Array of numbers, Array of strings COMP 102

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Array of numbers, Array of stringsCOMP 102 21
2008 T2
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Outline

• Using Arrays
• loops for arrays
• Exercises
• Administrivia
• A7 handout
• Friday lecture
• common problems in term test,
• Array exercises
• Pick up term test from School Office at CO358
• Model solutions are online

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Declaring and Creating Arrays

• Type of an array
• ?Type of element gt
• To use an array
• must declare a variable or field to hold the
  array
• MovingShapes allShapes
• int numbers
• must create an empty array
• allShapes new MovingShapes20
• numbers new int 1000
• must put values into the elements of the array
• numbers0 5
• numbers100 numbers6 numbers80

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Initialising Arrays (LL p379)

• Java has a special syntax for initialising
  arrays
• int outOf 10, 10, 15, 8, 20
• char vowels a, e, i, o, u
• String courses "COMP102", "COMP103",
  "MATH114",
  SWEN102", "STAT131", ENGR102"
• Square saver new Square(10, 10, 10,
  Color.blue), new
  Square(10, 10, 10, Color.red),
  new Square(10, 10, 10, Color.green)
  
• String siblings
• Can only be used in the declarations!

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Using an Array loops

• Usually, want to act on lots of elements
• ? access and assign array elements inside a
  loop
• Print out all the marks
• int num 0
• while (num lt marks.length)
• System.out.printf( Student 3d got 4.1f, num,
  marks num )
• num num 1
• Compute final marks from essay marks and exam
  marks
• int num 0
• while (num lt marks.length)
• marksnum essaynum0.6 examnum0.4
• num num1

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Using an Array for loops

• Standard pattern for processing each element of
  an array
• int i 0
• while (i lt data.length )
• data i
• i i1
• The for loop is a shorthand
• for (int i 0 i lt data.length i i1)
• data i
• or
• for (int i 0 i lt data.length i)
• data i

See LL 5.8
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Using an Array using for loops

• Same as before, but with for loop
• ? easier to read
• Print out all the marks
• for (int num 0 num lt marks.length num)
• System.out.printf( Student 3d got 4.1f, num,
  marks num )
• Compute final marks from essay marks and exam
  marks
• for (int i 0 i lt marks.length i)
• marks i essay i 0.6 exam i 0.4

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Exercise

• Write code to fill an array of numbers then print
  them out
• The array should hold doubles.
• The numbers should be the square roots of the
  integers from 1 to 100.
• public void squareRootTable()
• double table new double 110
• for ( int i 1 i lt 100 i)
• table i Math.sqrt(i)
• for ( int i 1 i lt 100 i)
• System.out.printf(Sqrt of d is 6.3f \n, i,
  table i )

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Reading data from a file into an array

• Suppose grades.txt contains student grades (in
  order of ID)
• String grades new String 200
• try
• Scanner sc new Scanner (new File(grades.txt))
  
• int n 0
• while ( sc.hasNext() n lt grades.length)
• gradesn sc.next()
• n
• catch (IOException e) System.out.printf( File
  failure s\n, e)

A C B B- A A-
dont go past end of array
dont go past end of file
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Reading data from a file into an array

• Suppose tutor1.txt contains student numbers and
  grades
• String grades new String 200
• try
• Scanner sc new Scanner (new File(tutor1.txt))
  
• while ( sc.hasNext() )
• int id sc.nextInt()
• String grade sc.next()
• if ( 0ltid id lt grades.length)
• gradesid grade
• catch (IOException e) System.out.printf(
  File failure s\n, e)

• 7 A
• 3 C
• 27 B
• 154 B-
• A
• 103 A-

null
null
C
null
null
null
A
?
null
null
null
null
1
2
3
4
5
6
7
199
0
200
length
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Arrays with meaningful indexes

• For some arrays, the index represents a
  meaningful value
• eg, the student number.
• every element of the array is meaningful.
• If no value has been assigned to an array
  element,then it will contain null (if an
  object) or 0 (if a number)
• Be careful to check if element is empty before
  using it
• Print out all the grades
• for (int num 0 num lt grades.length num )
• if ( gradesnum ! null )
• System.out.printf( Student 3d got 4.1f,
• num, grades num )

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Exercise

• Write a method that will read a sequence of stock
  prices from a file into an array and return the
  array.
• public double readPrices(String fname)
• double prices new double 200
• try
• Scanner sc new Scanner (new File(fname))
• int count 0
• while ( sc.hasNext() count lt 200)
• prices count sc.nextDouble()
• sc.close()
• return prices
• catch (IOException e) System.out.printf(
  File failure s\n, e)