[Page 308]
7.1. Creating and Accessing Arrays
A
variable
(or simple variable) is a
name
to which Visual Basic can assign a single value. An
array variable
is a collection of simple
variables
of the same type to which Visual Basic can
efficiently
assign a list of values.
Consider the following situation: Suppose that you want to evaluate the exam grades for 30 students. Not only do you want to compute the average score, but you also want to display the names of the students whose scores are above average. You might place the 30 pairs of student
names
and scores in a text file and run the program outlined:
Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
Dim student1 As String, score1 As Double
Dim student2 As String, score2 As Double
Dim student3 As String, score3 As Double
.
.
Dim student30 As String, score30 As Double
'Analyze exam grades
Dim sr As IO.StreamReader = IO.File.OpenText("SCORES.TXT")
student1 = sr.ReadLine
score1 = CDbl(sr.ReadLine)
student2 = sr.ReadLine
score2 = CDbl(sr.ReadLine)
.
.
student30 = sr.ReadLine
score30 = CDbl(sr.ReadLine)
sr.Close()
'Compute the average grade
.
.
'Display names of above average students
.
. End Sub
This program is going to be uncomfortably long. What's most frustrating is that the 30 Dim statements and 30 pairs of statements reading from the file are very similar and look as if they should be condensed into a short loop. A shorthand notation for the many
related
variables would be welcome. It would be nice if we could just write
For i As Integer = 1 To 30
studenti = sr.ReadLine
scorei = CDbl(sr.ReadLine)
Next
Of course, this will not work. Visual Basic will treat
studenti
and
scorei
as two variables and keep
reassigning
new values to them. At the end of the loop, they will have the values of the thirtieth student.
[Page 309]
Declaring an Array Variable
Visual Basic provides a data structure called an
array
that lets us do what we tried to accomplish in the loop. The variable names, similar to those in the
preceding
program, will be
student(0), student(1), student(2), student(3), ..., student(29)
and
score(0), score(1), score(2), score(3), ..., score(29)
We refer to these collections of variables as the array variables
student
() and
score
(). The
numbers
inside the parentheses of the individual variables are called
subscripts,
and each individual variable is called a
subscripted variable
or
element
. For instance,
student
(3) is the fourth subscripted variable of the array
student
(), and
score
(20) is the 21st subscripted variable of the array
score
(). The elements of an array are assigned successive memory locations. Figure 7.1 shows the memory locations for the array
score
().
Array variables have the same kinds of names as simple variables. If
arrayName
is the name of an array variable and
n
is an Integer literal, variable, or expression, then the declaration statement
Dim
arrayName(n)
As
varType
reserves
space in memory to hold the values of the subscripted variables
arrayName
(0),
arrayName
(1),
arrayName
(2), ...,
arrayName
(
n
). The value of
n
is called the
upper bound
of the array. The number of elements in the array,
n
+1, is called the
size
of the array. The subscripted variables will all have the same data type; namely, the type specified by
varType
. For instance, they could be all String variables or all Integer variables. In particular, the statements
Dim score(29) As Double Dim score(29) As Double
declare the arrays needed for the preceding program.
Values can be assigned to individual subscripted variables with assignment statements and displayed in text boxes and list boxes just as values of ordinary variables. The default initial value of each subscripted variable is the same as with an ordinary variable; that is, the keyword Nothing for String types and 0 for numeric types. The statement
Dim score(29) As Double
[Page 310]
sets aside a portion of memory for the Integer array
score
() and
assigns
the default value 0 to each element.
The statements
score(0) = 87 score(1) = 92
assign values to the zeroth and first elements.
The statements
For i As Integer = 0 To 2
lstBox.Items.Add(score(i)) Next
then produce the following output in the list box:
87 92 0
As with an ordinary variable, an array declared in the Declarations section of the Code window is class-level. That is, it will be visible to all procedures in the form, and any value assigned to it in a procedure will persist when the procedure terminates. Array variables declared inside a procedure are local to that procedure and
cease
to exist when the procedure is exited.
