Arrays

1
Arrays

• ITK 169
• Fall 2003

2
Single Dimension Arrays

• In C an array consisting of 5 variables of type
  int is declared
• int score5
• The value in the brackets is called the declared
  size of the array.
• Such a declaration creates 5 int type variables
  which are accessed as
• score0, score1, score2, score3,
  score4
• The number in the brackets is called an index or
  subscript.
• Array subscripts start at 0 and run to one less
  than the declared size of the array.
• All indexed variables in a particular array must
  have the same type.
• This type is called the base type of the array.
• Be careful not to confuse the declared size in a
  declaration of an array with the index value in
  an array reference.
• int array_namedeclared_size
• array_nameindex // 0 lt index lt
  declared_size

3
Be Very Careful!

• The most common programming error when using
  arrays is the attempt to reference a non-existent
  array index.
• The array definition
• int a6
• declares ONLY the indexed variables a0 through
  a5.
• An index value outside 0 to 5 is an out of range
  error, i.e., illegal.

4
Arrays in Functions

• Arrays are passed by reference automatically (DO
  NOT use ).void my_funct(int anArray )
• If you do not want the array to be changed you
  must hold it constant.void my_funct(int const
  anArray )
• An array cannot be a return type.

5
PracticeGiven a Square Array

• Write a function to find the sum of the main
  diagonal of an array. (Pass the array in and hold
  it constant. Return the sum.)
• Change the function to find the sum of the second
  diagonal.
• Change the function to find the sum of a single
  row (passing in the index of the row).

6
More Practice

• void fill_array(int myArr , int size, int
  number_used)
• //Precondition size is the declared size of the
  array myArr.
• //Postcondition number_used is the number of
  values stored in myArr.
• // myArr0 through myArrnumber_used-1 have
  been filled
• // with nonnegative integers read from the
  keyboard.
• int search(const int myArr , int number_used,
  int target)
• //Precondition number_used is lt the declared
  size of myArr.
• //Also, myArr0 through myArrnumber_used -1
  have values.
• //Returns the first index such that myArrindex
  target,
• //provided there is such an index, otherwise
  returns -1.

7
Multidimensional Arrays

• It is useful to have an array with more than one
  index. This is called a multidimensional array.
• Such an array is declared as following
• char page30100
• This array has 30 rows and 100 columns.
• There are 30100 indexed variables for this
  array. The indexed variables for this array are
• page00, page00, . . . .
  page099
• page10, page11, . . . .
  page199
• page20, page21, . . . .
  page299
• . .
  .
• . .
  .
• . .
  .
• page290, page291, . . . page2999

8
Multidimensional Arrays as Parameters

• When passing a multidimensional array into a
  function, the first dimension is not given a
  size, but the second MUST have the size.void
  myFunct(char page 100)
• Remember, this array is passed by reference and
  changes will be returned to the calling
  function.

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10
Practice

• const int NUMBER_STUDENTS 4, NUMBER_QUIZZES
  3
• //Reads quiz scores for each student into the
  two-dimensional array grade
• void read_scores(int grade NUMBER_QUIZZES)
• //Precondition Global constant NUMBER_STUDENTS
  and NUMBER_QUIZZES
• //are the dimensions of the array grade.
• //Postcondition Each of the indexed variables
  gradest_num-1, quiz_num-1 contains the score
  for student st_num on quiz quiz_num.
• // Computes the average score for each student
  and
• //the average score for each quiz. Displays the
  quiz scores and the averages.
• void compute_st_ave(const int gradeNUMBER_QUIZZ
  ES, double st_ave)
• //Precondition Global constant NUMBER_STUDENTS
  and NUMBER_QUIZZES
• //are the dimensions of the array grade. Each of
  the indexed variables
• //gradest_num-1, quiz_num-1 contains the score
  for student st_num on quiz
• //quiz_num.
• //Postcondition Each st_avest_num-1 contains
  the average for student number
• //matching the row number of the grade
  array.
• Write a third function to compute the quiz
  averages. (see next slide).

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12
Arraysand Class Objects

• Need practice problem for an array of objects
• Need practice problem for a class with an array
  member