolean value that can be read from s
• similar methods for char, int, long, float double
• error occurs if the string can not be matched to
  type