Example 1.
(This item is displayed on pages 310 - 311 in the print version)
|
The following program creates a string array consisting of the names of the first four Super Bowl winners. Figure 7.2 shows the array created by the program.
[Page 311]
|
Object
|
Property
|
Setting
|
|
frmBowl
|
Text
|
Early Super Bowls
|
|
lblNumber
|
Text
|
Number from 1 to 4:
|
|
mtxtNumber
|
Mask
|
|
|
btnWhoWon
|
Text
|
Who Won?
|
|
lblWinner
|
Text
|
Winning Team:
|
|
txtWinner
|
ReadOnly
|
True
|
Private
Sub btnWhoWon_Click(
...
)
Handles
btnWhoWon.Click
Dim
teamName(3)
As String
Dim
n
As Integer
'Place Super Bowl Winners into the array
teamName(0) =
"Packers"
teamName(1) =
"Packers"
teamName(2) =
"Jets"
teamName(3) =
"Chiefs"
'Access array
n =
CInt
(mtxtNumber.Text)
txtWinner.Text = teamName(n - 1)
End Sub
[Run, type 2 into the masked text box, and click the button.]
|
The Load Event Procedure
In Example 1, the array
teamName
was assigned values within the btnWhoWon_Click event procedure. Every time the button is clicked, the values are reassigned to the array. This manner of assigning values to an array can be very inefficient,
especially
in programs with large arrays where the task of the program (in Example 1, looking up a fact) may be repeated
numerous
times for different
user
input. When, as in Example 1, the data to be placed in an array are known at the time the program begins to run, a more efficient location for the statements that fill the array is in the form's Load event procedure. The Load event occurs automatically when the form is loaded into memory, before it becomes visible on the screen. The header and End Sub statements for the Load event procedure are placed in the Code window when you double-click on the form. A typical header looks like
Private Sub frmName_Load(...) Handles MyBase.Load
The keyword MyBase is similar to the Me keyword and refers to the form being loaded. Example 2 uses the frmBowl_Load procedure to improve on Example 1.
[Page 312]
Example 2.
|
The following improvement on Example 1 makes
teamName
() a class-level array and assigns values to the elements of the array in the frmBowl_Load procedure:
Dim
teamName(3)
AsString
Private
Sub btnWhoWon_Click(
...
)
Handles
btnWhoWon.Click
Dim
n
As Integer
n =
CInt
(mtxtNumber.Text)
txtWinner.Text = teamName(n - 1)
End Sub
Private Sub
frmBowl_Load(
...
)
HandlesMyBase
.Load
'Place Super Bowl Winners into the array
teamName(0) =
"Packers"
teamName(1) =
"Packers"
teamName(2) =
"Jets"
teamName(3) =
"Chiefs"
End Sub
|
Like ordinary variables, array variables can be declared and assigned initial values at the same time. A statement of the form
Dim
arrayName
() As
varType = {value0, value1, value2, ..., valueN}
declares an array having upper bound
N
and assigns
value0
to
arrayName
(0),
value1
to
arrayName
(1),
value2
to
arrayName
(2), ..., and
valueN
to
arrayName
(
N
). For instance, in Example 4, the Dim statement and frmBowl_Load event procedure can be
replaced
by the single line
Dim teamName() As String = {"Packers", "Packers", "Jets", "Chiefs"}
The GetUpperBound Method
The value of
arrayName
.GetUpperBound(0) is the upper bound of the array. For instance, in Example 1, the value of
teamName.GetUpperBound(0)
is 3.
ReDim Statement
After an array has been declared, its size (but not its type) can be changed with a statement of the form
ReDim arrayName(m)
where
arrayName
is the name of the already declared array and
m
is an Integer literal, variable, or expression.
Note:
Since the type cannot be changed, there is no need for an "As
dataType
" clause at the end of the ReDim statement.
