ITM 172

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ITM 172

• Java Programming
• Chapter 5
• Arrays Part A

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What is an array?

• An array is a data structure. A data structure is
  a space that can be reserved in memory for
  storing one or more values. For example, an int
  cell, a float cell, and a double cell are data
  structures.
• To instruct the computer to setup or allocate
  space for a data structure (such as an int,
  float, or char cell), programmers write
  declaration statements (e.g. int x).
• Programmers can also declare arrays. An array is
  a data structure that contains multiple int,
  float, or double cells.
• For example, if your program needed to store 10
  different integer values, you can declare an
  integer array of size 10.

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Reference variables

• A reference variable is a variable that contains
  the address of a data structure.
• In Java, when we declare an array, we also
  declare a reference variable and assign the
  starting address of the array to the reference
  variable.

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Declaring an array

• The following declaration statement declares a
  variable of type reference-to-integer-array
• int myGrades // creates a reference variable
• By itself, this statement is insufficient because
  it declares a reference variable that doesnt
  reference anything (yet).
• The following declaration statement (1) allocates
  space in memory for an array of 7 integer cells
  and (2) creates (declares) a variable of type
  reference-to-integer-array and (3) assigns the
  starting address of the array to the reference
  variable.
• int myGrades new int7

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int myGrades new int7
myGrades is a variable who contains the address
of a data structure (the array). myGrades is
setup in the programs (local) memory space the
array is actually allocated memory on the heap.
myGrades
00000001
The array
00000001 00000010 00000011 00000100
00000101 00000110 00000111
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The size of the array

• In the previous example, the size of the array is
  7.
• The size must be specified when the array is
  declared.
• The size must be a constant not a variable.
• The size cant change during program execution
  the programmer must commit to a size at the
  beginning of the program.

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Referring to the elements of the array
myGrades
myGrades0 myGrades1 myGrades2 myGrades3 my
Grades4 myGrades5 myGrades6
of array
Use the index (the number in the square brackets)
to refer to the individual array elements. The
first element is always 0 the last element is
(length 1).
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Declaring an array with initialization

• We already learned to declare an array (actually
  an array reference) and allocate space in memory
  for the array as follows
• int myGrades new int7
• But if you are going to assign initial values to
  the array when you declare it, you can do this
• int myGrades 89, 87, 90, 93, 83 or
• int myGrades new int 89, 87, 90, 93,
  83

You dont have to specify size if providing
initial values because the compiler can count.
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An array is actually an object

• When you declare an array, you are actually
  creating an object (an instance) of class Array.
• Any object of class Array has a data attribute
  named length which contains the size of the
  array. Therefore, once you declare an array
  (myGrades), you can refer to the size as
  myGrades.length.

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Example 5.X InputtingArray(not in text)

• This program declares an array of size 5 integers
  and prompts the user for 5 values to store in the
  5 integer cells.

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Not in text
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Example 5.1 TestArray

• This program declares an integer array of size 6.
• It prompts the user for the six values.
• It finds the maximum value entered.
• It finds the number of times the maximum occurs.
• It displays the maximum and the frequency that
  maximum occurred.

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Page 185-6
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Lab 5.2 AssignGrade

• This program prompts the user for a number of
  students and then creates an integer array whose
  length the number of students.
• Then it prompts the user for numeric grades and
  converts them to letter grades.
• The teacher is grading on a curve. To determine a
  students letter grade, if the numeric grade is
  within 10 of the highest grade, you get an A,
  if the numeric grade is within 20 of the highest
  grade, you get a B, within 30 for a C, within
  40 for a D.

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Page 187-189
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Homework J3

• 5.1 on page 226
• (Prompts the user for 10 integer values, stores
  them in an array, and then display array elements
  in reverse order).

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Passing Arrays to Methods

• Whole arrays and individual elements of arrays
  can be passed to methods as arguments.
• If you pass an individual element of an array to
  a method, its like passing any other individual
  int or float value A copy of the value is passed
  to the method. (If int x5 and x is passed to a
  method, that means that it is actually a copy of
  5 that is being passed to the method.)
• If you pass a whole array to a method, youre
  actually passing the address of the array, not a
  copy of the values stored in the array.

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Call by value vs. call by reference

• When individual values are passed to methods, we
  call that call by value
• max(x, y) // x and y are individual int
  cells
• When a reference variable to an object is passed
  to a method, we call that call by reference
• max (a) // a is a reference to an int array

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Lab 5.3 TestPassArray

• The following program demonstrates both call by
  value and call by reference.
• It declares an array of size 2 int. First it
  passes the two individual int values to a method
  swap( ). Then it passes the entire array to a
  method swapFirstTwoInArray( ).
• In the first case, swapping values in the swap( )
  method has no effect on the values in the array
  in the second case, swapping values in the
  swapFirstTwoInArray( ) method does swap the
  values in the array.

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Page 190-1
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Lab 5.4 Deviation

• This program declares and initializes an array,
  finds the mean and standard deviation of the
  array values, and prints the array values.
• It contains two versions of the deviation( )
  method and two versions of the mean( ) method
  one for double-valued arrays and one for
  int-valued arrays, and a print( ) method.

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Page 195-196
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Additional Practice

• 5.4 on page 226
• Write a method that returns an array that is a
  reversal of the original array

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Homework J4

• Write a program that creates an array of size 10
  integers, prompts the user for the 10 values and
  then calls the following three methods
• findAverage( ) calculates and displays the
  average value in the array.
• findMinimum( ) seeks and displays the mininum
  value in the array.
• printArray( ) prints the entire array.
• (See next slides for sample of input and output.)

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The program should prompt the user for 10 numbers
(one at a time)
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The program should display the following output