Introduction to Arrays

Chapter 7

Why use arrays?

• To store & represent lists of homogeneous values

• To simplify program code

• To eliminate the need to reread data

The kinds of problems that benefit from array representation of data• Roll a die (6 sides) 1000 times & record the

frequency of each outcome. Display it.

• Shuffle a deck of 52 cards & then choose 1 card. Record the frequency of each outcome.

• Read in 100 integers (e.g., test scores, prices). Sort them in ascending (or descending) order.

142 175 182162 149 129

Details & terminology• To use an array, we must establish its

subscript range - this must agree with the number of elements the array will have.

• Subscript = position in the array

• Element = the contents of the array at a particular position

• Ex: num(5) is the 5th element in the array called num

Array declaration

• All arrays must be declared using a Dim statement:

Dim num(1 To 25) As Single

Array name

Subscript range

Element type

Where to declare arrays

• Most of the time, arrays are declared as form-level variables

• This means that we Dim them in the “Declarations” section of (General)

– this is done in code view

• Any form-level variable can be used by any procedure or function without being declared as a parameter.

Declaring & referencingDim x (1 To 8) As Single

x: 3 5 12 0 43 -4 82 1

Dim y (1 To 5) As String

y: “abc” “3” “you” “hi” “#”

• x(3), x(7), x(2) + x(5), x(2 + 5)

• y(1), y(4), y(2) & y(3), len(y(5))

• if k = 3, what is x(k)? y(k)? x(k) + 1? x(k+1)? y(k-1) & y(k+1)? y(x(8))?

Common array activities

• Read data values from a user or an input file into an array and process them– some processing requires visiting a data value

more than once, so arrays are recommended or required

• Search an array

• Sort array elements– there are many sorting algorithms to use

Array processing requires iteration (looping)

• If we must process every element of an array, then we use a For…Next loop

• If we will process the array while or until a certain condition is met, then we use a While or Until loop

• Some of this iterative processing requires doubly nested loops– Sorts

Fill an array with user’s dataDim a (1 To 10) As Integer

…...

For j = 1 To 10

a(j) = Val (InputBox(“Enter an integer.”)

Next j

Because we must process every element, we use For...Next

Fill an array with data from an input file

Dim a (1 To 10) As Integer…...

Open “Integers.txt” for input as #1

For j = 1 To 10

Input #1, a(j)

Next j

Close #1Because we must process every

element, we use For...Next

Find the largest element in an array

Dim num (1 To 100) of Any_Type

………...

max = num(1)

For k = 2 to 100

if num(k) > max then

max = num(k)

endif

next kBecause we must process every

element, we use For...Next

Filling arrays at the time that the program is loaded

• Since the array has been declared as a form-level variable, we can use the Form_Load event procedure to fill an array when the program is first loaded to be run:

Private Sub Form_Load()

• See p. 307, Example 2

To recap...

• We have several ways to enter data into an array:– from an input file

– from interactive user input

– from the form_load event procedure

Searching

&

Sorting

Searching

• There is a variety of searching algorithms.

• The fastest ones require that the array elements be sorted (usually in ascending order) first.

• We study the sequential search, which makes no assumptions about the order of elements.

Find the position of the first occurrence of “$$$”

Dim ch (1 To 50) of String…

Let pos = 0Let found = “no”Do While pos < 50 and found = “no”

Let pos = pos + 1if ch(pos) = “$$$” then

Let found = “yes”LoopIf found = “yes” then

Picture1.Print “$$$ was found in position ”; pos

Because we need to process only until we find what we’re looking for, we use a While (or Until) loop

Convert the previous code to a function

• Write a function that receives an array and an object to search for. Return the position of the first occurrence of the item; if the search is unsuccessful, return 0.

• The function header:Private Function find (object As String) As Integer

• The function call:

picBox.Print find (x)(where x = “$$$” from our previous example)

Using & searching parallel arrays

• An Inventory File:Dim product_name (1 To 30) As StringDim product_id (1 To 30) As IntegerDim product_cost (1 To 30) As SingleDim num_in_stock (1 To 30) As IntegerDim num_on_order (1 To 30) As Integer

• We can ask:“How many widgets do we have in stock?”

Sorting• There are many sorting algorithms from

which to choose.

• The most straightforward ones are not usually the fastest, but they get the job done & they’re relatively easy to program.

• Bubble Sort is in this category.– More to come on this

Control arrays (Sec. 7.3)

• We can organize form objects such as text boxes & labels into arrays.– See Example 1, p. 339: Suppose we have five

departments - each one gets its own text box with its own label