Visual Basic allows you to declare an array without specifying an upper bound with a statement of the form
Dim arrayName() As varType
Later, the size of the array can be stipulated with a ReDim statement.
[Page 313]
The ReDim statement has one shortcoming: It causes the array to lose its current contents. That is, it resets all String values to Nothing and resets all numeric values to 0. This situation can be remedied by following ReDim with the word Preserve. The general form of a ReDim Preserve statement is
ReDim Preserve
arrayName(m)
Of course, if you make an array smaller than it was, data in the eliminated elements will be lost.
Example 3.
(This item is displayed on pages 313 - 314 in the print version)
|
The following program reads the names of the winners of Super Bowl games from a file and places them into an array. The user can type a team's name into a text box and then display the numbers of the Super Bowl
games
won by that team. The user has the option of adding winners of
subsequent
games to the array of winners. The program uses the text file SBWINNERS.TXT whose lines contain the names of the winners in order. That is, the first four lines of the file contain the names Packers, Packers, Jets, and Chiefs.
Dim
teamName()
AsString
Dim
upperBound
As Integer
Private Sub
frmBowl_Load(...)
HandlesMyBase
.Load
Dim
sr
As
IO.StreamReader = IO.File.OpenText(
"SBWINNERS.TXT"
)
Dim
name
As String
, numLines
As Integer
'Count number of lines in the file and assign it minus one to upperBound
numLines = 0
Do While
(sr.Peek <> -1) name = sr.ReadLine numLines += 1
Loop
upperBound = numLines -1 sr.Close()
'Specify the title bar of the form
Me
.Text =
"First "
& upperBound + 1 &
" Super Bowls"
'Specify the caption of the Next Winner button
btnNextWinner.Text =
"Add Winner of Game "
& upperBound + 2
'Specify the size of the array and fill it with the entries of the file
ReDim
teamName(upperBound) sr = IO.File.OpenText(
"SBWINNERS.TXT"
)
For
i
As Integer
= 0
To
upperBound teamName(i) = sr.ReadLine
Next
sr.Close()
End Sub
[Page 314]
Private Sub
btnDisplay_Click(...)
Handles
btnDisplay.Click
'Display the numbers of the games won by the team in the text box
lstGamesWon.Items.Clear()
For
i
As Integer
= 0
To
upperBound
If
teamName(i) = txtName.Text
Then
lstGamesWon.Items.Add(i + 1)
End If
Next
End Sub
Private Sub
btnNextWinner_Click(...)
Handles
btnNextWinner.Click
'Add winner of next Super Bowl to the array
Dim
prompt
As String
upperBound += 1
'Add one more element to the array
ReDim Preserve
teamName(upperBound)
'Request the name of the next winner
prompt =
"Enter winner of game #"
& upperBound + 1 teamName(upperBound) = InputBox(prompt, ,
""
)
'Update the title bar of the form and the caption of the button
'Note: "Me" refers to the form.
Me
.Text =
"First "
& upperBound + 1 &
" Super Bowls"
btnNextWinner.Text =
"Add Winner of Game "
& upperBound + 2
End Sub
[Run, type "49ers" into the text box, and press the Display button. Then, feel free to add subsequent winners. Your additions will be taken into account when you next press the Display button.]
|
[Page 315]
Using an Array as a Frequency Table
An array can be used as either a checklist or a frequency table, as in the next example.
Example 4.
|
The following program
requests
a
sentence
as input and records the frequencies of the
letters
occurring in the sentence. The first element of the array
charCount
() holds the frequency with which the letter
A
occurs in the sentence, and so on. Recall that the function Asc
associates
each character with its position in the ANSI table.
Private Sub
btnAnalyze_Click(...)
