CS1101: Programming Methodology Recitation 5

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CS1101 Programming MethodologyRecitation 5
Arrays and Collections
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Array Basics

• Index of first position in an array is 0.
• Length value of an array
• An array is a reference data type
• double A new double12 //array of double
• int size A.length
• Do not mix up with length method of a String
  object
• String s This is a string
• int size s.length()
• Array of objects

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Constructors for Person class

• public class Person
• private int age
• private char gender
• private String name
• public Person()
• // Default age is 0
• // Gender of a person F for female, M for male,
  U for unknown.
• // Default name is set to NotGiven.
• this (0, U, Not Given)
• public Person(int age, char gender, String name)
  
• this.setAge (age)
• this.setGender (gender)
• this.setName (name)

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Public Methods of Person class

• public void setAge (int personAge)
• age personAge
• public void setGender (char personGender)
• gender personGender
• public void setName (String personName)
• name personName
• public int getAge ()
• return age
• public char getGender ()
• return gender
• public String getName ()
• return name

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• Create a Person object

• Person latte
• latte new Person()
• latte.setName(Ms. Latte)
• latte.setAge(20)
• latte.setGender(F)
• System.out.println (Name latte.getName()
  )
• System.out.println (Age latte.getAge()
  )
• System.out.println (Sex
  latte.getGender() )

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• Create and manipulate an array of Person objects

• Person person //declare person array
• person new Person20 //and then create it
• Note The elements, Person objects, are not yet
  created.
• person0 new Person()
• person0.setName (Ms. Latte)
• person0.setAge (20)
• person0.setGender (F)

null
Person0 new Person (20, F, Ms. Latte)
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Set up person array

• String name
• int age
• char gender
• for (int i0 i lt person.length i)
• System.out.println (Enter name )
• name stdin.readLine()
• System.out.println (Enter age )
• age Integer.parseInt(stdin.readLine())
• System.out.println (Enter gender )
• gender stdin.readLine().charAt(0)
• personi new Person()
• personi.setName(name)
• personi.setAge(age)
• personi.setGender(gender)

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Find average age

• double sum 0 aveAge
• for (int i0 i lt person.length i)
• sum personi.getAge()
• aveAge sum / person.length

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Find youngest and oldest persons

• int minIdx, maxIdx //Index to youngest and
  oldest persons
• minIdx maxIdx 0
• for (int i0 i lt person.length i)
• if (personi.getAge() lt personminIdx.getAge())
  
• //found a younger person
• minIdx i
• else if (personi.getAge() gt personmaxIdx.getA
  ge())
• //found an older person
• maxIdx i
• System.out.println (Oldest
  personmaxIdx.getName() is
  personmaxIdx.getAge() years old.)
• System.out.println (Youngestt
  personminIdx.getName() is
  personminIdx.getAge() years old.)

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Find youngest and oldest persons - an array of
Person objects and two Person var

• Person youngest, oldest //Points to youngest and
  oldest persons
• youngest oldest person0
• for (int i0 I lt person.length i)
• if (personi.getAge() lt youngest.getAge())
• youngest personi
• else if (personi.getAge() gt oldest.getAge())
• oldest personi
• System.out.println (Oldest oldest.getName()
  is oldest.getAge() years old.)
• System.out.println (Youngestt
  youngest.getName() is youngest.getAge()
  years old.)

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An array of Person objects and two Person
variables
0 1 2 3
17 18 19
person
Person Jane 87 F
Person Ms. Latte 20 F
oldest
youngest
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Find a particular person

• int i 0
• while (i lt person.length //still more persons
  to search
• !personi.getName().equals(Latte))
• i
• if (i person.length) //unsuccessful search
• System.out.println (Ms. Latte was not in the
  array)
• else //found successful search
• System.out.println (Found Ms. Latte at position
  i)

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Delete a particular person

• Two approaches
• Reset array element to null.
• Leave holes in array
• Pack elements so that real references occur at
  the beginning and null references at the end
• If object at position j is removed, then elements
  from position j1 up to last non-null reference
  are shifted one position lower
• Order of Person objects is maintained.
• Replace removed element by last element in the
  array

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0 1 2
3
person
Person B
Person C
Person A
Person D
int delIdx 1 persondelIdx null
0 1 2
3
person
Person C
Person A
Person D
15
0 1 2
3
person
Person B
Person C
Person A
Person D
int delIdx 1 int last 3 persondelIdx
personlast personlast null
0 1 2
3
person
Person C
Person A
Person D
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Complete program Illustrate processing of an
array of Person objects

• public class testPerson
• public static void main(String args)
• Person person //declare person array
• person new Person5 //and then create it
• //--------------create person
  array-----------------------//
• String name
• int age
• char gender
• for (int i0 i lt person.length i)
• System.out.println (Enter name )
• name stdin.readLine()
• System.out.println (Enter age )
• age Integer.parseInt(stdin.readLine())
• System.out.println (Enter gender )
• gender stdin.readLine().charAt(0)

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• //--------------compute average
  age----------------------//
• float sum 0 aveAge
• for (int i0 i lt person.length i)
• sum personi.getAge()
• aveAge sum / (float) person.length
• System.out.println (Average age aveAge)
• //--------------find oldest and youngest
  person----------//
• Person youngest, oldest //Points to youngest
  and oldest persons
• youngest oldest person0
• for (int i0 I lt person.length i)
• if (personi.getAge() lt youngest.getAge())
• youngest personi

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• //--------------search for a particular
  person---------------------//
• System.out.println (Name to search )
• String searchName stdin.readLine()
• int i 0
• while (i lt person.length //still more persons
  to search
• !personi.getName().equals(searchName))
• i
• if (i person.length) //unsuccessful search
• System.out.println (searchName was not in
  the array)
• else //found successful search
• System.out.println (Found searchName at
  position i)