Handles
btnAnalyze.Click
'Count occurrences of the various letters in a sentence
Dim
index
As Integer
Dim
sentence, letter
As String
Dim
charCount(25)
As Integer
'Examine and tally each letter of the sentence
sentence = (txtSentence.Text).ToUpper
For
letterNum
As Integer
= 1
To
sentence.Length letter = sentence.Substring(letterNum - 1, 1)
If
(letter >=
"A"
) And (letter <=
"Z"
) Then index = Asc(letter) - 65
'The ANSI value of "A"is 65
charCount(index) += 1
End If
Next
'List the tally for each letter of alphabet
lstCount.Items.Clear()
For
i
As Integer
= 0
To
25 letter = Chr(i + 65)
If
charCount(i) > 0
Then
lstCount.Items.Add(letter & " " & charCount(i))
End If
Next
End Sub
[Run, type a sentence into the text box, and press the button.]
|
[Page 316]
Comments
-
Using a subscript greater than the upper bound of an array is not allowed. For instance, the following lines of code produce the error dialog box shown immediately thereafter:
Dim trees() As String = {"Sequoia", "Redwood", "Spruce"}
txtBox.Text = trees(5)
-
If
arrayOne
() and
arrayTwo
() have been declared with the same data type, then the statement
arrayOne = arrayTwo
makes
arrayOne
() an exact duplicate of
arrayTwo
(). It will have the same size and contain the same information.
-
An array can require a large block of memory. After the array is no longer needed, the statement
Erase
arrayname
can be executed to release all memory allocated to the array.
Practice Problems 7.1
|
1.
|
Write code to declare an array with upper bound 10 and place the names of the five Great Lakes into the first five elements. Use one or two lines of code.
|
|
2.
|
In Example 3, rewrite the btnDisplay_Click event procedure so that only the first Super Bowl won by the
requested
team is displayed. The loop should terminate when the team is found.
|
|
3.
|
When should arrays be used to hold data?
|
[Page 317]
Exercises 7.1
|
1.
|
What is the size of an array whose upper bound is 100?
|
|
2.
|
What is the upper bound of an array whose size is 100?
|
In Exercises 3 through 12, determine the output displayed when the button is clicked.
|
3.
|
Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
Dim num As Integer = 2
Dim spoon(num) As String
spoon(0) = "soup"
spoon(1) = "dessert"
spoon(2) = "coffee"
txtOutput.Text = "Have a "& spoon(num - 1) & " spoon." End Sub
|
|
4.
|
Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
Dim a(20) As Integer
a(5) = 1
a(10) = 2
a(15) = 7
lstOutput.Items.Add(a(5) + a(10))
lstOutput.Items.Add(a(5 + 10))
lstOutput.Items.Add(a(20)) End Sub
|
|
5.
|
Dim sq(5) As Integer Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
Dim t As Integer
For i As Integer = 0 To 5
sq(i) = i * i
Next
lstOutput.Items.Add(sq(3))
t = 3
lstOutput.Items.Add(sq(5 - t)) End Sub
|
|
6.
|
Dim fh(3) As String Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
Dim n As Integer
Dim sr As IO.StreamReader = IO.File.OpenText("HORSEMEN.TXT")
For i As Integer = 0 To 3
fh(i) = sr.ReadLine
Next
[Page 318]
sr.Close()
lstOutput.Items.Add(fh(fh.GetUpperBound(0)))
n = 1
lstOutput.Items.Add(fh(2 * n)) End Sub
(Assume that the four lines of the file HORSEMEN.TXT contain the following entries: Miller, Layden, Crowley, Stuhldreher.)
|
|
7.
|
Dim s(3) As Integer Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
Dim t As Integer
Dim sr As IO.StreamReader = IO.File.OpenText("DATA.TXT")
t = 0
For k As Integer = 0 To 3
s(k) = CInt(sr.ReadLine)
t += s(k)
Next
txtOutput.Text = CStr(t)
sr.Close() End Sub
(Assume that the four lines of the file DATA.TXT contain the following entries: 3, 5, 2, 1.)
|
|
8.
|
Dim a(3), b(3) As Integer Dim c(3) As Double Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
Dim sr As IO.StreamReader = IO.File.OpenText("DATA.TXT")
For i As Integer = 0 To 3
a(i) = CInt(sr.ReadLine)
b(i) = CInt(sr.ReadLine)
Next
sr.Close()
For i As Integer = 0 To c.GetUpperBound(0)
c(i) = a(i) * b(i)
lstOutput.Items.Add(c(i))
Next End Sub
(Assume that the eight lines of the file DATA.TXT contain the following entries: 2, 5, 3, 4, 1, 3, 7, 2.)
|
|
9.
|
Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
'Compare the values of two chess pieces
Dim chess() As string = {"king", "
queen
", ""}
chess(2) = "rook"
ReDim Preserve chess(6)
chess(3) = "
bishop
"
txtOutput.Text = "A "& chess(2) & " is worth more than a "_
& chess(3) End Sub
|
|
|
[Page 319]
|
|
10.
|
Dim grade(1) As Double Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
Dim average As Double
ReDim grade(3)
grade(2) = 70
grade(3) = 80
average = (grade(0) + grade(1) + grade(2) + grade(3)) / 4
txtOutput.Text = "Your average is "& average End Sub Private Sub frmGrades_Load(...) Handles MyBase.Load
grade(0) = 89
grade(1) = 91 End Sub
|
|
11.
|
Dim score() As Double = {71, 68, 69} Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
'Find the sum of four scores
Dim total As Double
ReDim Preserve score(3)
score(3) = 72
total = score(0) + score(1) + score(2) + score(3)
txtOutput.Text = "Your total is "& total & "." End Sub
|
|
12.
|
Dim
band
() As String = {"soloist", "duet", "trio", "quartet"} Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
Dim num As Integer
ReDim Preserve band(9)
band(4) = "quintet"
band(5) = "sextet"
band(6) = InputBox("What do you call a
group
of 7
musicians
?")
num = CInt(InputBox("How many musicians are in your group?"))
txtOutput.Text = "You have a "& band(num - 1) & "." End Sub
(Assume that the first response is
septet
and the second response is
3.)
|
{% if main.adsdop %}{% include 'adsenceinline.tpl' %}{% endif %}
In Exercises 13 through 18, identify the errors.
|
13.
|
Dim companies(100) As String Private Sub frmFirms_Load(...) Handles MyBase.Load
Dim recCount Integer
Dim sr As IO.StreamReader = IO.File.OpenText("FIRMS.TXT")
recCount = CInt(sr.ReadLine)
ReDim companies(recCount - 1) As String
For i As Integer = 0 To recCount - 1
companies(i) = sr.ReadLine
Next
sr.Close() End Sub
|
[Page 320]
(Assume that the file FIRMS.TXT contains a list of companies and the first line of the file gives the number of companies.)
|
14.
|
Dim p(100) As Double Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
For i As Integer = 0 To 200
p(i) = i / 2
Next End Sub
|
|
15.
|
Dim a(10) As Integer Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
Dim sr As IO.StreamReader = IO.File.OpenText("DATA.TXT")
For i As Integer = 0 To 8
a(i) = CInt(sr.ReadLine)
Next
sr.Close()
For k As Integer = 0 To 8
a(k) = a(5 - k)
Next End Sub
(Assume that the nine lines of the file DATA.TXT contain the following entries: 1, 2, 3, 4, 5, 6, 7, 8, 9.)
|
|
16.
|
Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
Dim
films
() As String = {"Gladiator", "Titanic"}
lstOutput.Items.Add(films) End Sub
|
|
17.
|
Private Sub btnDisplay_Click(...) Handles btnDisplay.Click
Dim amount(2) As Integer = {7, 11, 14}
Dim total As Integer = 0
For i As Integer = 0 To 2
total += amount(i)
Next
txtOutput.Text = CStr(total) End Sub
|
|
18.
|
Private Sub frmNames_Load(...) Handles MyBase.Load
Dim recCount As Integer
Dim sr As IO.StreamReader = IO.File.OpenText("DATA.TXT")
recCount = CInt(sr.ReadLine)
ReDim names(recCount - 1) As String
For i As Integer = 0 to recCount - 1
names(i) = sr.ReadLine
Next
sr.Close() End Sub
(Assume that the four lines of the file DATA.TXT contain the following entries: 3, Tom, Dick, Harry.)
|
|
|
[Page 321]
|
|
19.
|
Assuming that the array
river
() is as
follows
, fill in the empty rectangles to
illustrate
the progressing status of
river
() after the execution of each program segment:
temp = river(0) river(0) = river(4) river(4) = temp
temp = river(0) For i As Integer = 0 To 3
river(i) = river(i + 1) Next river(4) = temp
|
|
20.
|
Assuming the array
cat
() is as follows, fill in the empty rectangles (on the next page) to show the final status of
cat
() after executing the nested
loops
:
For i As Integer = 0 To 2
For j As Integer = 0 To 3 - i
If cat(j) > cat(j + 1) Then
temp = cat(j)
cat(j) = cat(j + 1)
cat(j + 1) = temp
End If
Next Next
[Page 322]
Dim gap As Integer = 2 Do While gap <= 1
Do
doneFlag = True
For i As Integer = 0 To 3 - gap
If cat(i) > cat(i + gap) Then
temp = cat(i)
cat(i) = cat(i + gap)
cat(i + gap) = temp
doneFlag = False
End If
Next
Loop Until doneFlag = True
gap = CInt(gap / 2) Loop
|
|
21.
|
The subscripted variables of the array a() have the following values: a(0) = 4, a(1) = 6, a(2) = 3, a(3) = 1, a(4) = 2, a(5) = 5, a(6) = 8, a(7) = 7. Suppose i = 2, j = 4, and k = 5. What values are assigned to
n
when the following assignment statements are executed?
-
n = a(k) - a(i)
-
n = a(k - i) + a(k - j)
-
n = a(k) * a(i + 2)
-
n = a(j - i) * a(i)
|
|
22.
|
The array
monthName
() holds the following three-character strings:
monthName(0)="Jan", monthName(1)="Feb", ..., monthName(11)="Dec"
-
What is displayed by the following statement?
txtMonth.Text = monthName(3) & " " & monthName(8)
-
What value is assigned to
fall
by the following statement?
fall = monthName(8) &", " &monthName(9) &", " &monthName(10)
|
|
23.
|
Modify the program in Example 3 to display "Never Won a Super Bowl" if the specified team has never won a Super Bowl game.
|
|
24.
|
Modify the program in Example 3 so that the text file SBWINNERS.TXT is only read once.
|
[Page 323]
In Exercises 25 through 36, write a line of code or program segment to complete the stated task.
|
25.
|
Suppose the Integer array
quantities
() has been declared with an upper bound of 100 and data have been placed into several elements of the array. Write one line of code that will restore the values of all the elements to their default value of 0.
|
|
26.
|
Write code to declare an array of size 20 and place the names Jerry, George, Elaine, and Kramer into the first four elements. Use one or two lines of code.
|
|
27.
|
Declare the string array
marx
() with upper bound 3 so that the array is visible to all
parts
of the program. Assign the four values Chico, Harpo, Groucho, and Zeppo to the array before any
buttons
are pressed.
|
|
28.
|
Declare the string array
stooges
() to have size 3 so that the array is local to the event procedure btnStooges_Click. Assign the three values Moe, Larry, and Curly to the array as soon as the button is clicked.
|
|
29.
|
The arrays
a
() and
b
() have been declared to have upper bound 4, and values have been assigned to
a
(0) through
a
(4). Store these values in array
b
() in reverse order.
|
|
30.
|
Given two arrays of type Double,
p
() and
q
(), each with upper bound 20, compute the sum of the products of the corresponding array elements, that is,
p(0)*q(0) + p(1)*q(1) + p(2)*q(2) + ... + p(20)*q(20)
|
|
31.
|
Display the values of the array
a
() having upper bound 29 in five
columns
, like the following:
|
a(0)
|
a(1)
|
a(2)
|
a(3)
|
a(4)
|
|
a(5)
|
a(6)
|
a(7)
|
a(8)
|
a(9)
|
|
·
|
·
|
·
|
·
|
·
|
|
·
|
·
|
·
|
·
|
·
|
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·
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·
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·
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·
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·
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a(25)
|
a(26)
|
a(27)
|
a(28)
|
a(29)
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32.
|
A list of 20 digits, all between 0 and 9, is contained in a text file. Display the frequency of each digit.
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33.
|
Compare two arrays
a
() and
b
() of size 10 to see if they hold identical values, that is, if for all
i
from 0 through 9.
|
|
34.
|
Calculate the sum of the entries with odd subscripts in an integer array
a
() of size 10.
|
|
35.
|
Twelve exam grades are stored in the array
grades
(). Curve the grades by adding 7 points to each grade.
|
|
36.
|
Read 10 digits contained in a text file into an array and then display three columns in a list box as follows: column 1 should contain the original 10 digits, column 2 should contain these digits in reverse order, and column 3 should contain the averages of the corresponding digits numbers in columns 1 and 2.
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[Page 324]
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37.
|
Thirty scores, each lying between 0 and 49, are given in a text file. Write a program that uses these scores to create an array
frequency
() as follows:
frequency102 = # of scores < 10
frequency112 = # of scores such that 10 < = score < 20
frequency122 = # of scores such that 20 < = score < 30
frequency132 = # of scores such that 30 < = score < 40
frequency142 = # of scores such that 40 < = score < 50
The program should then display the results in tabular form as follows:
|
Interval
|
Frequency
|
|
0 to 9
|
frequency(0)
|
|
10 to 19
|
frequency(1)
|
|
20 to 29
|
frequency(2)
|
|
30 to 39
|
frequency(3)
|
|
40 to 49
|
frequency(4)
|
|
|
38.
|
Write a program that asks the user for a month by number and then displays the name of that month. For instance, if the user inputs 2, the program should display February.
Hint:
Create an array of 12 strings, one for each month of the year.
|
|
39.
|
The file USPRES.TXT contains the names of the 43 U.S.
Presidents
in the order they
served
. Write a program that places the names in an array, supplies all presidents having a requested first name, and
supplies
all presidents for a requested range of numbers. The following is one possible outcome:
|
|
40.
|
The file COLORS.TXT contains the names of the colors of Crayola
crayons in alphabetical order. Write a program to read the colors into an array and then display the colors beginning with a specified letter. The program should declare an array with upper bound 50 and then increase the size of the array by 10 elements whenever it runs out of space to store the colors. One possible outcome is the following:
[Page 325]
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41.
|
An
anagram
of a word or phrase is another word or phrase that uses the same letters with the same frequency. Punctuation marks and spaces are ignored. Write a program that requests two words or phases as input and determines if they are anagrams of each other. (Test the program with the pair of phrases DWIGHT DAVID EISENHOWER and HE DID VIEW THE WAR DOINGS.)
|
Solutions to Practice Problems 7.1
|
1.
|
Two possible answers are as follows:
Dim lakes() As String = {"Huron", "Ontario", "Michigan", _
"Erie", "Superior", "", "", "", "", "", ""}
Dim lakes() As String = {"Huron", "Ontario", "Michigan", _
"Erie", "Superior"}
ReDim Preserve lakes(10)
|
|
2.
|
This task consists of searching through an array and is carried out just like the task of searching a text file for a specified item. Replace the For loop with the following code:
Do While (teamName(i) <> txtName.Text) And (i > upperBound)
i += 1
Loop
If teamName(i) = txtName.Text Then
lstGamesWon.Items.Add(i + 1)
End If
|
|
3.
|
Arrays should be used when
-
several pieces of data of the same type will be entered by the user; or
-
computations
must be made on the items in a text file after all of the items have been read.
|
|
